In flowering plants, flowers have taken the place of conventional sex organs, and the ovules which later on develop into seeds are enclosed within an ovary. The sexual reproduction in angiosperms involves the fusion of two gametes, the male and the female. The male gametes are produced in the pollen grains and the female gametes are produced in the ovule. The fusion of the two gametes results in the formation of a zygote. The zygote develops into an embryo within the seed. This embryo, when the seed germinates grows into a new plant.
The flower is concerned with the function of sexual reproduction in angiosperms.
Pollen grains are produced in the anther. A very young anther comprises a mass of undifferentiated thin - walled cells bounded by an epidermis. The anther becomes four lobed as the anther grows, and the lobes are joined by a sterile tissue called connective. Each lobe contains an elongated chamber which is termed as microsporangium or pollen sac.
(A) T.S. of young anther showing four microsporangia and a vascular stand; (B) Detailed structure of one microsporangium; (C) T.S. of mature antherA cross section of a young anther reveals that the microsporangium is surrounded on the outside by a single layered epidermis. Beneath the epidermis is a layer of cells called the endothecium. The sporangial wall encloses a mass of cells characterised by their large size abundant cytoplasm and prominent nuclei. These cells are called sporogenous cells. These sporogenous cells undergo a few mitotic divisions to increase their number and form the microspore mother cells or microsporocytes. The microsporocytes are diploid and usually closely packed. Each diploid microsporocyte undergoes meiosis, giving rise to a group of four haploid cells, known as microspores or pollen grains. Such a group of 4 spores is known as a tetrad.
(A-C), Pollen grains or microspores.(A-B), Entire pollen grains;C, Pollen grain in section
At maturity, the partition wall between two adjacent microsporangia breaks down and the cavities of the two microsporangia merge into one pollen sac. When the pollen grains are fully formed, the ripe anther splits to liberate the pollen.
At one or more places, the exine is very thin or absent. These spots are called germ pores and are the points from which the pollen tubes emerge during pollen germination.
Development of embryo sac and female gamete (in an anatropous ovule)A hypodermal cell of the nucellus enlarges and becomes differentiated into a megaspore mother cell or megasporocyte. This diploid megaspore mother cell increases in size and undergoes meiosis to form a linear tetrad of 4 haploid megaspores, 3 of which degenerate and the 4th becomes the functional megaspore.
(A I), (A - D), stages in the development of an ovule and functional megaspore;E I, stages in the development of an embryo sac from the functional megasporeThree nuclei are towards the micropylar end, and form the egg apparatus with a central egg cell surrounded by 2 synergids. Three nuclei are at the chalazal end and form the antipodals. One nucleus from each pole moves to the central position of the embryo sac and are called as polar nuclei. These nuclei may fuse together and form a diploid secondary nucleus. A fully developed embryo sac with the nucellus, integuments and funiculus, together forms the structure called the mature ovule.