POLEN

Polenin ve Polen tüpü

Polen development:

The microsporophyll has been reduced to the stamen. The microsporangium is the tapetum of the anther sac. The microsporocytes are numerous and undergo meiosis early. The tetrad of microspores from each microcsporocyte separate from each other and divide inside the microspore wall by mitosis. Thus the microgametophyte is endosporic and is called a pollen grain. The microgametophyte has been reduced from a complete thallus with antheridium as in the mosses, to just an antheridium as in Selaginella, to four cells as in pine, and now is reduced to just two functional cells. In angiosperms the pollen grain now depends upon a vector to get it close to the female gametophyte. Thanks to angiospermy, the pollen can only get as close as the stigma of the megasporophyll. From there the tube cell must digest a path to the egg. The tube cell depends upon the maternal sporophyte to nourish it as is grows from stigma to egg to deliver the sperm cells. The gametophyte has gone from autotrophic, to heterotrophic, to parasitic. The sperm have lost their motility completely. The pathway from deposition by the pollinator to the egg is long. However, as we have seen, the generative cell produces two sperm and both participate in syngamy (one with the egg and the other with the central cell). Reducing the gametophyte stages, where recessive traits would be expressed to the detriment of the species, to just a few cells with the genome mostly shut down as heterochromatin is quite adaptive. Getting it away from the need for external or internal water is adaptive.

In the pollen sac (microsporangium) of the anther, haploid microspores (A) are formed by meiosis. Mitosis then follows to produce a two-celled pollen grain (B) with a small generative cell and a large vegetative cell. This generative cell will undergo further mitosis to form two male gametes (nuclei). The pollen tube grows though a pore in the pollen grain, with the tube (vegetative) nucleus at its tip and the male nuclei behind.