Module-1-Unit-I-Part-II Plant Biology PDF

Unit I. INTRODUCTION 1. Levels of plant organization 2. Organs found in a vascular plant body 3. Features of vascular plants 4. Types of steles 5. Alternation of Generations 6. Overview of Extinct and Extant Vascular Plant Intended Learning Outcomes (ILOs) At the end of the unit, the...

Unit I. INTRODUCTION 1. Levels of plant organization 2. Organs found in a vascular plant body 3. Features of vascular plants 4. Types of steles 5. Alternation of Generations 6. Overview of Extinct and Extant Vascular Plant Intended Learning Outcomes (ILOs) At the end of the unit, the student must have: 1. discussed the levels of plant organization. 2. identified the organs found in a vascular plant body. 3. enumerated the features of vascular plants. 4. identified and described the type of steles. 5. discussed alternation of generations. 6. identified the extinct and extant vascular plants. Learning Activities/Outputs Lecture Discussion Portfolio/e portfolio ( Types of Stele) Group Role Playing on the Life Cycle of different Plant groups Assignment on the Types of stomata, tracheary elements and xylem development. Worksheets (Basic morphological and anatomical terms) Topics/Content ORGANS FOUND IN THE VASCULAR PLANT BODY VEGETATIVE ORGANS  Root  Stem(Rhizome/Tuber)  Leaf(Megaphyll/Microphyll) REPRODUCTIVE ORGANS  Flower  Fruit  Cones  Strobilus  Sporophyll  Sori/Spores Microphylls are defined as leaves of small size, with simple venation (one vein) and associated with steles that lack leaf gaps (protosteles). By contrast, megaphylls are defined as leaves of generally larger size, with complex venation and associated with leaf gaps in the stele. Sporophyll  a leaf that bears sporangia.  both microphylls and megaphylls can be sporophylls.  In heterosporous plants, sporophylls (whether they are microphylls or megaphylls) bear either megasporangia and thus are called megasporophylls, or microsporangia and are called microsporophylls. Strobilus  A strobilus (pl.: strobili) is a structure present on many land plant species consisting of sporangia-bearing structures densely aggregated along a stem. Strobili are often called cones, but some botanists restrict the use of the term cone to the woody seed strobili of conifers. Strobili are characterized by a central axis (anatomically a stem) surrounded by spirally arranged or decussate structures that may be modified leaves or modified stems. Sporangia-bearing stems are called sporangiophores.  The reproductive structures in gymnosperms are called cones or strobili. The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili. The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili. Synangium  The synangia appear to be the product of three sporangia which became fused over the course of evolution, and are borne on the tip of a short lateral branch. This is another feature in which the psilophytes differ from other living vascular plants; all other such plants produce their sporangia on their leaves. Sori  Sori (singular: sorus) are groups of sporangia (singular: sporangium), which contain spores. Sori are usually found on the underside of the blade. Young sori are commonly covered by flaps of protective tissue called indusia (singular: indusium). Spores  a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, fungi and protozoa.  Ferns produce sexually by releasing spores which a haploid gametophyte. The gametophyte contains male and female structures called the antheridium and the archegonium which produce sperm and eggs. In the presence of water, the sperm swims to the egg to fertilize which produces a diploid sporophyte. FEATURES OF A VASCULAR PLANT 1. The cuticle is the outer casing of some parts of the plant. Most of these plant parts are above the ground such as leaves, non-woody stems, fruits, and flowers. The cuticle is characterized by being a protective impermeable repellent to water, preventing valuable water from being lost. The epidermis, which is considered one of the most important outer layers of plants, is responsible for secreting the cuticle. The epidermis is the barrier between the outer environment and the plant. 2. Stomata The stomata are minute pores or openings found in the epidermis of leaves, stems, and other plant organs that allow gases such as carbon dioxide, oxygen, and water vapor to diffuse into and out of the internal tissues of the plant.Stomata is the plural form of the stoma. In greek, stoma means “mouth”. The Stomata are apertures in the epidermis of the plants. Structure Each stoma or pore remains surrounded by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the stomatal opening. The shape of guard cells usually differs in monocots (dumbbell shape) and dicots (kidney shape) but their mechanism is the same. The wall of guard cells surrounding the pore is thickened and the rest of the wall is thin. Guard cells contain a single nucleus and a number of chloroplasts. But the RUBISCO enzyme is absent, so guard cells have very poor photosynthesizing capability. Two or more cells adjacent to the guard cells appear to be associated functionally