Angiosperm Reproduction and Microsporangium Anatomy
Reproductive Life cycle of Angiosperms
The predominant plant body in all angiosperms is the diploid sporophyte. This stage of the life cycle is responsible for undergoing meiosis to produce haploid spores. These haploid spores subsequently form the gametophyte, which is significantly reduced in size and complexity compared to the sporophyte. The gametophytes develop entirely within the structure of the flower and are the specialized structures that produce gametes.
Anatomy and Morphology of the Androecium
The male reproductive whorl of a flower is known as the androecium, and its individual members are called stamens. Each stamen is composed of three primary parts: the filament, the connective, and the anther. The anther is usually characterized by having two lobes, a condition described as dithecous. Because each of these two lobes contains two pollen sacs, the anther is typically tetrasporongiate, meaning it contains a total of four pollen sacs.
During the immature stage of development, the anther is a group of parenchymatous tissue surrounded by a single-layered epidermis. The process of differentiation, or heterogeneity, begins when specific hypodermal cells transform into archesporial cells.
Tissue Organization and T.S. of Anther
The internal structure of the anther, as seen in a transverse section (T.S.), reveals a specific arrangement of tissue layers originating from the archesporial cell. The archesporial cell divides into an inner sporogenous cell and an outer primary parietal cell. The sporogenous cell eventually forms the sporogenous tissue, where every individual cell is capable of giving rise to a microspore tetrad. The primary parietal cell undergoes further divisions to form the distinct wall layers of the mature anther.
The wall of a mature anther consists of four distinct layers:
- Epidermis: This is the outermost Protective layer and is composed of tabular or flattened cells.
- Endothecium: Located directly beneath the epidermis, this sub-epidermal layer consists of radially elongated cells characterized by fibrous thickenings.
- Middle Layers: These are located inner to the endothecium and consist of to layers of thin-walled cells. These layers may disintegrate as the anther reaches maturity.
- Tapetum: This is the innermost layer of the anther wall and serves a nutritive function. It immediately encloses the sporogenous tissue, which contains the microspore mother cells.
Microsporogenesis and Pollen Grain Structure
Microsporogenesis is the process by which microspores are formed. Each microspore mother cell undergoes meiotic division to produce a tetrad consisting of four haploid microspores, which are also known as pollen grains. A typical pollen grain is a unicellular, non-motile, haploid body that contains a single nucleus.
The pollen grain is protected by a two-layered wall called the sporoderm. The outer layer, known as the exine, is thick and composed of a complex, non-biodegradable substance called sporopollenin. Sporopollenin is highly resistant to chemicals, which allows pollen grains to remain well-preserved as fossils over long periods. The exine can be smooth or exhibit sculptured patterns specific to the species. At certain points, the exine is very thin; these areas are known as germ-pores and are the sites where the pollen tube emerges during germination. The inner layer of the wall is the intine, which is composed of cellulose and pectin.
Gynoecium and Female Reproductive Structures
The female reproductive whorl of a flower is the gynoecium, also commonly referred to as the pistil. The individual members of the gynoecium are called carpels or megasporophylls. In some flowers, there may be many carpels that remain free from one another, a condition described as apocarpous.