Pollen grains are used to initiate the development of the male gametophyte while it is still present in the microsporangium or pollen sac (precocious germination). The microspore divides only twice during its life cycle. During the first mitotic division, a vegetative cell is formed, followed by the formation of a generative cell.

Pre Pollination Development stages.

1. The size of the pollen grain nucleus increases in size and then shifts to one side, close to the wall.
2. The protoplast divides into two unequal cells – the small generative cell and the large tube cell or vegetative cell.
3. By developing a layer of callose on the pollen grain wall, the generation cell is separated from the pollen grain wall.
4. The dissolution of callose allows naked generative cells to freely move around in the cytoplasm of the tube cell in the following stages. The generative cell has an outline that ranges from lenticular to spherical.
5. Before the dehiscence of the anther and the release of the pollen grains, the generative cell in some species divides into two male gametes. As a result, at the time of pollination, the pollen grain is either two-celled or three celled depending on the species. There are two types of cells in two-celled pollen grains: a tube cell and a generative cell. One tube cell and two male gametes are present in each of the three-celled pollen grains.
6. Pollen grains are referred to as monosiphonous when each pollen grain produces a single pollen tube, as is the case in monocots.
7. Pollen grains are referred to as polysiphonous when each pollen grain produces more than one pollen tube, which is typically found in dicots and other similar plants. Examples include Malva, Althaea (Malvaceae), and members of the Cucurbitaceae family.

Tube Cell or Vegetative Cell

A tube cell is another name for a vegetative cell. It is larger than the reproductive cell, contains abundant food reserves, and has a large irregularly shaped nucleus. These cells do not have a cell wall and are therefore represented solely by their cell membranes.

In the space between the two cells, a temporary callose wall is constructed. To finally pinch it off, the callose wall spreads between generative cells and intine. Within a short period, this callose wall dissolves, and the generative cell can move freely within the cytoplasm of the vegetative cell

Lenticular or spindle-shaped generating cells are possible shapes for generating cells. The cytoplasm of generative cells is almost completely devoid of stored food material and is almost completely devoid of water. The fat, starch, and protein granules found in the larger vegetative cell are all stored in the smaller vegetative cell.

It is usually during this two-celled stage that pollen grains are released from pollen sacs in the anther lobes of the flowering plants. Occasionally, generative cells will divide further to produce two male gametes. Following pollination, the stigma undergoes additional stages in the development of the pollen tube and other structures. Pre-pollination stages include all of the stages in the development of the male gametophyte and are grouped.

Stages of Development

Post-pollination changes occur as a result of pollen grains falling from the stigma. The pollen grain absorbs water and nutrients from the stigmatic secretion through its germ pores, which are located on the surface of the grain. The intine protrudes out of one of the germ pores or through a germ furrow, depending on which is present.

The generative nucleus divides into two male nuclei, each of which is surrounded by cytoplasmic masses and appears as two distinct male gametes as a result of this division. Forming male gametes can take place in vegetative cells or pollen tubes, among other places.

A typical scenario is for the generative cell to descend into pollen tubes and then divide to produce male gametes. The pollen tube’s hinder region has a high concentration of vacuolated air. The development of callose separated this region from the anterior portion. Angiosperms have a significantly reduced number of male gametophytes.

Asexual reproduction in flowering plants occurs through a very simple system when compared to the sporophyte’s development system. Even though it appears to contain all of the genetic information necessary to grow a normal plant, all it does is send out a tube that grows in length for several millimetres. This growth causes the generative nucleus in the tube, which is already in the late interphase or early prophase of mitosis, to undergo a division during the process of development. Many species’ pollen has already experienced this division in the pollen grain before anthesis, indicating that this division has already occurred. It appears that the only synthetic processes taking place in the pollen tube are those involving the synthesis of tube wall polysaccharides and the synthesis of membranes, both of which are unlikely. All other normal sporophyte developmental processes are suppressed as a result of this condition.

Conclusion

The theory of variable or differential gene activity is currently considered to be the most likely explanation for cellular differentiation, according to current thinking. Molecular relationships between the structure of the various proteins found in the cell and the structure of the chromosomal DNA are now well understood. The characteristics of a particular cell type are determined by how these proteins perform their functions. As a result, the patterns of RNA and protein synthesis must be examined to find answers to questions about the development of the pollen tube.