Plant Anatomy and Morphology Course Specifications (B111) PDF

Department of Botany and microbiology (B111) First term 2022-2023 1  Course specifications Menoufia University Faculty of Science Programme on which the course is given B.Sc. Botany, Botany & Chemistry Maj...

Department of Botany and microbiology (B111) First term 2022-2023 1  Course specifications Menoufia University Faculty of Science Programme on which the course is given B.Sc. Botany, Botany & Chemistry Major or minor element of programmes Major Department offering the programme Botany Department offering the course Botany Academic year / Level First (1 *) Basic Information Title: Plant anatomy and Morphology Code: B111 Credit Hours: 3 h Lecture: 2h Practical: 2h A: Overall aims of course After studying this course, the student should be able to define the plant shapes and its external structure, the different plant seeds, and their differences in both structure and function. The course aims also at development the student’s knowledge about the differences of internal structure between the dicotyledonous and monocotyledonous plants (roots, stems and leaves). The students should be also able to: - Compare and contrast between the different plants and plant seeds based on the macro morphology - Summarize the functions of each tissue in the plant body - Compare between the dicotyledonous and monocotyledonous plants from their internal structure. - Identify the different taxa of tracheophyta by using their external and internal structure and use this as a tool in plant identification. B- Professional Information 1- Overall aims of course After studying this course, the students should be able to: i) Define the morphological characteristics of the plant and its external structure. ii) Differentiate between seeds in different plants and their differences in both structure and function. iii) Differentiate the different germination stages. 2  iv) Know about the differences of the internal structure (roots, stems and leaves) between the dicotyledonous and monocotyledonous plants. 2- Intended learning outcomes of course (ILOs) a. Knowledge and understanding The students should be able to: a1- Define plant seeds and differentiate between different plant organs. a2- Describe the external structure of both dicotyledonous and monocotyledonous plants a3- Summarize the differences between dicotyledonous and monocotyledonous plants a4- State the internal structure of both dicotyledonous and monocotyledonous plants a5- Illustrate the differences between dicotyledonous and monocotyledonous plants based on the differences between them in internal structure. a6- List the common terminologies of external and internal plant structure. b. Intellectual skills The students should be able to: b1- Differentiate between the different plant seeds based on the physical characteristics b2- Compare between the dicotyledonous and monocotyledonous plants from their external and internal structure. b3- Determine the different tissues in the plant body. c. Professional and practical skills The students should be able to: c1-Use appropriate lab equipment to Identify the different taxa of Tracheophyta c2- Collect and preserve the dissected and prepared plant specimens (as permanent slides) 3 Contents No. Topics page Introduction 1 4 Plant seed structure and seed types 2 6 Germination 3 9 Root system 4 27 Shoot system 5 35 Leaves 6 47 The plant cell 8 64 Formation of cell wall 9 67 Cytoplasm and membranous system in the cell 10 74 Tissues and tissue systems 11 81 Permanent tissue (epidermal tissue, Fundamental tissue) 12 86 Vascular or conducting tissue 13 103 Internal structure of stems 14 120 Internal structure of roots 15 130 Internal structure of leaves 16 139 References 17 146 4  Part I: Plant Morphology Introduction The branch of biology that deals with the structure of animals and plants is Morphology While, Plant morphology (phytomorphology) is the general term for the study of physical form and external structure of plants. The study of plant morphology is useful in the identification of plants. Plant morphology represents a study of the development, form, and structure of plants, and, by implication, an attempt to interpret these on the basis of similarity of plan and origin. There are major areas of investigation in plant morphology, and each overlaps with another field of the biological sciences. First, morphology is comparative, in which the morphologists examine structures in many different plants of the same or different species, then draws comparisons and formulates ideas about similarities. Secondly, plant morphology studies both the vegetative (somatic) structures of plants, as well as the reproductive structures. The 5 vegetative structures of vascular plants include the study of the shoot system, ( stems and leaves), as well as the root system. The reproductive organs are more varied, and are usually specific to a particular group of plants, such as flowers and seeds. The detailed study of reproductive structures in plants led to the discovery of the alternation of generations found in all plants and most algae. The study of plant morphology overlaps with the study of biodiversity and plant systematic. Thirdly, plant morphology studies plant structure at a range of scales. At the largest scale is the study of plant growth habit, the overall architecture of a plant. The pattern of branching in a tree varies from species to species, as will the appearance of a plant as a tree, herb, or grass. Fourthly, plant morphology studies the pattern of development, the process by which structures originate and mature as a plant grows. Plants constantly produce new tissues and structures throughout their life. A living plant always has embryonic (meristematic) tissues. The way in which new structures mature as they are 6 produced may be affected by the stage in the plants life when they begin to develop, as well as by the environment to which the structures are exposed. The characters used in descriptions or for identification are called diagnostic or key characters, which can be either qualitative or quantitative. 1. Quantitative characters are morphological features that can be counted or measured for example a plant species has flower petals 5-8 mm wide. 2. Qualitative characters are morphological features such as leaf shape, flower color or hairiness. Both kinds of characters can be very useful for the identification of plants. A vascular plant begins from one celled zygote, formed after fertilization of an egg cell by a sperm cell. From that point, it begins to divide to form an embryo through the process of embryogenesis. Through this happens, the resulting cells will organize so that one end becomes the first root, while the other end forms the tip of the shoot. 7 In seed plants, the embryo will develop one or more seed leaves (cotyledons). By the end of embryogenesis, the baby plant will have all the parts necessary to begin in its life enclosed in solidified integuments (seed coat) which is called in general the seed. Seeds and seed functions A seed is an embryonic plant enclosed in a protective outer covering (solidified ovule integuments). The formation of the seeds is part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiosperm plants. Seeds are the product of the fertilized ripened ovule, after fertilization by male gametes from pollen grain and some growth within the mother plant. The embryo is developed from the zygote and the seed coat from the integument (s) of the ovule. Seeds with testa protect and nourish the embryo or baby plant. Seeds usually nourish and give a seedling fast because of the larger food reserves in the seed. Different plants have evolved many ways to disperse their population through their seeds. 8 Seed dispersal is often attributed mainly to fruits, however many seeds aid in their own dispersal An important function of most seeds is delaying germination (dormancy) to allow time for dispersal and to prevent all seeds from germinating at once when conditions are favorable. Economic importance of seeds Many seeds are edible (used in nutrition). Seeds also provide most used oils. Many important nonfood oils are extracted from seeds, many beverages, spices, and some important food additives. Seeds are used to propagate many crops such as legumes, forest trees, turf grasses ‫العشب‬, and pasture grasses ‫المرعى‬. Some seeds are poisonous. The most important clothing fiber in the world grows attached to cotton seed. Seeds are the source of some medicines such that of castor oil, tea tree oil. 9 Germination Germination is usually the emergence of radicle and plumule as a result of the growth of a baby plant (Embryo) contained within a seed; it results in the formation of the seedling. Seeds remain dormant or inactive until conditions are favourable for germination. All seeds need water, oxygen, and proper temperature to germinate. Some seeds require proper light also. Some seeds germinate better in full light while others require darkness to germinate. When a seed is exposed to the proper conditions, water and oxygen are taken in through the micropyle in seed coat. The embryo's cells start to divide and enlarge. Then the seed coat breaks open and a radicle emerges first, followed by plumule that contains the leaves and stem. Many things can cause poor germination such as overwatering that causes the plant to have not enough oxygen. Planting seeds too deeply may cause them to use all stored energy before reaching the soil surface. Dry conditions mean the plant does not have enough moisture to start the germination process and keep it going. 10 Some seed coats are so hard that water and oxygen cannot get through until the coat breaks down. Other seeds need to be exposed to proper temperatures. Factors affecting seed germination The factors affecting seed germination can be Abiotic (environmental) or Biotic (internal). A) Abiotic factors that control or affect seed germination can be summarized as: 1) Temperature: every species of seed, have an optimal soil temperature for germination, and at that temperature, the maximum number of seeds will germinate in less time than at any other temperature. Cold (low) temperature is not favorable for seed germination. Seeds prefer higher temperature. The rate of seed germination is directly proportional to the rise in temperature. 2) Moisture or water: Dry seeds do not germinate. They must first imbibe water to start the process of germination. For non-dormant seeds, germination starts when a seed is provided with water as long as the temperature is appropriate. The uptake of water by dry seed is called imbibition (imbibition means to drink: seeds imbibe water). As seeds imbibe water, they expand and 11 enzymes and food supplies become hydrated. Hydrated enzymes become active and the seed increase its metabolic activities to produce energy for the growth process. In addition, the water causes turgor pressure to increase in the cells and they are able to enlarge. The first part of the seedling to emerge from the seed coat is the root (radical). The emergence of the root is typically used as the first indication that a seed is viable. Eventually the shoot will also expand and emerge from the seed. 3) Light: Plants and seeds can sense light quality (wavelength), light direction, periodicity, and intensity. Light is not required for germination only but later on it plays a dominant role. The effects of light vary between different plant species and varieties. Some seed germination is stimulated, and others are inhibited by light. 