with them and are morphologically distinct from the other epidermal cells. Such cells are called subsidiary or accessory cells. Subsidiary cells contain no chloroplast. They support the movement of guard cells. Types of Stomata  Metcalfe and Chalk (1950), based on the number and arrangement of the subsidiary cells that surround the two guard cells. a. Anomocytic (irregular-celled) Anomocytic stomata have guard cells that are surrounded by a limited number of subsidiary cells that have the same size, shape, and arrangement as the rest of the epidermal cells. These are also called the ranunculaceous type. This type of stomata can be found in more than a hundred dicot families including Apocynaceae, Boraginaceae, Chenopodiaceae, and Cucurbitaceae. b. Anisocytic (unequal-celled) In this type, the stoma has guard cells that are surrounded by three unequally sized subsidiary cells of which one is distinctly smaller than the other two. It is also called the cruciferous type. This type of stomata can be found in more than thirty dicot families, such as Brassicaceae, Solanaceae, and Crassulaceae. c. Paracytic (parallel-celled) Paracytic stomata have one or more subsidiary cells parallel to the opening between the guard cells. The subsidiary cells may reach beyond the guard cells or not. These are also called the rubiaceous type. This type of stomata can be found in more than a hundred dicot families including Rubiaceae, Convolvulaceae, and Fabaceae. d. Diacytic (cross-celled) Diacytic stomata have guard cells surrounded by two subsidiary cells, that each encircle one end of the opening and contact each other opposite to the middle of the opening. These are sometimes called the caryophyllaceous type. This type of stomata can be found in more than ten dicot families such as Caryophyllaceae and Acanthaceae. e. Actinocytic (star-celled) The stomata have guard cells that are surrounded by at least five radiating cells forming a star- like circle. This is a rare type of stomata that can for instance be found in the family Ebenaceae. f. Cyclocytic (ring-celled) This type of stomata is surrounded by four or more subsidiary cells forming a narrow ring around the guard cells. Examples: Palmae, Pandanus, Cyclanthaceae. g. Gramineous (grass-like) Gramineous or graminoid stomata have two guard cells surrounded by two lens-shaped subsidiary cells. The guard cells are narrower in the middle and bulbous on each end. This middle section is strongly thickened. The axis of the subsidiary cells is parallel to the stoma opening. This type can be found in monocot families including Poaceae and Cyperaceae. 3. Vascular tissue Vascular tissue is comprised of the xylem and the phloem, the main transport systems of plants. They typically occur together in vascular bundles in all plant organs, traversing roots, stems, and leaves. Xylem is responsible for the transport of water and dissolved ions from the roots upwards through the plant. Xylem Tissue  Xylem is a complex tissue made up of several types of cells. All xylem tissue contains tracheids and some plants have both tracheids and vessels in their xylem. All xylem also contains living cells, xylem parenchyma, and sometimes fibers (xylary fibers).  Xylem originates from the Greek word “xylon” that means wood. Carl Nägeli coined the word xylem. Xylem is a type of vascular tissue present in plants, which primarily transports water and nutrients from roots to stem and leaves. They also provide mechanical strength to the plants. On the basis of origin, there are two types of xylem cells: a. Primary xylem: originating from procambium, further divided into protoxylem and metaxylem Protoxylem Proto means ‘first’ and Xylem means ‘wood’; the first primary xylem that develops first during primary growth. The primary xylem that forms initially during primary growth is called the protoxylem. Prior to plant organ elongation, protoxylem develops. Protoxylem is found toward the outside of a stem. It comprises more compact cells. In other words, it has tracheids and components of thin vessels. As a result, the cells' lumen is small. In the protoxylem cells, lignification is also not very extensive. The secondary cell walls of protoxylem vessels exhibit annular and spiral thickenings. Additionally, protoxylem lacks xylem fibres and has a significant quantity of parenchyma. Protoxylem is less effective in conducting water than metaxylem. Metaxylem Meta means ‘last’ and Xylem means ‘wood’; the last primary xylem. The primary xylem's metaxylem is the portion that forms the following protoxylem. After the plant organs have finished growing, the metaxylem develops. It can be found near the stem's interior. Larger cells like tracheids and broader arteries are found in metaxylem. The lignification of metaxylem cells is also quite widespread. The secondary cell walls of metaxylem vessels have scalariform, reticulate, and pitted thickenings. Metaxylem also has a small amount of parenchyma cells and xylem fibres. Protoxylem is less effective than metaxylem for conducting water and minerals. Therefore, compared to the lumen of protoxylem cells, the lumen of cells is bigger. b. Secondary xylem: originating from the vascular cambium Xylem is composed of four different kinds of elements: 1. Tracheids: Dead, tube-like cells with a tapering end. They are present mostly in gymnosperm and lower angiosperm. They have a thick lignified wall and lack protoplasm. Their main function is water and mineral transportation. 