Tag: sexual reproduction in flowering plants

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.