4) Soil: It is not required initially, but as the seedlings grow they require mineral elements for further growth. This requirement can be fulfilled only by soil. 12 B) Biotic factors that control seed germination 1) Viability of the seeds: A viable seed is one, which is capable of germination under suitable conditions. While, non-viable seed, is one which fails to germinate even under optimal conditions. The practical viability indicates the ability of the seed to germinate and the ability of the seedling to establish itself in the environment in which the seed finds itself. However, when seeds are utilized, to produce a crop, for example, then viability is a measure of the suitability of the seed batch to produce a satisfactory crop. After the seeds are produced, they remain viable up to certain period that varies from plant to plant or seed to seed. Indeed ‫في‬ ‫ الحقيقة‬if a seed is not allowed to germinate (sprout) within some certain length of time, the embryo inside will die. Each species of seed has a certain length of viability. Some plant species have seeds that need to sprout within few weeks of being dispersed, or they die. Lotus seeds have viability period of more 13 than 800 years. Once that period expires, the seed germination is difficult. 2) Seed dormancy Seed dormancy is defined as a seed failing to germinate under environmental conditions optimal for germination. It is often confused with seed quiescence ‫السكون‬, which is a seed failing to germinate because environmental conditions are inappropriate for germination. A seed in dormancy certainly looks dead. It does not seem to grow, nor do anything. In fact, even with biochemical tests for the metabolic processes associate with life (respiration, etc.) the rate of these processes is so slow that it would be difficult to determine whether there really was anything alive in a seed. Reasons of seed dormancy There are several mechanisms that permit seeds to be truly dormant. 14 1- Seed Coat Many kinds of seeds have very thick seed coats. These obviously keep water out of the seed, so the embryo cannot get the water needed to activate its metabolism and start growing. Some species might use some pounding along a river or drop seeds into seacoast surf ‫ األمواج‬to abrade ‫ حك‬the thick seed coat. Other seeds might need a vertebrate or other animal to attack the seed coat and thereby weaken the coat. The process of nicking ‫ خدش‬the thick seed coat to initiate germination is called scarification. There are five basic mechanisms of coat-imposed dormancy: 1. Prevention of water uptake. Prevention of water uptake by the seed coat is a common cause of seed dormancy in families found in arid and semiarid regions, especially among legumes. Waxy cuticles, suberized layers, and lignified sclereids all combine to restrict the penetration of water into the seed. 2. Mechanical constraint. The first visible sign of germination is typically the radicle breaking through the seed coat. In 15  some cases, however, the seed coat may be too rigid for the radicle to penetrate. Nuts with hard, lignified shells are examples of dormancy caused by mechanical constraint. Such shells must be broken by biotic or environmental forces for the seed to germinate. 3. Interference with gas exchange. Some seed coats are considerably less permeable to oxygen than an equivalent thickness of water is. This lowered permeability to oxygen suggests that the seed coat inhibits germination by limiting the oxygen supply to the embryo. 4. Inhibitor production. Seed coats and pericarps may contain relatively high concentrations of growth inhibitors that can suppress germination of the embryo. Abscisic acid is a common germination inhibitor present in these maternal tissues. In certain cases where repeated washing (leaching) removes dormancy, the effect is thought to be due to the loss of such inhibitory compounds. 5. Retention of inhibitors. The seed coat may prevent the escape of inhibitors from the seed. For example, growth inhibitors 16  readily diffuse out of isolated Xanthium embryos, but not from intact seeds. 2- Presence of inhibitors Many plant species invest chemicals in the developing seeds, and these chemicals inhibit the development of the embryos. They keep the embryos dormant. Obviously, the seed must have some way to eliminate these chemicals before they can sprout. Abscisic acid induces dormancy in the embryo of many temperate zone species that use inhibitors. Abscisic acid is produced in abundance in the late summer and early fall. The seeds in the fruits become dormant so, they cannot sprout. During the winter, enzymes in the seeds degrade the abscisic acid. By spring, the abscisic acid is gone and the seed can sprout. This process is called vernalization. In laboratory the seeds are put in moist soil and refrigerated for about four weeks. This is sufficient time to degrade the abscisic acid. Then the planted seeds are placed in a warm greenhouse. The seeds assume winter is over, spring has come, and they begin to sprout. 17 Phenolic Compounds: Plants that live in deserts have a different problem. Deserts typically have very long dry seasons and a short wet season accompanied by flash floods and so on. These plants use phenolic compounds, to keep their seeds dormant until the proper season for germination. Phenolic compounds are freely water-soluble. Leaching simply requires washing seeds with fresh water. It may be accomplished by running water over. 3- Insufficient Development If a seed's embryo is not completely developed, some additional maturation may be needed before the seed can sprout. These seeds do not germinate immediately after they are produced. They require a resting period (after ripening period) during which they remain dormant and once that period is over, they are ready to germinate. Based on the timing of dormancy different types of seed dormancy can be distinguished. 18   Seeds that are released from the plant in a dormant state are said to exhibit primary dormancy.  Seeds that are released from the plant in a non-dormant state but which become dormant if the conditions for germination are unfavorable exhibit secondary dormancy. Types of germination  Hypogeal germination implies that the cotyledons stay below the ground. The epicotyl (part of the stem above the cotyledon) grows, while the hypocotyl (part of the stem below the cotyledon) stays the same length. In this way, the epicotyl pushes the plumule above the ground. Normally, the cotyledon is fleshy, and contains many nutrients that are used for germination. No photosynthesis takes place within the cotyledon  Epigeal germination implies that the cotyledons are pushed above ground. The hypocotyl elongates while the epicotyl stays the same length. In this way, the hypocotyl pushes the cotyledon upward. 19  Normally, the cotyledon itself contains very little nutrients in plants that show this kind of germination. Instead, the first leaflets are already folded up inside it. Photosynthesis starts to take place in it rather quickly. Types of seeds Endospermic seed structure: In mature seeds, the embryo is surrounded by 1-2 cell layers of endosperm. Seeds exhibit, two- step germination process with distinct testa rupture and endosperm rupture. Endospermic dicotyledonous seed structure; Castor bean The castor bean (Ricinus communis) plant is a perennial shrub with large, palmately lobed leaves and sharply toothed leaf margins. The leaves are usually deep green, but in some strains, they have a reddish cast. The fruit is round, spiny capsule, often reddish, containing up to three shiny, smooth, mottled ‫ مزركشة‬seeds. Castor bean seeds contain an elaiosome. The particular elaiosomes of Euphorbiaceae seeds are called caruncle. Elaiosomes are fleshy structures that are attached to seeds of many plant species. They are rich in lipids and proteins an have different 20 shapes. In many cases elaiosomes attract ants, which disperse the seed. Castor bean seeds are endospermic, the embryo is spatulate. The cotyledons are thin and broad and the endosperm is the major storage tissue. Castor bean seed germination is epigeal and the cotyledons of the seedling absorb the nutrients from the endosperm, which encloses the cotyledons until it is obliterated ‫تختفى‬. The hypocotyl grows due to which two papery cotyledons enclosed by endosperm are pulled out of the soil. Cotyledons come out of the endosperm when it is consumed. The cotyledons become green and leaf-like, while the plumule slowly develops into leafy shoot. The remanents of endosperm withers and drop off 21 Non-endospermic dicotyledonous seed structure: Fabaceae The cotyledons serve as sole food storage organs. During embryo development, the cotyledons absorb the food reserves from the endosperm. The endosperm is almost degraded in the mature seed and the embryo is enclosed by the testa. The legume family plants are examples. Broad bean (Vicia faba) is a stiffly erect plant 0.5–1.8 m tall, with stout stems of a square cross-section. The leaves are 10–25 cm long, compound pinnate with 2–7 leaflets, and of a distinct grey- green color. The leaves do not have tendrils for climbing over other vegetation. The flowers are 1–2.5 cm long, with five petals, the standard petal white, the wing petals white with a black and the keel petals are white. The flowers have a strong and sweet scent ‫الرائحة‬, which is attractive to bees and other pollinators. The fruit is a broad, leathery pod, green maturing to blackish- brown; the pods are 5–10 cm long and 1 cm diameter. Each pod contains 3–8 seeds. Seeds may be round to oval, usually flattened 22 and up to 20–25 mm long, 15 mm broad and 5–10 mm thick in food cultivars. The seed is made up of two parts, the testa and all that within the testa (the embryo). The testa is of a uniformly yellowish brown colour, broken by a sharply defined, long black band at one end. This mark is the hilum and denotes the region where first the ovule, and later the seed, was attached to the funicle, the boat-shaped stalk that formed the attachment to the pod. On the curved rim of the seed, above one end of the hilum, a triangular bump marks the position of the radicle. Between the apex of the bump and the end of the hilum there is a minute hole, or micropyle, through which the pollen-tube entered the ovule in fertilization. The presence of the micropyle is made obvious by soaking the seed in water. Gentle pressure near the hilum will cause drops of water to exude from the micropyle. It is obvious that the hole is well placed in relation to the tip of the radicle that must, eventually, emerge. 23 In such soaked seeds it is an easy to remove the testa. On one curve, the radicle fits into an inner pocket. The radicle is in a somewhat unprotected position because of being so near the micropyle, which is an area of weakness, and this pocket gives it extra protection. When the testa is removed, the naked embryo is fully exposed. It consists of three parts: (1) Two seed- leaves, or cotyledons, which are attached laterally to a very short primary axis. (2) The radicle, or young root, which points directly downwards from the primary axis. (3) The plumule, or rudimentary bud, which grows directly upwards from the primary axis. It lies between the cotyledons at right angles to the radicle. The plumule develops into the plant’s whole system of leaf- bearing, flower-bearing, and fruit-bearing parts. In germination the pressure of the growing root tears the testa at the point of weakness and its tip emerges in the region that was once the micropyle. When the root is well established in the ground, the plumule grows up. Its tip is bent like a hook, so that the delicate leaves of this first 24 bud are not subjected to friction ‫اإلحتكاك‬. Once the tip has broken through the surface of the soil it becomes erect by the straightening of the stem. The first leaves borne on the stem are simply stipules. These are succeeded by compound leaves made up of two leaflets and a rudimentary tendril. At the base of each compound leaf is a pair of stipules. The later leaves have three pairs of leaflets and the rudimentary tendril is still present. The elongation of the stem of the plumule, the primary bud of the plant, like the corresponding elongation of the axis of a resting- bud, separates the nodes widely. The leaves, which were originally crowded together, are now separated one from another by long internodes, and the only crowding is that of the young leaves, continually being formed from the growing point. 