2. Vessels: They are present in angiosperms. These are a long cylindrical structure having tube- like appearance. The walls are lignified and have a large central cavity. They are also dead and lack protoplasm. They have many cells called vessel members which are interconnected through a perforation in common walls. Mostly involved in the conduction of water, minerals and give mechanical strength to the plant. 3. Xylem Fibre: Dead cell with lignified walls and a central lumen. Involved in water transportation and providing mechanical support. 4. Xylem Parenchyma: Only living cells of xylem and store starch and fat. They assist in the short distance transportation of water. Types of Tracheary elements (TEs)  Tracheary elements (TEs), which are the distinctive cells of the xylem, are characterized by the formation of a secondary cell wall with annular, spiral, reticulate, or pitted wall thicken- ings.  Tracheary elements are dead, hollow cells with patterned cell walls comprising xylem vessels and tracheids, which function as conductive hollow tubes for water and nutrient transport throughout the plant body. Tracheid types based on lignin deposition on primary wall. a. Annular  Deposition of lignin in the inner surface of cell wall in the form of ring. b. Spiral  Deposition of lignin in the inner surface of cell wall in the form of spiral band. c. Scalariform  Deposition of lignin in the inner surface of cell wall in the form of transverse rod. d. Reticulate/Pitted  Deposition of lignin in the inner surface of cell wall in the form of net. Xylem development Xylem development can be described by four terms: centrarch, exarch, endarch and mesarch. As it develops in young plants, its nature changes from protoxylem to metaxylem (i.e. from first xylem to after xylem). The patterns in which protoxylem and metaxylem are arranged is important in the study of plant morphology. Patterns of protoxylem and metaxylem There are four main patterns to the arrangement of protoxylem and metaxylem in stems and roots. 1. Centrarch refers to the case in which the primary xylem forms a single cylinder in the center of the stem and develops from the center outwards. The protoxylem is thus found in the central core and the metaxylem in a cylinder around it. This pattern was common in early land plants, such as "rhyniophytes", but is not present in any living plants] The other three terms are used where there is more than one strand of primary xylem. 2. Exarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the outside inwards towards the center, i.e. centripetally. The metaxylem is thus closest to the center of the stem or root and the protoxylem closest to the periphery. The roots of vascular plants are normally considered to have exarch development. 3. Endarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the inside outwards towards the periphery, i.e. centrifugally. The protoxylem is thus closest to the center of the stem or root and the metaxylem closest to the periphery. The stems of seed plants typically have endarch development. 4. Mesarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the middle of a strand in both directions. The metaxylem is thus on both the peripheral and central sides of the strand with the protoxylem between the metaxylem (possibly surrounded by it). The leaves and stems of many ferns have mesarch development.  Patterns of xylem development: xylem in brown; arrows show direction of development from protoxylem to metaxylem. Phloem Tissue Composition: 1. Sieve elements 2. Companion cell 3. Phloem parenchyma 4. Phloem fiber or bast fiber 1. Sieve element  Main conducting element of phloem.  Elongated tube like structure. Sieve area:  Depressed area with group of perforation or pore or connecting area.  Sieve area is present in the end wall.  Perforation is used for transport. Through this, protoplasm and nutrients of different cells are connected and passed.  The pores are encircled by chemical substances such as Beta-1,3-glucan.  In a sieve area, connecting strands are enclosed by chemical substances which are called callose. The wall of sieve area is a double structure which is composed of two primary walls. Sieve elements have two forms: 1. Sieve cell  Primitive form.  Single cell.  No series of united cells.  Found in lower group of organisms such as pteridosperm and gymnosperm.  In pteridosperm, sieve cell and phloem parenchyma are present.  In gymnosperms, sieve cell, phloem fibre and phloem parenchyma are present. 2. Sieve tube  Advanced and specialized form.  Series of united cell.  In angiosperms, all are present.  Lack nucleus. Companion cell  Special type of parenchyma cell.  Always associated with sieve tube.  Living (abundance of cytoplasm).  