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Development of male gametophyte

This document provides information about embryology of angiosperms. It discusses the key topics in plant embryology including microsporogenesis, megasporogenesis, pollination, fertilization, and embryo development. The history of embryology is explored beginning with Aristotle and Theophrastus in ancient Greece. Important later figures who advanced the understanding of plant sexuality and reproduction are mentioned like Nehemiah Grew, R.J. Camerarius, and J.G. Kolreuter. The structures of the anther and pollen grain are described along with the development of the male gametophyte within the pollen.

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B.Sc.- II, Sem- III Unit – VI Embryology of Angiosperm DR. SWATI PUNDKAR ASSISTANT PROFESSOR DEPARTMNET OF BOTANY SHRI SHIVAJI SCIENCE COLLEGE, AMRAVATI
. 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. Embryology played a significant role in consideration of systemic botany. Plant embryogenesis is a process that occurs after the fertilization of an ovule to produce a fully developed plant embryo Plant embryology deals with study of all events starting from microsporogenesis, megasporogenesis. Pollination and fertilization till the development of mature embryo.
Fritillaria tenella and Lilium martagon Histrory The Greek philosopher Aristotle (384-322 B.C.) is the ‘Father of the Study of Natural Sciences’. Later Theophrastus (370-287 B.C), the ‘Father of Botany’ in third century B.C. referred the pollination of the date palm. the nature of sexuality of plants was forgotten for about 2,000 years and his report was considered as useless Nehemiah Grew who first clearly stated, in 1682, that the stamens are nothing but the male sex organs of a plant. In 1694, R. J. Camerarius suggested that the anthers are the male sex organs of a plant, while the ovaries with the styles were the female sex organs. J. G. Kolreuter (1761) fully confirmed the idea of Camerarius.
FLOWER STRUCTURE
Sexual reproduction takes place with slight variations in different groups of plants. Plants have two distinct stages in their lifecycle: the gametophyte stage and the sporophyte stage.
MALE GAMETOPHYTE
A. Epidermis It is the outermost layer of a young anther and undergoes only anticlinal divisions. It is single layered. Function: It performs the role of protection. B. Endothecium Immediately below the epidermis, the cell layers are radially elongated and develop fibrous thickenings bands which U shaped. These cells are hygroscopic in nature and help in dehiscence of anther and discharge of pollen. It is generally single layered but some plants it may be multilayered. C. Middle layer Usually one to three middle layers are found below the endothecium. The cells are paremchymatous which present between endothecium and tapetum from middle layer. These cells are ephemeral and degenerate completely before pollen mother cell undergoes meiosis. (They become crushed at the time of meiotic division in the pollen.) Function: The middle layers store the food material which is utilized by pollen mother cells.
Tapetum The cells of the innermost parietal layer possess dense protoplasm and the food entering into the sporogenous cells pass through it. Thus it serves as a nutritive layer for the developing microspore. The tapetum may be glandular or amoeboid based on the behaviour of the cells during sporogenesis. Two types 1-Amoeboid or invasive or periplasmodial 2-Secretory or glandular tapetum
5. Tapetum is associated with the development of pollen grains because when tapetum is degenerate before microsporogenesis, the pollen grains produced are steril or abortive. 6. Pollen wall contains proteins derived from the gametophytes as well as the tapetal cells. These proteins are responsible for pollen allergy.
STRUCTURE OF POLLEN GRAINS Tube cell, which will eventually become the pollen tube, and a generative cell, which contains the male sperm nuclei needed for fertilization. There are three main components of a pollen grain. The inside of the grain is made up of cytoplasm. This fluid medium houses the aforementioned living cells, keeping them moist and alive. The outer shell is made up of two layers. The inside layer is aptly named the intine (think interior). It is composed partly of cellulose, a common component in the cell walls of plant cells.
The tough outer layer is known as the exine (think exterior). This highly sophisticated and complex outer layer is rich in a compound known as sporopollenin. On the surface of the pollen there is a sticky layer known as the pollen coat or pollen kit, which is a rich source of proteins, lipids, vitamins, phenolic compounds and minerals for the bees. The extent and chemistry of pollenkit varies from species to species and the bees prefer pollen rich in pollen kit.
STAGES OF DEVELOPMENT OF MALE GAMETOPHYTE
Vegetative cell - After pollen grains divide into two cells, the vegetative cell increases in size. The cell organelles increase in number and size. Vegetative cells includes organelles such as ribosomes, E.R., plastid, dictyosomes, and mitochondria. Nucleus is spherical or irregular in outline containing chromosomes and one or two nucleoli. In mature stages nucleus often lacks nucleolus.
Generative cell- (A reproductive cell, especially a cell of an angiosperm pollen grain that divides to produce two male gamete nuclei) Initially the generative cell is attached to intine of the pollen wall but later on it gets detached from the pollen wall. It lies free in the cytoplasm of vegetative cell but there is no cytoplasmic connection between two cells. The generative cell cytoplasm contains the organells Such as mitochondria, ribosomes, endosplasmic reticulum, microtubule, and dictyosomes (In the embryo sac, the double fusion of the generative cells with the egg cell and the two nuclei of the central cell give rise to the diploid zygote and the triploid endosperm, respectively.)
Formation of male gametes- In the anthers of a flower, certain diploid cells undergo meiosis, forming haploid spores (called microspores—"little spores"). These microspores divide by mitosis to form two-celled pollen, consisting of a vegetative cell and a generative cell. Or may be three celled (vegetative + 2 male gamets) at time of release from the anther. When the pollen grain release it landed on the stigma, The pollen grains has two cells (vegetative and generative cell). The generative cell forms two gametes. These are non motile. In angiosperms the female gametophyte (embryo sac) is situated at distance from the stigma and to transport the male gamets pollen tube is form. The pollen tube along with two male gamets grows down through the style and enters the ovule for the process of fertilization.