25 Endospermic seed structure (Monocots): Poaceae (cereal grain, caryopsis) – Zea mays and other cereals Monocot species like corn and other cereal species have caryopses (cereal grains) as propagation units. Caryopses are single-seeded fruits in which the testa (seed coat) is fused with the thin pericarp (fruit coat). Cereal grains have highly developed embryos and the triploid endosperm consists of the starchy endosperm (dead storage tissue) and the aleurone layer (living cells). Organs of the cereal embryo are the coleoptile (shoot sheath), the scutellum, the radicle and the coleorrhiza (root sheath). After germination, the scutellum functions in absorbing the nutrients from the starchy endosperm and delivering them to the growing seedling. The coleoptile protects the shoot meristem and plumule when growing through the soil. The coleorhiza has a similar function for the radicle prior to germination. The coleorhiza is the first structure that grows through the pericarp, the radicle (the completion of germination) then ruptures it. 26 Germination and seedling establishment of cereal grains is hypogenous. After the primary root emerges, the coleoptile is pushed upward by elongation of the mesocotyl. The coleoptile elongates and reaches the soil surface. Here it ceases to elongate and the first leaves of the plumule emerge through an opening at its tip. 27  Root System In trcheophyta (vascular plants), the root is the organ of a plant body that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial (growing above the ground) or aerating (growing up above the ground or especially above water). Therefore, it is better to define root as a part of a plant body that lacks nodes and bears no leaves. There are also important internal structural differences between stems and roots. The major functions of roots 1) Absorption and transportation of water and inorganic nutrients 2) Anchoring of the plant body to the ground 3) Roots often function in storage of food and nutrients. Roots have four regions: a root cap; a zone of division (growing zone); a zone of elongation; and a zone of maturation. Root cap is interpreted as structure protecting the apical meristem and assisting the growing root in penetrating the soil. It consists of 28 living parenchyma cells derived from the apical meristem by divisions contributing cells a way from the apex. This part of the apical meristem often appears as a distinct meristem called calyptrogen. As new cells are produced, cells on the periphery of the root cap are sloughed off...‫تنسلخ‬. Root tips growing in the soil are coated with more or less large amounts of mucilage. The root cap appears to be the main source of this mucilage, although the material occurs also on the surface of the young root at levels free of the root cap and extending to the root hair zone. The well-known effect of this coating mucilage is the adherence of soil particles to the root tips and the root hairs. The outer cells in the root cap secrete mucilage. The secretion process is a function of dictyosomes, which produce large vesicles containing the secretory product. Above the root cap is the zone of division and above that is the zone of elongation. The zone of division contains growing and dividing primary meristematic cells. After each cell division, one daughter cell retains the properties of the meristem cell, while the other daughter cell (in the zone of elongation) elongates sometimes 29 up to as much as 150 times. As a result, the root tip is literally pushed through the soil. In the zone of maturation, cells differentiate and serve such functions as protection, storage, and conductance. The zone of maturation of many roots has an outer layer (the epidermis), a deeper layers (the cortex), and a central region that includes the conducting vascular tissue. Early root growth is one of the functions of the apical meristem located near the tip of the root. The meristem cells more or less continuously divide, producing more meristem, root cap cells (these are sacrificed to protect the meristem), and undifferentiated root cells. The latter become the primary tissues of the root, first undergoing elongation, a process that pushes the root tip forward in the growing medium. Gradually these cells differentiate and mature into specialized cells of the root tissues. Roots will generally grow in any direction where the correct environment of air, mineral nutrients and water exists to meet the plant's needs. Roots will not grow in dry soil. Over time, given the right conditions, roots can crack foundations, snap water lines, and lift sidewalks. At germination, roots grow downward due to 30 geotropism, the growth mechanism of plants that also causes the shoot to grow upward. In some plants (such as ivy), the "root" actually clings to walls and structures. Types of roots A true root system consists of a primary root and secondary (lateral) roots. The primary root originates in the radicle of the seedling. It is the first part of the root to be originated. During its growth, it re-branches to form the lateral roots. It usually grows downwards. Generally, two categories are recognized: The taproot system: the primary root is prominent and has a single, dominant axis; there are fibrous secondary roots running outward. Usually allows for deeper roots capable of reaching low water tables. The taproot is most common in dicots. The main function of the taproot is to store food. The adventitious root system: the primary root is not dominant; the whole root system is fibrous and branches in all directions. The diffuse root is most common in monocots. The main function of the fibrous root is to anchor the plant. 31 The roots, or parts of roots, of many plant species have become specialized to serve adaptive purposes besides the two primary functions described above, these roots are modified for storage of food or water Storage roots: They include some taproots and tuberous roots. Storage taproot The primary root of taproot becomes fleshy and swollen due to storage of food. These are of following types: (a) Conical root is broad at the base and gradually tapers towards apex e.g. Daucus carota (Carrot). (b) Napiform. The food is accumulated only in upper parts to give it a top-shaped appearance e.g. Beta vulgaris (beetroot) and Brassica napus (turnip). (c) Fusiform. The root is swollen in middle and tapers towards the base and apex e.g. Raphanus sativus (radish). 32 Adventitious roots Adventitious root is any root arising, in an abnormal, unusual or unexpected position or place. Adventitious roots arise out-of-sequence from the more usual root formation of branches of a primary root, and instead originate from the stem, branches, leaves, or old woody roots. They commonly occur in monocots and pteridophytes, but also in many dicots, such as strawberry (Fragaria). Most aerial roots are adventitious. In some conifers, adventitious roots can form the largest part of the root system.  Respiratory or aerating roots (or pneumatophores): roots rising above the ground, especially above water such as in some mangrove genera (Avicennia, Sonneratia). In some plants like Avicennia the erect roots have a large number of breathing pores for exchange of gases.  Fibrous roots: A root system made up of many threadlike members of more or less equal length, as in most grasses. 33  Aerial roots: roots entirely above the ground, such as in ivy (Hedera) or in epiphytic orchids. They function as prop roots, as in maize or anchor roots or as the trunk in strangler fig.  Contractile roots: they pull bulbs or corms of monocots, such as hyacinth and lily, and some taproots, such as dandelion, deeper in the soil through expanding radially and contracting longitudinally. They have a wrinkled surface.  Climbing root: any plant that in growing to its full height requires some support. Climbing plants may clamber over a support (climbing rose), twine up a slender support, or grasp the support by special processes such as adventitious aerial roots (English ivy,).  Haustorial roots: roots of parasitic plants that can absorb water and nutrients from another plant, such as in mistletoe (Viscum album) and dodder.  Propagative roots: roots that form adventitious buds that develops into aboveground shoots, termed suckers, which form new plants, as in cherry and many others.  Prop roots: these are adventitious support roots, that develop on a trunk or lower branch that begin as aerial roots but eventually grow into a substrate of some type, common 34  among mangroves. They grow down from lateral branches, branching in the soil.  Tuberous roots: is a modified lateral root, enlarged to function as a storage organ. It is thus different in origin but similar in function and appearance to a tuber. Examples of plants with notable root tubers include the sweet potato, and Dahlia. It is a structure used to perennialize the plant for survival from one year to the next. 35  Shoot system The shoot system refers to new fresh plant growth and does include stems but to other structures like leaves or flowers. The shoot originates in the embryo at the end opposite the root and develops a complex shoot apex, different from that of the root. Characteristics of Shoot Systems The aboveground, conspicuous part of flowering plants constitutes the shoot system, which is composed of erect stems on which are attached leaves, flowers and buds. In most plants, stems are located above the soil surface but some plants have underground stems. A stem is one of two main structural axes of a vascular plant. The stem normally divided into nodes and internodes, the nodes hold buds, which grow into one or more leaves, inflorescence (flowers), or other stems etc. The internodes act as spaces that distance one node from another. The upper angle between the stem and the leaf at the node called the leaf axil. Axillary (lateral) buds located in the leaf axils give rise to vegetative branch stems or to flowers. 36 Terminal buds are present at the tips of the main stem and branches and contain the apical meristem tissues. Roots Shoots Apical covered by a cup- exposed, no cellular cap meristem shaped root cap Apical absent produce primordia appendages (leaves and buds) Orientation of occurs in all planes oriented; two locations: cell division inner ( corpus) and an outer ( tunica) Zones separate areas of no such recognizable division, elongation, zones present and differentiation Stems have four main functions, which are  Support for and the elevation of leaves, flowers and fruits. The stems keep the leaves in the light and provide a place for the plant to keep its flowers and fruits. 