Nucleus of the companion cell serves as the nucleus of the sieve tube.  Helps in transport.  No storage of starch grain.  Shape round, rectangular etc.  Small Origin  By longitudinal division of the sieve tube (Two cells are formed. One acts as sieve tube and another as companion cell). Number  Companion cell may divide by transverse division. So, companion cells may be one or more than one (row of companion cells).  Confined in number in particular genus and family.  Single companion cell in primary phloem and herbaceous plant.  More than one companion cells in secondary phloem and woody plants. Phloem parenchyma  Parenchyma cells present in phloem are known as phloem parenchyma.  Activity is concerned with living parenchyma.  Elongated and oriented or adjacent with sieve tube.  No chlorophyll.  Tannin and resin may be present.  Two systems of parenchyma:  Vertical system also called phloem parenchyma.  Horizontal system also called phloem ray parenchyma.  Numerous pits are present.  Phloem parenchyma is not found in many or most of the monocot plant. Phloem fiber or bast fiber  Fiber cells are usually elongated, tube like; pointed tipped; thick walled; have small lumen and sclerenchymatous.  Associated with sieve tube.  Phloem fiber is the most prominent part both in primary and secondary phloem of phloem tissue in most flowering plants.  Pteridosperm: Phloem fibre absent.  Gymnosperm: Phloem fiber present (may be absent too).  Pits always present.  Wall is lignified (e.g. Cannabis) or non-lignified (e.g. Linum).  In lignified wall, wall is thickened by lignin whereas in non-lignified wall, wall is thickened by cellulose or pectin. TYPES OF STELE  The central cylinder or core of vascular tissue, consisting of xylem, phloem, pericycle and sometimes medullary rays and pith, is technically called stele.  Van Tieghem and Douliot (1886) developed “Stellar Theory”.  They used the term stele in collective sense and mentioned that the stele is not only made up of xylem and phloem, but the tissue like pericycle, vascular rays and pith are also associated with it. According to them the cortex and the stele are the fundamental parts of a shoot and both these parts are separated from each other by the endodermis. According to stellar theory, primarily there is no fundamental difference in the gross anatomy of stem and roots, because in both of them a stele is surrounded by the cortex is present.  Although stele is real entity and present universally in all axes of the plants, I n higher vascular plants like ferns, gymnosperms and angiosperms the leaf traces are large and it plays an important role in the vascular system of the axis.  Foster and Gifford (1959) have mentioned that the most debated and controversial aspect of “stellar theory is the nature of the anatomical boundaries which separate the cortex from the stele”.  According to Van Tieghem and Douliot (1886) the endodermis represents the inner boundary of the cortex. The cells of the endodermal layer have the characteristic casparian strip. But in the stems of many seed plants, the characteristic endodermal layer is not present. Some have mentioned that in such cases the pericycle serves as separating layer between the stele and the cortex. TYPES OF STELES Jeffrey (1868) interpreted the stellar theory from the phylogenetic point of view. Based on stellar theory various types of vascular cylinders can be recognised in the roots and stems. Most of the workers recognised two main types of stellar organisations, Protostele and siphonostele. 1. PROTOSTELE  A stele in which the vascular cylinder consists of a solid core of xylem surrounded by phloem, pericycle and endodermis  is primitive type of stele in vascular plants.  There is no pith  In most of the pteridophytes, the stem remains protostelic at sporeling stage. TYPES OF PROTOSTELE A. HAPLOSTELE  The protostele with a smooth core of xylem surrounded by a uniform layer of phloem  It has been observed in fossil genera like Rhynia, Horneophyton and living genera like Selaginella chrysocaulos, S.kraussiana, S. selaginoides, S. willdenowi, Gleichenia dichotoma, Lygodium and Cheiropleuria. B. ACTINOSTELE  It is a protostele having a xylem core with radiating ribs. In this case the xylem core is star shaped or stellate.  In actinostele the phloem is not present in a continuous manner but in the form of separate groups which alternate with the distant ends of the star shaped xylem, eg. Asteroxylon, Psilotum, Lycopodium serratum and Sphenophyllum.  Because of the breaking of the xylem mass into different forms, the actinosteles shows following variations: C. PLECTOSTELE:  In plectostele the xylem gets broken into a number of parallel plates. Such xylem plates alternate with the phloem plates. E.g Lycopodium clavatum and L. volubile. D. MIXED PROTOSTELE:  In this type the xylem groups are uniformly scattered in the ground mass of the phloem. E.g. Lycopodium cernuum. III. MIXED PROTOSTELE WITH PITH:  Sometimes, thin parenchyma cells remain associated with the xylem. E.g. In genera like Hymenophyllum demissum, H. dilatatum, Lepidodendron intermedium and L. selaginoides the centre of the protostele is occupied by the parenchyma cells among the tracheids. 