Plant Morphology PDF

PLANT MORPHOLOGY Dr. Ashraf Tawfiek Soliman Cellphone: 01203305553 E-mail: [email protected]  Dr. Ashraf T. Soliman Plant morphology It is the study of the physical form and external structure of plants. It represents a study of the d...

PLANT MORPHOLOGY Dr. Ashraf Tawfiek Soliman Cellphone: 01203305553 E-mail: [email protected]  Dr. Ashraf T. Soliman Plant morphology It is the study of the physical form and external structure of plants. It represents a study of the development, form, and structure of plants Here, we are dealing with the flowering plants (Angiospermae) 2  Dr. Ashraf T. Soliman Structure of a typical flower The flower is a compressed shoot (compact lateral branch), serving for purposes of sexual reproduction. It is composed of a stalk called pedicel, with an expanded swollen end known as receptacle on which the floral leaves are inserted. These floral leaves are usually found in whorls one inside the other: a. Calyx: is the outermost whorl of leaves. It is composed of sepals which are usually green. pedicel b. Corolla: it follows usually the calyx internally. It is formed of leaf-like colored structures called the petals. 3  Dr. Ashraf T. Soliman c. Androecium: it lies inside the corolla and represents the male sexual organ. It is made up of a number of stamens. Each stamen is composed of a slender part the filament, ending in a lobe-like structure named the anther. Dehiscence (bursting) of the anther Stamen usually takes place for the liberation of pollen grains. 4  Dr. Ashraf T. Soliman d. Gynoecium: the innermost whorl, located inside the androecium and represents the female sexual organ. The Carpel gynoecium is composed of one to many carpels. The carpel of structure angiosperms encloses the ovules. The carpel (pistil) is composed of a swollen basal part called the ovary extending in short or long slender part named the style which ends in a globular or lobed papillated structure called the stigma. Structure of the ovule: The edges protrude inside the cavity of the carpel forming the placenta on which the ovules arise, each ovule is attached to the placenta by a stalk called the funicle. The central part of the ovule is known as the embryo sac. The embryo sac is surrounded by a tissue called the nucellus which is covered externally by one or two investments known as the inner and outer integuments. The gap left by the integuments at the top of the ovule is called the micropyle. 5 Cross section through ovule  Dr. Ashraf T. Soliman Pollination When the pollen grains are fully developed, the pollen sacs of the anther burst exposing the pollen grains. The transfer of the pollen grains from the stamen to the stigma (the receptive organ of the carpel) is called the pollination. Pollination includes wind (the most common type of pollination) or water pollination especially in water plants. Biotic pollination includes insect, animal or even man as pollinators. 6 Fertilization: Dr. Ashraf T. Soliman fertilization involves not only the union of the egg cell with the sperm cell but, in addition, the union of a second sperm cell with the endosperm mother cell to form the primary endosperm cell. Since there are two nuclei in the endosperm mother cell, the nucleus of the primary endosperm cell will have three sets of chromosomes. The fusions of two sperm cells, one with the egg, the second with the endosperm mother cell, is known as double fertilization. 7 Dr. Ashraf T. Soliman 8  Dr. Ashraf T. Soliman The resulting zygote generally develops directly into a small pro- embryo, from one end of which a typical embryo develops. The fate of the primary endosperm cell depends on the species. This cell gives rise to an endosperm that plays some role in nourishing the developing embryo. In a few genera, notably the grasses, the endosperm enlarges and persists in the seed to form the source of nourishment for the young seedling. 9  Dr. Ashraf T. Soliman Seeds and seed germination The seed can be defined as the mature fertilized ovule. The seed possesses only one scar (the hilum) which is the point of attachment to its stalk (funicle). Seeds of flowering plants differ greatly in form, color, size and other characteristics. They may be spherical, ovoid, elliptical, elongate, disc-like, or irregular in shape. The size ranges from the dust-like, very light seed e.g. orchids to the huge seeds of coconut. The outer wall of the seed may be smooth, rough or sculptured. It may be also provided with outgrowths in the form of spines, hooks or fibers which help in seed dispersal. 10  Dr. Ashraf T. Soliman Structure of the seeds: The mature seed consists of: (1) seed coat, (2) an embryo and (3) storage tissue. 1. Seed Coat: It is the protective wall surrounding the embryo and storage tissue. It is developed from the integuments of the ovule. The seed coat is usually called testa. The hilum is the point of attachment of the seed to the funicle. The micropyle is a minute pore and is mostly present in the testa of many seeds. 