37   Transport of fluids between the roots and the shoots in the xylem and phloem.  Storage of nutrients.  The production of new living tissue. The normal life span of plant cells is one to three years. Stems have cells called meristems that annually generate new living tissue. Duration of plants: it is the length of time from establishment to harvest Annual: plant, which lives for one year or season, reproduces, and then dies Biennial: plant, which lives for two years or seasons, reproduces, and then dies Perennials are plants, which live for several to many years or seasons. Perennials may be woody, with stems that persist aboveground over the winter, or they may be herbaceous, with stems that die back to the ground each year. Evergreen: having leaves, which persist for two or more seasons. Broadleaf evergreens usually have thick, leathery leaves. Deciduous: having leaves which die and fall in the cold or the dry season. 38 Plant Habit: refers to the overall shape of a plant. It has a number of components such as stem length and development, branching pattern, and texture. Herb: A plant in which all structures above the surface of the soil, vegetative or reproductive, die back at the end of the annual growing season, and never become woody. but may have underground perennating structures; may be annual, biennial, or perennial Subshrub (=suffrutescent): lower stems woody but upper stems herbaceous ("sub" means "almost") Shrub: a woody low-stature ‫ القوام المنخفض‬perennial plant with one to many slender trunks arising from near its base Tree: a large woody perennial plant with one to several relatively massive trunks and an elevated crown Succulent: plant possesses thick, usually soft, watery leaves and/or stems. Stem succulents & leaf succulents Vine: a woody or herbaceous plant with a long, slender, more or less flexible stem which cannot support itself 39 Stem may be: Erect having an essentially upright vertical habit or position Alternatively, prostrate laying flat on the ground. prostrate type of stems may be: Creeping growing along the ground and not producing roots at intervals along surface, or Runner an elongated, slender branch that roots in the soil at the nodes or tip, Metamorphosis of stems Stems are often metamorphosed for storage, asexual reproduction, protection or photosynthesis, including the following: I: Underground stems They are modified plant structures that derive from stem tissue but exist under the soil surface. Plants use under ground stems to multiply their numbers by asexual reproduction and to survive from one year to the next, usually over a period of dormancy. Plants produce these modified stems so they can survive a cold or dry period, which normally is a period of inactive growth, and when the cold or dry period is over the plants begin new growth from the underground stems. Being underground protects the stems from the elements during the dormancy period, such as freezing 40 and thawing in winter or extreme heat and drought in summer or fire. They can also protect plants from heavy grazing pressure from animals, the plant might be eaten to the ground but new growth can occur from below ground that can not be reached by the herbivores. A number of weedy species use underground stems to spread and colonize large areas since the stems do not have to be supported or strong, less energy and resources are needed to produce these stems, often these weeds have more mass under ground than above ground. Underground stem functions mainly in reproduction but also in storage 1- Bulb: a short vertical underground stem with fleshy storage leaves attached, e.g. onion, tulip. Bulbs often function in reproduction by splitting to form new bulbs or producing small new bulbs termed bulblets. Bulbs are a combination of stem and leaves so may better be considered as leaves because the leaves make up the greater part. 2- Corm: is a short, vertical, swollen underground plant stem that serves as a storage organ used by some plants to survive winter or other adverse conditions such as summer drought and heat. 41 A corm consists of one or more internodes with at least one growing point, with protective leaves modified into skins or tunics. The thin tunic leaves are dry papery, dead petiole sheaths, formed from the leaves produced the year before which acts as a covering that protects the corm from insects and water loss. Internally a corm is mostly made of starch-containing parenchyma cells above a circular basal node that grows roots. Corms grow two different types of roots; from the bottom of the corm normal fibrous roots are formed as the shoots grow, they are produced from the basal area at the bottom of the corm, the other type of roots are produced were the corm grows new corms. The second type of roots is thicker layered roots that are able to pull the corm deeper into the soil. These roots which are called contractile roots are produced in response to a fluctuating soil temperature and light levels e.g. Colocasia. 3- Rhizome: is a horizontal stem of a plant that is usually found underground, often sending out roots and shoots from its nodes. Rhizomes develop from axillary buds 42 In general, rhizomes have short internodes; they send out roots from the bottom of the nodes and new upward-growing shoots from the top of the nodes. The plant uses the rhizome to store starches, proteins, and other nutrients. These nutrients become useful for the plant when new shoots must be formed or when the plant dies back for the winter. This is a process known as vegetative reproduction and is used by farmers and gardeners to propagate certain plants. 4- Tuber: is a thickened part of stolon that has been enlarged for use as a storage organ. In general, a tuber is high in starch, for example, the common potato, which is a modified swollen, underground storage stem adapted for storage and reproduction. The tuber has all the parts of a normal stem, including nodes and internodes, the nodes are the eyes and each has a leaf scar. The nodes or eyes are arranged around the tuber in a spiral fashion beginning on the end opposite the attachment point to the stolon. Internally a tuber is filled with starch stored in enlarged 43 parenchyma like cells; also internally the tuber has the typical cell structures of any stem, including pith, vascular zones and a cortex. II: Metamorphosis for assimilation Cladodes: A flattened stem that appears leaf like and is specialized for photosynthesis with only one internodes (e.g. Asparagus) and phylloclade: A flattened stem that appears leaf like and is specialized for photosynthesis, with more than one internodes (Cactus pads, Ruscus). III: Climbing are stems that cling or wrap around other plants or structures. IV: Reduction of transpiration: The most Effective adaptation by which desert plants reduce their water loss and adjust to variations for water involves changes in the size of the transpiring body. Seasonal reduction in the transpiring surface is an important factor in maintaining the water balance of certain xerophytes As a seedling, the plant has broad leaves, which wither over time. When conditions are favorable in the wet season, the new growth has broad leaves and soft spines. At the advent of the dry season, however, adaptations that reduce water loss, leaves are shed, while 44 the spines are lignified and become stiff. The whole plant becomes spiny and without leaves. Thorn is a reduced stem with a sharp point and rounded shape. e.g. Alhagi, Zilla. Buds A bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of the stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. The buds of many woody plants are protected by a covering of modified leaves called scales, which tightly enclose the more delicate parts of the bud, many bud scales are covered by a gummy substance, which serves as added protection. In many plants, scales are not formed over the bud, which is then called a naked bud. Such naked buds are found in shrubs and in herbaceous plants. In many of the latter, buds are even more reduced, often consisting of undifferentiated masses of cells in the axils of leaves. 45 A terminal bud occurs on the end of a stem and lateral buds are found on the side. A head of cabbage is an exceptionally large terminal bud. Since buds are formed in the axils of leaves (axillary buds), their distribution on the stem is the same as that of leaves. There are alternate, opposite, and whorled buds, as well as the terminal bud at the tip of the stem. In many plants buds appear in unexpected places; these are known as adventitious buds. Types of buds Buds are often useful in the identification of plants, especially for woody plants in winter when leaves have fallen. Buds classified and described according to different criteria: location, status, morphology, function. Botanists commonly use the following terms:  For location o Terminal, when located at the tip of a stem (apical is equivalent but rather reserved for the one at the top of the plant), 46 o Axillary, when located in the axil of a leaf (lateral is equivalent but some adventitious buds may be lateral too), o Adventitious, when occurring elsewhere, for example on trunk or on roots (some adventitious buds may be former axillary ones reduced and hidden under the bark, other adventitious buds are completely new formed ones),  For morphology Scaly or covered (winter), when scales (which are in fact transformed and reduced leaves) cover and protect the embryonic parts, Naked (summer), when not covered by scales, and Hairy, when also protected by hairs (it may apply either to scaly or to naked buds),  For function o Vegetative, if only containing vegetative pieces (embryonic shoot with leaves), Reproductive, if containing embryonic flower(s) (a flower bud) and Mixed, if containing both embryonic leaves and flowers. 47  Leaves A leaf is an aboveground plant organ, typically flat (laminar) and thin, to expose the cells containing chloroplast to light over a broad area, and to allow light to penetrate fully into the tissues specialized for photosynthesis. Leaves are also the sites in most plants where transpiration and guttation take place. Leaves can store food and water, and are modified in some plants for other purposes. Parts of a Typical Leaf The leaf consists of three parts namely, leaf base (usually provided with a pair of stipules), petiole and leaf blade or lamina. (i) Leaf base (Hypopodium): Leaf base is the lower most part of the leaf meant for attachment. It acts as a leaf cushion. In most of the plants it is indistinct. Sometimes leaf base shows different variations as follows: (a) Pulvinate leaf base: In members of leguminosae the leaf is swollen. Such swollen leaf bases are called pulvinate leaf bases as seen in mango leaves. 