Tag: modes of reproduction

Siphonogamy is the process of fertilization by means of a pollen tube. This is a characteristic feature of seed plants, which includes gymnosperms, monocots, and dicots.

In sexual reproduction of plants, the sperm is transferred to the egg through a long, polarized tube of a pollen cell called as the pollen tube. The growth of the pollen tube is apical. It grows at its tip at very rapid rate. The rapid growth is due to a highly polarized fusion of apical vesicles. These transport cell wall components to the growing tip via cytoplasmic streaming and are incorporated into a zone of elongation in the apical region of the pollen tube. Hence, the pollen tube extends its length by apical growth in a relatively small region.

After reaching ovary, the pollen tube enters the ovule. When the pollen tube enters the ovule through micropyle, it is called porogamy. It is the most common type. e.g., Lily. 

Apical zone of pollen tube contains abundant vesicles but lacks cell organelles. It is hemisphere in shape and transparent in nature. This transparent apical growing region of pollen tube is called as cap block. It exists only until the tube is growing. It disappears as soon as the growth ceases. 

Callose is a beta-1,3-glucan. It is the major component of the pollen tube cell wall. It is also deposited within the cytoplasm as distinct callose grains. This leads to the formation of callose plugs. Callose plugs separate the living, growing and functional part of the pollen tube in the tip from the remaining degenerating part.

Site of meiosis in Angiosperms is the spore mother cell. 

In plants, meiosis can be observed in sporangia. A microsporangium is the part of plant is suitable for the study of meiosis. A typical anther contains four microsporangia. The microsporangia form sacs or locules in the anther. The two separate locules on each side of an anther may fuse into a single locule. Each microsporangium is lined with a nutritive tissue layer called the tapetum and initially contains diploid pollen mother cells. These undergo meiosis to form haploid spores. The spores may remain attached to each other in a tetrad or separate after meiosis.

The site of meiosis in higher plants is the spore mother cell.  In land plants and some algae, there is an alternation of generations such that meiosis in the diploid sporophyte generation produces haploid spores. These spores multiply by mitosis, developing into the haploid gametophyte generation, which then gives rise to gametes directly. In both animals and plants, the final stage is for the gametes to fuse, restoring the original number of chromosomes.

Meiosis occurs in microspores mother cells which are diploid cells; to produce microspores or pollen grains. Each microspore mother cell produces four microspores after a meiotic division.