11  Dr. Ashraf T. Soliman a. The testa: It develops from the integuments of the ovule. The integumentary tissues harden, their cells become thick-walled and serve only for protection of the embryo and the stored food. The inner integument may persist in some seeds as a distinct layer of the testa known as tegmen. The testa may be leathery (broad bean); hard brittle and sculptured (castor oil seed); fleshy (Magnolia); Membranous and easily detached as in peanut (Arachis) or it is woody (apricot). In maize (Zea mays) grains, there is no testa. The developing embryo consumes all the tissues of the integument. The seed is only protected by the developed and hardened ovary wall i.e. the pericarp of the fruit. While, the grain coat of wheat (Triticum) includes some of the layers of the integument fused with the pericarp of the fruit. 12  Dr. Ashraf T. Soliman b. The hilum: It is the scar which marks the point of attachment to the funicle. During maturation of the fruit, the funicle gradually dries up, a separating layer is developed between the funicle and the base of the seed which helps abscission (falling off) of the seed. 13  Dr. Ashraf T. Soliman c. The micropyle: Is a very minute pore resulting from the incomplete fusion of the integuments of the ovule and is usually locates between hilum and the radicle. The micropyle allows for communication between the nucellus and the outer surface of the ovule. The pollen tube penetrates through the micropyle to the embryo sac for fertilization of the ovule. At germination of the seed, water absorption takes place mainly through the micropyle, especially when the testa is impermeable. The micropyle may be concealed (hidden) in some seeds due to the occurrence of outgrowths called Caruncle as in castor oil seed or it may be absent as in Zea mays. 14  Dr. Ashraf T. Soliman 2. The embryo: The embryo axis is the elongated portion of the embryo, at one of this axis there is a plumule and at the other there is the radicle. One or two cotyledons are attached to the embryo axis by very fine and short cotyledonary stalks. a. The plumule: is the terminal upper portion of the embryo axis. It is considered as the first bud of the young embryonic plant. Apex is the growing shoot apex including embryonic meristem (or apical meristem of the shoot). Epicotyl: is the small portion of the plumule above the cotyledonary stalk, it is considered as the main axis of the plumule. b. The radicle: is the terminal lower portion of the embryo axis. From the radicle arises the primary root of the plant. It usually grows downward the soil and away from light. Hypocotyl: is the small portion of the radicle below the cotyledonary stalk, it is considered as the main axis of the radicle. 15  Dr. Ashraf T. Soliman c. The cotyledons: they are the young leaves of the embryo. They are termed as seed leaves. The number, size and shape of cotyledons varies in different groups of the flowering plants. Accordingly, higher plants may be: - Monocotyledonous plants: the seed possesses only one cotyledon (e.g. corn (Zea mays) & date palm (Phoenix dactylifera). - Dicotyledonous plants: the seed possesses two cotyledons, e.g. broad bean (Vicia faba) & castor oil bean (Ricinus communis). 16  Dr. Ashraf T. Soliman 3. The storage tissue: There are two types of seeds according to the food storage tissue. The storage tissue is termed as endosperm. a. Exendospermous (Exabluminous) seeds: are the seeds lacking endosperm in the mature state. The embryo is large in relation to the seed and the food reserves are stored in the cotyledons. Exalbuminous seeds usually possess large and stout cotyledons as they store much food (e.g. Vicia faba, & Phaseolus vulgaris). b. Endospermous (Albuminous) seeds: are the seeds with endosperm. In the seeds of such type, the embryo is comparatively small and varies in relation to the amount of endosperm left at maturity. Albuminous seeds possess papery cotyledons; they do not store food (e.g. Ricinus communis & Gossypium barbadense). 17  Seed Germination Dr. Ashraf T. Soliman The growth and development of seed into seedling: Growth is the increment of size and dry weight of the growing structures. Development is the differentiation of the embryo into the different organs of mature plant Conditions necessary for seed germination A. Internal conditions: 1-Vitality Vitality or Viability is the germinating capacity of the seeds & the ability of a seed to germinate when provided with optimum condition, it is dependent upon its stored food, size, health, etc. Environmental factors and storage conditions will have a decisive effect on the life span of any given seed, whether the seed will remain viable for the longest period of time or whether it loses its viability at some earlier stage. Most of the crop plants lose their viability within 2-5 years. Some seeds may remain viable for more than hundreds of years. 18  Dr. Ashraf T. Soliman 2- Dormancy It is the failure of fully developed, mature, viable seed to germinate even under favorable physical conditions (moisture and temperature). These internal restrictions must be offset before seed germination. There are several types of dormancy exhibited by seed at maturity, they are: Physiological (e.g., presence of some inhibiting chemicals), Physical (e.g., hard testa, impermeable to water), Morphological (e.g., immature embryo) and Morphophysiological. 