48 (b) Sheathing leaf base: In grasses and many monocots, the leaf base is broad and surrounds the stem as an envelope, such a leaf base is called sheathing leaf base, e.g., Maize, Wheat and Palms. In grasses (Maize, Wheat etc.) the sheathing leaf base protects the intercalary meristem. (c) Modified leaf base: The leaf bases in few plants perform accessory functions and show modifications. In (Onion), the leaf bases store food materials and become fleshy. They are arranged concentrically to form a tunicated bulb. In Platanus and Robenia, the leaf bases protect the axillary buds and grow around them to form cup like structures. The petiole attaches to the stem at a point called the "leaf axil". A petiole may be absent, or the blade may not be laminar (flattened). After a period (i.e. seasonally, during the autumn), deciduous trees shed their leaves. These leaves then decompose into the soil. 49 Arrangement on the stem Different terms are usually used to describe leaf placement (phyllotaxis): Alternate: leaf attachments are singular at nodes, and leaves alternate direction, to a greater or lesser degree, along the stem. Opposite: The leaves on this plant are arranged in pairs opposite one another, with successive pairs at right angles to each other ("decussate") along the stem or superposed if not rotated, but two-ranked (in the same geometric flat-plane). Whorled: three or more leaves attach at each point or node on the stem. Opposite leaves may appear whorled near the tip of the stem. A rosette: is a circular arrangement of leaves, with all the leaves at a similar height. Though rosettes usually sit near the soil, their structure is an example of a modified stem. Divisions of the lamina (blade) Two basic forms of leaves described considering the way the blade is divided. 50 A simple leaf has an undivided blade. However, the leaf shape may be formed of lobes, but the gaps between lobes do not reach to the main vein. A compound leaf has a fully subdivided blade, each leaflet of the blade separated along a main or secondary vein. Because each leaflet can appear to be a simple leaf, it is important to recognize where the petiole occurs to identify a compound leaf. Compound leaves are a characteristic of some families of higher plants, such as the Fabaceae. The middle vein of a compound leaf, when it is present, is called a rachis. Palmately compound leaves have the leaflets radiating from the end of the petiole, like fingers off the palm of a hand, e.g. Cannabis (hemp). Pinnately compound leaves have the leaflets arranged along the main or mid-vein. This may be: Odd-pinnate (imparipinnate): Leaflets are attached along an extension of the petiole called a rachis; there is a terminal leaflet and therefore an odd number of leaflets, e.g. Rosa. 51 Even-pinnate (paripinnate): Leaflets are attached along an extension of the petiole called a rachis; there is an even number of leaflets, e.g. Cassia. Bipinnately compound leaves twice divided: The leaflets also divided into leaflets, the leaflets arranged along a secondary vein that is one of several branching off the rachis. Each leaflet called a "pinnule". The pinnules on one secondary vein are called "pinna"; e.g. Albizia (silk tree). Trifoliate: a pinnate leaves (also known as ternate leaves) are a leaf shape characterized by a leaf divided into three leaflets, e.g. Trifolium (clover). Characteristics of the petiole Petiolated leaves have a petiole. Sessile leaves do not. The blade attaches directly to the stem. In clasping or decurrently leaves, the blade partially or wholly surrounds the stem, often giving the impression that the shoot grows through the leaf. In peltate leaves, the petiole attaches to the blade inside from the blade margin. In some Acacia species, the petioles are expanded or broadened and function like leaf blades; these are called phyllodes. 52 Stipules The stipules are the small lateral appendages present on either side of the leaf base. They protect the young leaf or leaf primordia. Leaves with stipules are called stipulate and those without them are called exstipulate. The stipules are commonly found in dicotyledons. In some grasses (Monocots) an additional outgrowth is present between leaf base and lamina. It is called ligule. The leaves having ligules are called ligulate. Sometimes, small stipules like outgrowths are found at the base of leaflets of a compound leaf. They are called Stipules. Types According to their shape, size and location may be of the following types: Adnate stipules: These are two lateral stipules attached to the petiole up to some distance but the anterior part remains free e.g., Rose and Lupin. Ochreate: When stipules of the leaf make a hollow tube, which encircles (surrounds) the stem up to the upper part of node, the structure is known as ochrea, e.g., 53 Polygonum Rumex etc. Hairy stipules: These are hair like stipules which are dry in nature, e.g.,Corchorus Foliaceous stipules: These are two leafs by broad and green stipules. Due to the absence of axillary buds in their axil these can be distinguished from the leaves. In Lathyrus aphaca stipules are modified into leaves and leaves into tendrils. In Pisum sativum foliaceous stipules are much developed and upper leaflets are modified into tendrils. Main function of these structures is to synthesize food. Tendrillar stipules: are thin and modified into wire like structure. Tendrillar stipules help in climbing of plants. These are usually present in tendril climbing plants e.g., Smilax. Spiny stipules: When stipules are converted into sharp pointed structures, these are known as spiny stipules. These may be long (as in Mimosa and Acacia) or curved (in Ziziphus). They protect plants from wild animals. Venation Venation is the pattern of veins in the blade of a leaf. The veins consist of vascular tissues, which are important for the transport 54 of food and water. Leaf veins connect the blade to the petiole, and lead from the petiole to the stem. The venation pattern of a leaf is classified as reticulated, parallel, or dichotomous. In reticulated venation, the veins are arranged in a net-like pattern, in that they are all interconnected like the strands of a net. Reticulated venation is the most common venation pattern, and occurs in the leaves of nearly all dicotyledonous Angiosperms, whose embryos have two cotyledons (seed leaves). In parallel venation, the veins are all smaller in size and parallel or nearly parallel to one another, although a series of smaller veins connects the large veins. Parallel venation occurs in the leaves of nearly all monocotyledonous Angiosperms, whose embryos have one cotyledon, as in grasses. In dichotomous venation, the veins branch off from one another like the branches of a tree. This is the rarest venation pattern, and occurs in the leaves of some ferns and in the gymnosperms. 55 Leaf Blade Shape: The overall shapes of leaves and leaflets are often characteristic of individual species of plants and, together with reproductive features, are used in plant identification. The terminology below is used to describe the shape of laminae of simple and compound leaves as well as the laminae of the leaflets of compound leaves. Acicular: have a long and very narrow leaf shape, with sides that are almost parallel to each other and are usually more than ten times longer than broad. Acicular leaves are often borne on short lateral branches called fascicles; Linear leaves: have a long and very narrow leaf shape, with sides that are almost parallel with one another and are usually more than four times longer than broad. Oblong leaves: have a rectangular leaf blade two to four times longer than it is wide, with sides that are almost parallel to each other. Lanceolate leaves: have a lance-shaped leaf, with the widest part of the leaf near the base and the narrowest part near the apex. Oblanceolate leaves: have a lance-shaped leaf, with the widest part of the leaf near the apex and the narrowest part near the base. 56 Orbicular leaves: have a more or less circular leaf shape in which the width and length of the lamina are equal, or nearly so. Elliptical leaves: have a shape that looks like an ellipse, twice as long as broad, with the widest part of the leaf near the middle. Ovate leaves: are egg-shaped, with the widest part of the leaf below the middle toward the base. Obovate leaves: are egg-shaped, with the widest part of the leaf above the middle toward the apex. Reniform leaves: have a shape like a kidney. Cordate leaves: are shaped like a heart, with the lobes of the heart at the base of the leaf and the pointed portion at the apex. Obcordate leaves: are also heart-shaped, but the lobes are at the apex, and the basal lamina tapers into the petiole. Sagittate leaves: have a shape like an arrowhead. Hastate leaves: are also shaped like an arrowhead, but the basal lobes diverge or extend away from the midrib, giving an outline that resembles a halberd. 57 Spatulate leaves: have a spoon-shaped or spatula-shaped leaf where the lamina is widest near the rounded apex. Deltoid leaves: are delta-shaped, resembling an equiangular triangle. Often the sides of the deltoid leaves are slightly curved toward the apex. Subulate leaves: are short, narrow, flat, stiff, awl-shaped leaves that taper to a sharp point. Scale leaves: are small, inconspicuous leaves that are typically appressed tightly to the stem and have overlapping margins. Lyrate: When the leaf blade is like a lyre ‫القيثارة‬, i.e., contains a big terminal lobe and many smaller lateral lobes, e.g., Raphanus sativus. Leaf apices The apex of a leaf lamina is opposite the petiole. Leaves apexes vary greatly from plant to plant and can be useful in classification and identification. Acute: sharp, ending into a pointed apex with margins that form an acute angle between 45 and 90 degrees. e.g., Mangifera indica. 58 Acuminate: a sharp-pointed apex with straight or convex margins that form an angle less than 45 degrees. Obtuse: a blunt apex, rounded end with margins that form an angle greater than 90 degrees. e.g., Cassia obtusifolia. Retuse: a rounded summit like obtuse but with a shallow depression (shallow notch) at the apex, e.g., Pistia stratiotes. Emarginate: with a shallow depression at the apex, not exceeding 1/15 of the distance to the centre of the leaf blade. e.g., Bauhinia Mucronate: Rounded apex with a small extension of the midrib barely extending beyond the blade apex e.g., Calotropis gigantea. Leaf margins The margin of a leaf is another name for the structure of the leaf's edge. There are many different variations, and they can be: Entire: A leaf margin that has a continuous, unbroken and smooth edge, without teeth, lobes or indentations 59 Crenate: the margins of a leaf (or leaflet or other organ ) have relatively large rounded, blunt projections, but not so large as to be considered lobes. Serrate: A leaf margin forming a row of small sharp outward projections pointing toward the apex of the leaf resembling the teeth of a saw. Dentate: A leaf margin with sharp tooth-like projections pointing outward Sinuate: Strongly wavy margins with shallowly rounded divisions within the same plane of the blade. Spinose: Has spines along leaf margins. Often these spines occur on the margin where the leaf veins terminate at serrations at the leaf margin. 60 Terminology in plant morphology The seeds are the dispersal and propagation units of the Spermatophytes (seed plants): Gymnosperms (conifers and related groups) and angiosperms (flowering plants). Seeds are mature, fertilized ovules. Ovules are structures of seed plants containing the female gametophyte with the egg cell, all being surrounded by the nucellus and 1-2 integuments. Double fertilization results in formation of the diploid embryo and the triploid endosperm. Testa (seed coat): Outer protective layer of the seed developed from the integuments of the ovule, diploid maternal tissue, Seed coats help protect the embryo from injury and also from drying out. Seed coats can be thin and soft as in beans or thick and hard as in locust or coconut seeds. Hilum and funiculus: Funicular scar on seed coat that marks the point at which the seed was attached via the funiculus to the ovary tissue. 