Introduction to Plant Embryology Dr

Introduction to Plant embryology Dr. Pallavi J.N.L. College Khagaul Plant Embryology • Embryology is the study of structure and development of embryo, including the structure and development of male and female reproductive organs, fertilisation and similar other processes. • Father of Indian Plant empryology- Panchanan Maheshwari • Plant embryogenesis is a process that occurs after the fertilization of an ovule to produce a fully developed plant embryo. This is a pertinent stage in the plant life cycle that is followed by dormancy and germination. • The zygote produced after fertilization, must undergo various cellular divisions and differentiations to become a mature embryo. An end stage embryo has five major components including the shoot apical meristem, hypocotyl, root meristem, root cap, and cotyledons. Unlike animal embryogenesis, plant embryogenesis results in an immature form of the plant, lacking most structures like leaves, stems, and reproductive structures. • The Phanerogams (the flowering-plants) are also called spermatophytes (the seed bearing plants). These plants propagate mainly through seeds. The seed is a structure in which the embryo is enclosed. Adjacent to the embryo, foods are stored either inside the endosperm (albuminous) or in cotyledon (exalbuminous) for future use. Life cycle of flowering plants • Alternation between a dominant sporophytic generation and a highly reduced gametophytic generation. Dominant sporophytic generation is diploid and reduced gaThe normal sexual cycle (amphimixing) involves two important processes: • (i) Meiosis and • (ii) Fertilization • In meiosis also known as reduction division, a diploid sporophytic cell spore mother cell) • gets converted into four haploid gametophytic cells. (“2n” number of chromosomes becomes half i.e. “n” number of chromosome) gametophytic generation is haploid. Microsporogenesis- formation of spores • Development of anther • 1. It is a multicellular, four-cornered structure, surrounded by a layer of epidermis. • 2. In each corner develops one or more archesporial initials. • 3. These initials divide by a periclinal wall into outer primary parietal cell and inner primary sporogenous cell. • 4. Primary parietal cell divides periclinally as well as anticlinally and form 3 to 5 concentric layers of cells. • 5. Innermost wall layer is called tapetum which is nutritive in function. • 6. From the sporogenous tissue develop the pollen grains. • 7. Some cells form the procambial strand in the centre of the anther. • Mature Anther • It is a four-cornered structure containing a pollen sac • 2. Anther is surrounded by a layer of epidermis throughout. • 3. Each pollen sac is surrounded by epidermis, an endothecial layer, one to three middle layers or wall layers and innermost layer of tapetum. • 4. In each pollen sac or pollen chamber are present many pollen tetrads which on separation form microspores. • 5. A joint in the form of connective is present in the centre. • T.S. Mature Anther Showing Dehiscence: • 1. It is a four-cornered, four-chambered, multicellular body surrounded by a layer of epidermis. • 2. Partition wall between the two pollen sacs is dissolved • 3. Many pollen grains or microspores are present in the pollen sacs in the form of fine, powdery or granular mass. • 4. Endothecium, middle layers and tapetal layers are present below the epidermis. • 5. Along the line of dehiscence of each lobe, thin- walled cells of endothecium form the stomium. • 6. A connective is very clear. • Pollen Tetrads: • (A) Isobilateral Tetrad: • All the four spores are formed in one plane because the spindles of first and second meiotic division remain at right angle to one another (Fig. 184), e.g.,Zea mays. • (B) Decussate Tetrad: • Out of the two lower spores, only one is visible. Both the upper ones are clear (Fig. 184), e.g.,Magnolia. • (C) T-shaped Tetrad: • In meiosis II upper cell divides to form two cells present side by side and the lower cell forms two cells lying one above the other, e.g., Aristolochia. • (D) Linear Tetrad: • All the four spores are present one above the other in a linear fashion, e.g., Halophila. • (E) Compound Pollen Grain: • Sometimes microspore tetrads adhere to each other (Fig. 184) and form the compound pollen grain, e.g., Typha, Cryptostegia. • (F) Pollinium: • Pollen grains of a pollen sac sometimes remain together to form a single mass called pollinium. Each pollinium (Fig. 184) consists of carpusculum, caudicle and pollinia, e.g., Asclepiadaceae. • Pollen Grain: • 1. It is a unicellular, uninucleate structure (Fig. 185). But pollen grains are always 2- or-3 nucleate, when shed. • 2. It is surrounded by a double-layered wall, i. e., outer exine and inner intine. • 3. Exine is thick, cutinized, pigmented, sculptured and perforated by germ pores. • 4. Intine is thin, colourless, smooth and consists of cellulose. • 5. In the cytoplasm are present water, protein, fats, carbohydrates, etc. Types of ovule • (A) Orthotropous: • (Ortho, straight; tropous, turned). When micropyle, chalaza and funicle lie in one straight line; e.g., Polygonaceae, Urticaceae. • (B) Anatropous: • (Ana, backwards; tropous, turned). Here, the body of the ovule turns backwards by an angle of 180° and so the micropyle becomes close to the hylum and placenta; Sympetalae. • (C) Hemitropous: • (Hemi, half; tropous, turned). Here the body of the ovule is placed transversely or somewhat at right angle to the funicle. Chalaza and micropyle are present here in one straight line (Fig. 186); e.g., Ranunculus. • (D) Campylotropous: • (Kampylos, curved). Here the body of the ovule is curved in such a way that the chalaza and the micropyle do not lie in the same straight line; e.g., Leguminosae. • (E) Amphitropous: • Here the curvature of ovule is more pronounced and embryo sac becomes horseshoe shaped (Fig. 186); e.g., Butomaceae. • (F) Circinotropous: • Here the funicle is very long and the ovule rotates by an angle of 360° in such a fashion that it is completely circled around by the funicle. Micropyle faces upward; e.g., Cactaceae. .

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