19  Dr. Ashraf T. Soliman B. External ‘Environmental’ conditions 1. Water: It is required for germination. Mature seeds are often extremely dry and need to take in significant amounts of water, relative to the dry weight of the seed, before cellular metabolism and growth can resume. Most seeds need enough water to moisten the seeds but not enough to soak them. The uptake of water by seeds is called imbibition, which leads to the swelling and the breaking of the seed coat. When seeds are formed, most plants store a food reserve with the seed, such as starch, proteins, or oils. This food reserve provides nourishment to the growing embryo. When the seed imbibes water, hydrolytic enzymes are activated which break down these stored food resources into metabolically useful chemicals. After the seedling emerges from the seed coat and starts growing roots and leaves, the seedling's food reserves are typically exhausted; at this point photosynthesis provides the energy needed for continued growth and the seedling now requires a continuous supply of water, nutrients, and light. 20  Dr. Ashraf T. Soliman B. External ‘Environmental’ conditions 2. Oxygen is required by the germinating seed for metabolism. Oxygen is used in aerobic respiration, the main source of the seedling's energy until it grows leaves. Oxygen is an atmospheric gas that is found in soil pore spaces; if a seed is buried too deeply within the soil or the soil is waterlogged, the seed can be oxygen starved. Some seeds have impermeable seed coats that prevent oxygen from entering the seed, causing a type of physical dormancy which is broken when the seed coat is worn away enough to allow gas exchange and water uptake from the environment. 21  Dr. Ashraf T. Soliman B. External ‘Environmental’ conditions 3. Temperature affects cellular metabolic and growth rates. Seeds from different species and even seeds from the same plant germinate over a wide range of temperatures. Seeds often have a temperature range within which they will germinate, and they will not do so above or below this range (maximum and minimum values). Temperature also affects the degree of viscosity of water and leads to tenderness of the testa which becomes more easily ruptured by the developing embryo. Seeds may be named after the seasons that they prefer to grow into summer seeds that prefer warmer months of the year to germinate as maize and cotton or winter seeds that prefer cold months of the year as bean, wheat and clover. 22  Dr. Ashraf T. Soliman B. External ‘Environmental’ conditions 4. Light or darkness can be an environmental trigger for germination and is a type of physiological dormancy. Most seeds are not affected by light or darkness, but many seeds, including species found in forest settings, will not germinate until an opening in the canopy allows sufficient light for growth of the seedling. Accordingly, seeds are classified into 3 types: a. Light seeds: Germination of these seeds is favored by their exposure to light even for short period. These are known as positively photoblastic seeds. b. Dark seeds: that will not germinate at all in the presence of light, e.g., mistletoe (Viscum album). These are termed negatively photoblastic seeds. c. Indifferent seeds: these are indifferent towards light or darkness, e.g., many cereals and legumes. These are known as non-photoblastic seeds. 23  Dr. Ashraf T. Soliman Changes occurring during seed germination Several changes occur growth and development of the embryo of a germinated seed: 1. Physical changes: these include a. Swelling up of the embryo. b.Rupture of the testa due to the imbibition of water. It is to be noticed that viable and dead seeds behave the same during the first period of water imbibition. 24  Dr. Ashraf T. Soliman Changes occurring during seed germination 2. Biochemical changes: Transformation of complex and insoluble reserve materials of the seed to simple and easily soluble substances are purely chemical changes. Hydrolysis of complex stored food through the activity of enzymes present in the viable embryo of the seed, e.g. Amylases, proteases and lipases. The soluble products of enzymatic reactions are translocated to the growing apices of the radicles and plumules for building up the primary body of the young plant. Other biochemical changes are the partial consumption and combustion of the hydrolyzed or degraded complex material for obtaining the energy needed for other vital processes e.g. respiration. 25  Dr. Ashraf T. Soliman Changes occurring during seed germination 3. Biotic changes: These include changes occurring seed germination e.g. growth and development of the embryo. The plumule and radicle increase in size, become gradually differentiated to shoot and root systems. Seedling is the young plant from the time the radicle emerges out of the seed coat to the time it becomes independent in its nutrition. 26  Dr. Ashraf T. Soliman - Hypogeal germination “below ground”: the hypocotyl doesn’t elongate or its elongation is insignificant, the cotyledons remain under the soil surface. For example, broad bean (Vicia faba), maize (Zea mays). 27  Dr. Ashraf T. Soliman - Epigeal Germination “above ground”: here the hypocotyl elongates carrying the cotyledons together with the plumule above the soil surface. For example, kidney bean (Phaseolus vulgaris) and castor oil bean (Ricinus communis). 28 Dr. Ashraf T. Soliman 29