61 The Micropyle is a canal or hole in the coverings (seed coat) of the nucellus through which the pollen tube usually passes during fertilization. Later, when the seed matures and starts to germinate, the micropyle serves as a minute pore through which water enters. The micropylar seed end has been demonstrated to be the major entry point for water during seed imbibitions and germination. During germination the testa ruptures at the micropylar end and the radicle protrudes through the micropylar endosperm. Chalaza: Non-micropylar end of the seed. At the base of an ovule, bearing an embryo sac and surrounded by integuments. Fruits are mature, ripened ovaries containing seeds. The pericarp ("fruit coat") is diploid maternal tissue Embryo: Young saprophyte, diploid (2n), results of fertilization. The mature embryo consists of cotyledons (seed leaves), hypocotyls (stem-like embryonic axis below the cotyledons), and radicle (embryonic root). Endosperm: Food storage tissue, triploid (3n), results of double fertilization. A temporary food supply is packed around the embryo in the form of special leaves called cotyledons or seed 62 leaves. These generally are the first parts visible when the seed germinates. Endospermic seeds: The endosperm is present in the mature seed and serves as food storage organ. Non-endospermic seeds: The cotyledons serve as sole food storage organs as in the case of pea (Pisum sativum). During embryo development the cotyledons absorb the food reserves from the endosperm. The endosperm is almost degraded in the mature seed and the embryo is enclosed by the testa. 63  Part II: plant anatomy Introduction The plant anatomy is the study of the internal structure of the various parts of the plant, and began a little over 300 years ago with the work of Grew and Malpighi. Their work involved careful well-illustrated descriptions of plant material. The highly organized plant body of a seed plant begins its existence usually with the fertilized egg, the zygote, which develops into the embryo by characteristic steps resulting in the adult organization. The root represented by its formative tissue of root, the radical. Similarly, the apical formative tissue of the shoot may or may not initiate the development of a shoot above the cotyledons. If a primordial shoot is present, it is called epicotyls, plumule. Cell divisions in the embryo initiate the organization of tissue systems. The component tissues are still formative (meristematic) but their position and cytological characteristics indicate a relation to mature tissues appearing in the subsequently developing seedling. 64 The plant cell Cells make up the smallest level of a living organism. The cell is called the fundamental unit of life because the cellular level of an organism is where the metabolic processes occur that keep the organism alive. Each cell is capable of not only make up living things but also it is living things. The cell is the unit of construction of plant, just as atoms are the units of molecules. Plants, like animals, are composed of cells. Some consists of only one cell, but the flowering plants, with which our study is concerned, are made up of many cells, which at maturity differ greatly in structure. Recognition of the cellular construction of plants goes back to the seventeenth century and knowledge of the cell and its contents has progressed hand-in hand with the development of the microscopes employed for its observation. By the middle of the nineteenth century, it had been realized that all organisms consist of cells, and further, that all such units are derived from the division of existing cells. It was shown that 65 chromosomes were present in the nucleus, and that these divided during nuclear divisions. The most important characteristic of a cell is that it can reproduce by dividing. Cell division is the process by which cells duplicate and replace themselves. Nuclear divisions were of two kinds: 1- Mitosis division; those that gave rise to the somatic cells of the plant, in which, the chromosomes duplicated and the daughter cells had the same number of chromosomes as the original cells. 2- Meiosis division; those that gave rise to the reproductive cells of the plant, in which, the daughter cells had only half the original number of chromosomes. The cell was known to consist of a cell wall and its inner contents, the protoplast. The protoplast comprised a more or less spherical body, the nucleus, containing chromosomes, which were the bearers of the hereditary units or genes, embedded in the granular matrix, the cytoplasm. Various other inclusions were also observed with the microscope, cell organelles. The various components of the plant cell that are known at the present time will now be discussed. 66 As summarized below the main components of the plant cell are the cell wall, cytoplasm, and nucleus. The cytoplasm includes the different cell organelles. In the following, we will discuss briefly each of the cell components. The plant cell The cell wall The protoplast Protoplasm Ergastic substances The cytoplasm The nucleus Hyaloplasm Cell organelles 67 The cell wall is not a living system, but in absence of the protoplast that formed it, is merely a non- living shell. The wall is formed during the growth of the cell. Plant cell walls are of considerable importance to man. Cell walls themselves constitute timbers and also used directly as fibers, cotton, etc.; materials extracted from them serve as glue, food and food additives. Formation of cell wall The cell wall consists of a crystalline poly-saccharide, which in higher plants is cellulose. The cellulose occurs in bundles of chains which comprise fiber like structures, the microfibrils. The cell wall is formed during the process of cell division. There is evidence that the presence of the nucleus is necessary for wall formation. The cell division takes place through two steps namely karyokinesis (nuclear division) and cytokinesis (division of the cytoplasm). During nuclear division, a plate is gradually produced at the position of the equator of the spindle. Vesicles formed by dictyosomes apparently fuse to form the cell plate, and this process continues until the cell plate reaches the exciting cell walls. 68 The cell plate gives rise to the middle lamella, which, is composed of pectic substances. The pectic substances react with calcium and magnesium ions already present in the cell, to form the middle lamella which become a rigid substance (cementing material). The middle lamella holds together the primary walls of adjacent cells. The primary cell wall The primary wall is the first wall to be formed by the cell, and is deposited on either side of the middle lamella by the contiguous cells. Chemically it consists mainly of cellulose, hemicellulose and other polysaccharides. Layers of pectic substances may separate the cellulose lamellae. All meristematic cells and many mature cells still have living contents, have primary walls. Since the wall is formed when the cell is young, it must undergo considerable growth. The wall is characterized by plasticity. 69 Secondary walls Secondary walls are usually formed after a cell has completed its elongation; therefore do not normally extend to any considerable degree. Where a secondary wall is formed it is deposited on the inner side of the existing wall, next to the cell lumen. It consists mainly of cellulose and other polysaccharides. Various other substances, notably lignin may be deposited in the wall. Among the organic substances, cutin, subrin, and waxes are found most commonly in the protective surface tissues of the plant which imparts rigidity to the cell wall The secondary wall often consists of three layers, so that a cell wall may consist altogether of five layers Pits: Pits are relatively thinner portions of the cell wall that adjacent cells can communicate or exchange fluid through. Pits are characteristic of cell walls with secondary layers. Generally each pit has a complementary pit opposite of it in the neighboring cell. These complementary pits are called "pit pairs". 70 Pits are composed of three parts: the pit chamber, the pit aperture, and the pit membrane. The pit chamber is the hollow area where the secondary layers of the cell wall are absent. The pit aperture is the opening at either end of the pit chamber. The pit membrane is the primary cell wall and middle lamella, or the membrane between adjacent cell walls, at the middle of the pit chamber. The primary cell wall at the pit membrane may also have depressions similar to the pit depressions of the secondary layers. These depressions are primary pit-fields, or primary pits. In the primary pit, the primordial pit provides an interruption in the primary cell wall that the plasmodesmata can cross. The primordial pit is the only aperture in the otherwise continuous primary cell wall. Plasmodesmata are thin sections of the endoplasmic reticulum that traverse pits and connect adjacent cells. These sections provide an avenue of transport through the pits and facilitate communication Types of pits Though pits are usually simple and complementary, a few more pit variations can be formed: 71  Simple pits: A pit pair in which the diameter of the pit chamber and the diameter of the pit aperture are equal.  Bordered pits: A pit pair, in which the pit chamber is over-arched by the cell wall, creating a larger pit chamber and smaller pit aperture.  Half bordered pits: A pit pair in which a bordered pit has a complementary simple pit. Such a pit pair is called half bordered pit pair.  Blind pits: A pit pair in which a simple pit has no complementary pit.  Compound pits: A pit pair in which one cell wall has a large pit and the adjacent cell wall has numerous, small pits. 72 The architecture of the cell wall Cellulose forms the framework interpenetrated by the matrix represented by the non-cellulosic carbohydrates. The cellulose framework is a system of fibrils composed of cellulose molecules. The fibrils are of different classes of magnitude. The largest are visible with a light microscope and are called macrofibrils. With an electron microscope these fibrils are resolvable into microfibrils. Subunits of the microfibrils are referred to as micelles. Cellulose has crystalline properties as a result of orderly arrangement of cellulose molecules. Less regularly arranged glucose chains occur between and around the micelles and constitute the paracrystalline regions of the microfibril. Methods of wall building The mechanisms of wall building, to increase the thickness, occur by two methods of deposition of wall material: Apposition and intussusceptions. 73 In apposition, the building units are placed one on top of another and this is usually centripetal, i.e. it occurs from outside towards the center of the cell and so the lumen of the cell becomes narrow In intussusception, the units of new material are inserted into the existing structure. Intussusception is probably the rule when lignin or cutin is incorporated into the wall. Intercellular spaces A large volume of the plant body is occupied by an intercellular space system, which is most characteristic of mature tissues. The most common intercellular spaces develop by separation of contiguous primary walls through the middle lamella. Two types of intercellular space develop in plant tissues. 1- Schizogenous in which spaces result from separation of cell walls from each other along more or less extended areas of their contact. In such cases, the intercellular substance dissolves partly and an intercellular space develops and becomes quite big in size. Schizogenous cavities form an intercommunicating system of long intercellular canals, which facilitate diffusion of gases and liquids from one part of the plant body to the other. The resin ducts in the 74 Coniferales, and the secretory ducts in the Compositae are the typical examples. 