2 SIPHONOSTELE:  A medullated protostele.  In a siphonostele the centrally placed xylem core is replaced by parenchymatous cells called pith. The pith is surrounded by xylem. TYPES OF SIPHONOSTELE  Jeffrey (1898) has classified the siphonostele into the following two types on the basis of the position of phloem. A. ECTOPHLOIC SIPHONOSTELE:  In this case the phloem is restricted only to the external side of the xylem. The pith is central in position. The phloem is externally surrounded by the pericycle and endodermis. A leaf trace is also visible. E.g. Osmunda, Schizaea, etc. B. AMPHIPHLOIC SIPHONOSTELE/SOLENOSTELE: In this case, the phloem is present on both the external and internal side of the xylem. The pith is present in the centre. Eg. Rhizome of Marsilea. In this case xylem on its inner side remains surrounded by inner phloem, the inner pericycle, inner endodermis and centrally placed pith. On the outer side of the xylem are present the outer phloem, the outer pericycle, and the outer endodermis. E.g. Adiantum, Dipteris) on both the sides of xylem, e.g. Adiantum pedatum. C. DICTYOSTELE: This is a type of solenostele which is broken into a network of separate vascular strands, mainly because of the crowded leaf gaps (Brebner, 1902). Each separate vascular strand is known as meristele, e.g. Ophioglossum lusitanicum, Pteris, Adiantum capillaris-veneris, Dryopteris chrysocoma, D. rigidia and D. filix-max. The whole stele appears as a network of interconnected vascular strands. D. EUSTELE: If the stele is split into distinct collateral vascular bundles, it is called eustele. It is a modification of the ectophloic siphonostele. The splitting takes place because of the overlapping leaf gaps. E. ATACTOSTELE: Differing from the eustelic condition, the vascular strands in some cases are scattered. It occurs in monocotyledons. Atactostele 3. POLYSTELE  Sometimes more than one stele is present in the axis of some pteridophyte like Selaginella,  It is a type which must have been derived from a protostele because each such stele shows a protostelic condition. In Selaginella kraussiana, generally, two steles are present but in Selaginella laevigata as many as sixteen steles are present in the axis. ALTERNATION OF GENERATIONS  Alternation of generations is common in plants, algae, and fungi. This can be compared to the sexual reproduction in animals where both haploid and diploid cells are found in every generation.  Plants alternate between the diploid sporophyte and haploid gametophyte, and between asexual and sexual reproduction. Therefore, the life cycle of plants is known as alternation of generations. The ability of the plants to reproduce sexually and asexually helps them to adapt to different environments.  The alternation of generations depends upon the type of the plant. In Bryophytes, the dominant generation is haploid and the gametophyte comprises the main plant. In tracheophytes, the dominant generation is diploid and the sporophyte comprises the main plant.  The plants’ life cycle in one of the two generations is dominant over the other. The plants in the dominant generation grow larger and live longer. The plants in the non-dominant generations are small and hardly visible. On the contrary, the dominant generations are seen in the form of ferns, trees or other plants.  The dominant generation in vascular plants is the sporophyte, while in the non-vascular plants is the gametophyte. The alternation of generations include the following stages: 1. The diploid sporophyte has a structure called sporangium. 2. The sporangium undergoes meiosis and forms haploid spores. 3. The spore develops into a gametophyte which is haploid in nature. 4. The gametophyte has the reproductive organs which undergo mitosis to form haploid gametes. 5. The gametes fertilize to form a haploid zygote which matures into a mature sporophyte. 6. This cycle keeps repeating. Stages of Alternation of Generations Following are the two stages of alternation of generations: Sporophyte Generation Two haploid gametes fuse together to form a diploid zygote. This results in a sporophyte. The sporophyte is formed by multiple rounds of mitosis and is a multicellular organism. On reaching maturity, the sporophyte develops reproductive organs known as sporangia. This is one key point in the alternation of generations. These sporangia are used to create haploid spores. These spores are released and carried away by air and water and when the conditions are favourable they develop into a gametophyte. Gametophyte Generation This is the next generation in the alternation of generations. The spore is newly formed and has half the DNA as the parent organism. This spore undergoes mitosis multiple times to form a gametophyte. The gametophyte generation creates gametes. These gametes are produced by gametangia. These gametes are then transferred between plants or spread into the environment. When a gamete encounters a gamete of the opposite sex, it fuses with it to form a zygote which eventually becomes a sporophyte.