2- Lysigenous intercellular spaces (lysis, loosening, Greek). These spaces arise through dissolution of entire cells. These cavities of intercellular spaces store up water, gases and essential oils in them. The examples are commonly found in water plants and many monocotyledonous plants. The secretory cavities in Citrus and Gossypium are good examples. The cytoplasm The cytoplasm, physically, is a viscous substance, forming a colloidal system, which is more or less transparent in visible light. Chemically the structure of cytoplasm is very complex; the major component is water (forms 85-90% in active cells and about 10- 12% in cells of dormant seeds) with soluble and non-soluble components (hyaloplasm). Membranous system in the cell The cell contains a number of membranes forming system particularly in the well - organized cells. All the membranes have a 75 more or less the same chemical structure namely lipoprotein referred to as a unit membrane. Each of these membranous systems functions according to their location. The nucleus The nucleus is present in all different types of cells except the sieve tube cell of higher plants and the red blood cells. It is considered the most important component of the cell. It contains the hereditary material, which control the activity of the cell. Eukaryotes possess a definite nucleus. Prokaryotes such as bacteria and others do not have easily recognizable nuclei, but they contain certain nuclear material. The eukaryotic nucleus consists of nuclear membrane, chromatin material, nucleolus and nuclear sap. Endoplasmic reticulum It is a membranous system, which branches inside the cytoplasm to form a network-like structure. The endoplasmic reticulum is present in all adult plant. Under the electron microscope, it appears as double unit membranes, which, chemically made of lipoprotein. It forms channels that branch and penetrate throughout the 76 cytoplasm. Endoplasmic reticulum serves as a channel for transportation to various parts of the cell of different nutrients, protein including enzymes. Two types of endoplasmic reticulum are found: 1) Granular due to the presence of small dense granules, which are the ribosomes. The granular ER, is related to protein synthesis due to the presence of the ribosomes, 2) A granular endoplasmic reticulum, similar to the rough E. R., but lacks the ribosomes. The smooth ER is normally related to steroid, lipid and glycogen metabolism. The manufactured protein can penetrate into cavities of the endoplasmic reticulum. Plastids They are only present in plant cells. They appear under the light microscope as a small, rounded, oval or disc-shaped bodies embedded in the cytoplasm. The plastids are considered as part of the membranous system. The principal types of plastids are chloroplasts, chromoplasts and leucoplasts. Chromoplasts are usually yellow, orange, or red because of the carotenoid pigments. They occur in petals, in some ripe fruits, and some roots (e.g. carrot roots). Leucoplasts are non-pigmented 77 plastids, usually located in tissues not exposed to light; they store the plant products such as starch (amyloplasts), proteins (proteinoplasts), and fats (elaioplasts). Leucoplasts of tissues which become exposed to light may develop into chloroplasts (e.g. in potato tuber). Chloroplasts are green because of pigment chlorophyll, which predominates in them. The chloroplastids are bounded by an envelope consisting of two unit membranes. Internally the plastids contain a membrane system embedded in a proteinaceous matrix or stroma. The membrane system is in the form of flattened sacs called thylakoids. The thylakoid system consists of grana and frets (stroma thylakoids). The grana are interconnected by the frets that transverse the stroma. In addition to the light harvesting system, the chloroplasts contain the enzymes responsible for the fixation of carbon dioxide into sugar. Each granum is composed of a series of disc-like thylakoids stalked one upon the other like a pile of coins. 78 The Golgi apparatus The Golgi apparatus appears, under EM, as membranes in the form of a) System of flattened sacs (Cisternae) b) Vesicles and c) Large vacuoles surrounded by membranes. In many cells of higher plants and in some animal cells, the Golgi apparatus appear to consist of many separate units called dictyosomes. Golgi apparatus performs a number of important functions such as Secretion of large molecules in plant cells e.g. the growing region in root tips secretes pectic material which take part in cell plate formation and collection and accumulation of certain substances such as lipids, proteins and enzymes. Mitochondria They are found lying free in the cytoplasm of nearly all plant and animal cells. They appear, under the light microscope, as rods or spherical granules freely distributed in the cytoplasm. The mitochondria are surrounded by a double envelope consisting of an outer and an inner membrane. 79 The outer membrane is smooth and stretched around, while the inner one is greatly folded. These folds are known as mitochondrial cristae. The cristae produce a large surface to volume ratio, which facilitates the function of the mitochondrion. The interior of the mitochondria is filled with a certain liquid known as the matrix. The matrix includes variety of soluble substances such as sugars, organic acids, mineral salts, vitamins as well as many respiratory enzymes. The membranes of mitochondria are made of proteins and lipids. The main function of mitochondria is considered as the site where the respiratory reactions take place. The mitochondria contains the cytochromes (electron transfer agents), the dehydrogenises enzymes (oxidoreductases) associated with them, respiratory pigments and some enzymes involved in the krebs cycle. Ribosomes Ribosomes appear under the EM as small granules. They are found either attached to the endoplasmic reticulum or free in the cytoplasm. The ribosomes are being formed within the nucleolus. 80 They are chemically composed of RNA known as ribosomal RNA, and protein. Lysosomes Lysosomes are vacuoles surrounded by a unit membrane formed by the Golgi apparatus. There are primary and secondary lysosomes. The primary is formed on the rough ER (endoplasmic reticulum). The secondary lysosomes are formed on the smooth ER. Lysosomes are containing high concentration of number of digestive enzymes. These enzymes are capable of hydrolyzing different classes of macromolecules. The lysosomes control the release of these digestive enzymes to the cell. In case of abnormal situation, lysosomal enzymes may escape into the cytoplasm and either kill the cell (autolysis), or bring about dramatic metabolic changes 81 Tissues and tissue systems The morphologic units of multicellular organisms, the cells, are of many different kinds and acquire diversity in structure and function. Similar cells are associated in various ways with each other forming coherent masses or tissues. Besides being similar in form and function, the cells of a tissue have a common origin. A group of vegetative cells is a vegetative tissue, and group of reproductive cells is a reproductive tissue. The tissues of more highly organized plants are distinguished as regards their stage of development into: 1- Meristematic or formative in which growth is taking place. 2- Permanent tissues, those which lost, at least temporarily, their power of division. Meristematic tissues Meristems are specialized tissues responsible for the formation of new cells and localized in certain organs in plants. The cells of meristems differ from mature tissues and characterized by: 1- They have abundant cytoplasm. 2- Vacuoles small or lacking, 82 3- Large nuclei compared with cell size. 4- Thin cellulosic walls. 5- No intercellular spaces. 6- The ability of repeating division. According to the mode of their origin, meristematic tissues are classified as primary and secondary. 1- Primary meristems arise by the division of the germ cell and at the very early stage of plant life comprise the whole embryo. Later, they become localized at the apices of stems and roots and the primordial of leaves and similar appendages. In primary meristems, promeristem do always the earliest stage and transition stages to mature tissues constitute the remainder of the meristem. 2- Secondary meristem, on the other hand, are derived either from the inactive remains of the primary meristem or are newly originated from cells of the permanent tissues by cell division. Such secondary meristems, which get the name cambium, give rise to the secondary growth increasing thickness of woody plants, as represented by: 1- New tissues in medullary rays (interfascicular cambium) or 2- Periderm or cork tissue (cork cambium) 83 On the other hand, as regard their position in plant body meristems are classified into apical, intercalary and lateral. + Apical meristems: are those which lie at the apices of the axis and of the appendages and are commonly called growing points e.g. tips of roots, stems and often leaves of vascular plants. The activity of apical meristems brings increase in the length of these organs, building the primary body of the plant. + Intercalary meristems are portions of apical meristems that have become separated from the apex during development by layers of permanent tissues. The best known intercalary meristems are those which lie at the basal regions of internodes and leaves of many monocotyledons, e.g. grasses. Intercalary meristematic regions ultimately disappear, as being wholly transformed into permanent tissues. + Lateral meristems are situated laterally in an organ such as the cambium and the cork cambium. The cells of these latter meristems divide chiefly in one plane (periclinally), thus increasing the diameter of the organ in which they occur. 84 Structural development and differentiation of plant tissues In higher plants, the apical meristem consists of a small apical portion, the promeristem. The remainder of the apical meristem represents the early stages of the tissues formed by the promeristem. These tissues gradually pass over into the permanent stage, while the promeristem retains its power of division. In the stem apex, the region below the promeristem is distinguished into: 1- An outermost layer, protoderm which, in the mature region, can be recognized as the epidermis. This layer which is uniseriate external layer sometimes termed the dermatogen. 2- The inner cell become distinguished into procambium which appears as strands (procambial strands) very close to the apex of the stem and consists of elongated, slender cells. This procambium is a central core which forms the pith and the primary vascular tissue, this central core sometimes termed the plerome. 3- The remaining undifferentiated meristematic tissue is composed of layer of cells with intercellular spaces and is known as the ground meristem. From the ground meristem the primary cortex, 85 medullary rays and photosynthetic elements are all formed. This meristematic tissue is sometimes termed the periblem. Along the sides of the meristem are outgrowths of meristemaic cells which have arisen by the accelerated divisions of some of the outermost cells of the meristems. Such outgrowths are the beginnings of young leaves and thus referred to as leaf primordia. Branch primordial cells arise in the same way in the axils of immature leaves. The cells of the leaf, after a time, lose their meristematic properties, while the axillary branch retains its growing point. Unlike the growing point of the shoot, that of the root lacks lateral appendages and segregation into nodal and internodal regions. The thin-walled meristematic cells of the growing point are protected by a special organ composed of permanent tissue, and called the root cap, in which the outer cell walls become mucilaginous and this makes the forward passage of the root easier. The root cap has an independent origin, from initial cells called calyptrogen in monocotyledons, while in dicotyledons the formation of the root cap results from the periclinal division of the dermatogen itself. 86 Tissue systems In specialization of cells from meristem changes in shape, structure and nature of the cell wall and of the protoplast produce various cell types; accordingly, there are two tissue types: 1- Simple, when cells of a single type occur together in a uniform or homogenous mass, e.g. parenchyma, collenchyma and sclerenchyma. 2- Complex or Heterogenous, when cells of more than one type are associated in a tissue, e.g. xylem and phloem. Permanent tissues Permanent tissues are those in which growth has ceased at least temporarily. Permanent tissues may again become meristematic, also sometimes when permanent tissues mature their cells become completely dead. Permanent tissues are recognized into three tissue systems. 1) Epidermal tissue system (dermal – boundary tissue). 2) Ground tissue system (fundamental tissue). 3) Vascular tissue system (conducting tissue). 87 Epidermal or Boundary Tissue The epidermal tissue system consists of three elements; epidermal cells, stomata, and hairs or trichomes. The epidermis is the outermost layer or layers of cells on all plant parts during primary growth. It is thus in direct contact with the environment and is subject to structural modification by various environmental factors. The epidermis of the stem, leaves and floral parts originates from the surface layer of the shoot apical meristem. That of the root originates from a layer of cells in the root apical meristem that is covered by the root cap. Usually the epidermis consists of only one layer of cells, but in a few species the cells of this layer may divide periclinally to give rise to a several layered or multiple epidermis. The epidermis of both root and shoot may be differentiated into various kinds of cells. In both instances, epidermal cells may elongate at an angle to the surface of the organ to give rise to hairs. 88 In the epidermis of the leaf, and often also of the stem, stomata may be present; in some species cork cells containing silica are also differentiated. Epidermal cells are living cells tubular in shape, barrel-shaped or lens-shaped in cross section and sometimes elongated or nearly isodiametric. Each epidermal cell has a large central vacuole and thin peripheral cytoplasm. Minute leucoplasts are present and chloroplasts are absent except in hydrophytes and those of deeply shaded habitats. The epidermal cells in many leaves and petals have wavy lateral walls, which increase the firmness of the union of the cells. Epidermal cell walls vary in thickness in different plants and in different parts of the same plant. Epidermal cell wall may become silicified as in grasses. Primary pits and plasmodesmata generally occur in the anticlinal and inner periclinal walls of the epidermis. Very often, a layer of fatty material, or cutin, is deposited on the surface of the epidermal cell wall, forming the cuticle. This 89 substance is for the most part impervious to water and may have a protective function. The cuticle is extremely resistant to microorganisms, thus in the living plant it affords some protection, perhaps largely mechanical, against infection by pathogens, and in fossilized plant remains it may by extremely well preserved, being resistant to decay. In many plants, conspicuous deposits of wax are formed on the surface of the cuticle. It is this wax, which gives the “bloom” to some fruits, e.g. grapes. Waxes may be deposited in large or small flakes ‫ رقائق‬or granules, rods, tubes or sheets, which may then be sculptured into ridges. Cutin and waxes are synthesized in the living protoplasm and migrate to the surface through the cell wall. The cuticle, the cutinized cell wall beneath it and the surface wax serve to reduce loss of water. Stomata (sing. Stoma) occur on most of the aerial parts of plants, though predominantly on leaves and young stems. They are the ports for exchange of oxygen and carbon dioxide gas for photosynthesis, but also release excess water into the air. 90 This process of water loss maintains a steady flow of water and minerals from the roots to the leaves. To minimize the water loss, many plants regulate the duration and time of day when stomata are open. A stoma consists of a pore surrounded by two guard cells. The stoma arises by the division of one of the young epidermal cells (protodermal cell); one of the products of this division forms the guard mother cell. The guard mother cell divides by a vertical wall to form the two guard cells. The epidermal cells adjoining the guard cells often differ in size or arrangement from the rest of the epidermal cells; such cells called subsidiary cells. The stoma, together with the subsidiary cells, is sometimes termed the stomata complex. Stomata are clearly of great importance to the physiology of the plant, being sites of gaseous exchange during transpiration, photosynthesis and respiration. The guard cells differ from other cells of the epidermis in the more richly cytoplasm, the prominent nucleus and existence of chloroplasts and starch grains. 91 The guard cells of dicotyledons and monocotyledons except Gramineae (Poaceae) and Cyperaceae are commonly crescent- shaped (kidney-shaped), with rounded ends, as seen from the surface, and have ledges ‫ حواف‬of wall material on the upper or both upper and lower sides. The cells are covered with cuticle that extends over the surfaces facing the stomatal pore and the substomatal chamber. Stomata may be completely covered with wax. Among the guard cells of monocotyledons, those of Poaceae have a rather uniform, specific structure. As seen from surface, the cells are narrow in the middle and enlarged at both ends, i.e. dumb-bell-shaped. The middle portion is thick walled and rigid whereas the enlarged or bulbous ends are thin walled. Although the guard cells of the major taxa have their distinguishing characteristic, they chair the feature that the anticlinal wall away from the pore (dorsal wall) is thinner, and then more flexible than the other walls. This feature is commonly cited as having a causal relation to the ability of the guard cells to change their shape in response to turgor changes and thus to control the size of the stomatal opening. Guard cells may occur at the same level as the adjacent epidermal cells. These are usually present in mesophytes, while raised types 92 of stomata, which protrude above epidermal cells levels, are confined to some water plants (hydrophytes) and sunken stomata (below the surface of epidermis) are confined to desert plants (xerophytes). In leaves with parallel veins such as monocotyledons and some dicotyledons and in the needles of conifers, the stomata are arranged in parallel rows and attain the stripped pattern. While in leaves with reticulate venation (dicots) the stomata are distributed in no particular order and the stomata are scattered. The mechanism of stomata movement During stomata opening, starch disappears from chloroplasts, at the same time as K+ ions enter the guard cells, and during stomata closure, the reappearance of starch parallels the loss of K+ ions. The early theory that the breakdown of starch contributes to the increase of osmotic pressure in the guard cells, because of formation of sugar, has been replaced by the concept that starch hydrolysis may provide the organic anions with which potassium uptake is associated. 93 Types of stomata according to subsidiary cells In most species, the number and arrangement of the subsidiary cells around the stomata are relatively constant. Various classifications of stomata according to these arrangements have been drown up, and are sometimes useful to taxonomists. Four types of stomata complex have been described; these types have given descriptive names as follows. 1- Anomocytic or irregular-celled (Ranunculaceous): the surrounding cells are indefinite in number and do not differ from the other epidermal cells. 2- Anisocytic or unequal-celled (Cruciferous): usually three subsidiary cells surround the stoma, one cell being considerably smaller or larger than the other two. 3- Diacytic or cross-celled (Caryophyllaceous): two subsidiary cells surround the stoma with their common wall at right angles to the guard cells. 4- Paracytic or parallel-celled (Rubiaceous): one or more (often two) subsidiary cells are present, with their long axes parallel to the guard cells. 94 Trichomes or hairs Trichomes are highly variable appendages formed by the outgrowth of epidermal cells. They are formed in all parts of the plant, including stamens (e.g. Tradescantia) and seeds (e.g. cotton). Root hairs are morphologically typical hairs Hairs may be unicellular or cell division may take place, so that the hair becomes multicellular. The unicellular hair, which may have the form of papillae, represents the simplest form and they are common upon petals and many leaves. The unicellular hairs may be branched or unbranched. The more advanced types or forms of hairs are the multicellular, which range from simple linear hairs of few cells to complex branched or massive structures involving considerable areas of the epidermis. Stinging hairs (peltate hairs) and many others are complex multicellular structures. Unicellular, multicellular and peltate hairs may be glandular, producing secretions which are often of the nature of ethereal oils. The glandular head may be unicellular or multicellular, the head constitutes the secretory part of the 95 hair. The glandular hair formed of living cells and those of the head have dense protoplasmic contents and large nuclei. The cells of hairs may be dead in other types. The cell walls of trichomes are commonly of cellulose and are covered with a cuticle. They may be lignified. Plant hairs often produce thick secondary walls, e.g. the cottonseed hairs are cellulose, and the walls of hairs may be impregnated with silica or calcium, e.g. stinging hairs. Cystolith and other crystals may develop in hairs. Among the diverse types of hairs are those associated with absorption of water from the atmosphere, e.g. the unicellular absorbing hair of Diplotaxis harra. Hairs perform very different functions. 1- Dense coverings of hairs bring about a decrease in the rate of transpiration. 2- Root hairs serve for the absorption of water and other substances. 3- Vesiculated hairs on leaves of Atriplex halimus (salty plant) serve to remove salts from the leaf tissue and thus prevent an accumulation of toxic salts in the plant. 96 5- Trichomes may provide a defense against insects. Ground tissue or Fundamental tissue Fundamental or ground tissue is that in which the vascular bundles are embedded or is the tissue, which occupies all the spaces in the plant organs, which are not occupied by the vascular or conducting tissues. The ground tissue originates from the periblem and plerome i.e. it constitutes the cortex and pith. The ground tissue in the different plant organs is differentiated into cortex, pith, and medullary rays and in leaf is represented by the mesophyll, which is differentiated in dicotyledonous leaves into palisade and s