Jul 24, 2010

Development of angiosperm gametophytes and the process of pollination

Within the microsporangia (pollen sacs) of an anther are many diploid cells called microsporocytes, also known as microspore mother cells.

Each microsporocyte undergoes meiosis, forming four haploid microspores, each of which can eventually give rise to a haploid male gametophyte.

A microspore undergoes mitosis and cytokinesis, producing two separate cells called the generative cell and tube cell. Together, these two cells and the spore wall constitute a pollen grain, which at this stage of its development is an immature male gametophyte. The spore wall usually exhibits an elaborate pattern unique to the particular plant species. During maturation of the male gametophyte, the generative cell passes into the tube cell. The tube cell now has a completely free–standing cell inside it (the generative cell). The tube cell produces the pollen tube, a structure essential for sperm delivery to the egg. During elongation of the pollen tube, the generative cell usually divides and produces two sperm cells, which remain inside the tube cell. The pollen tube grows through the long style of the carpel and into the ovary, where it then releases the sperm cells in the vicinity of an embryo sac.

One or more ovules, each containing a megasporangium, form within the chambers of the ovary. One cell in the megasporangium of each ovule, the megasporocyte (or megaspore mother cell), grows and then goes through meiosis, producing four haploid megaspores.

The details of the next steps vary extensively, depending on the species. In most angiosperm species, only one megaspore survives. This megaspore continues to grow, and its nucleus divides by mitosis three times without cytokinesis, resulting in one large cell with eight haploid nuclei. Membranes then partition this mass into a multicellular female gametophyte—the embryo sac. At one end of the embryo sac are three cells: the egg cell and two cells called synergids. The synergids flank the egg cell and function in the attraction and guidance of the pollen tube to the embryo sac. At the opposite end of the embryo sac are three antipodal cells of unknown function. The remaining two nuclei, called polar nuclei, are not partitioned into separate cells but instead share the cytoplasm of the large central cell of the embryo sac. The ovule, which will eventually become a seed, now consists of the embryo sac and two surrounding integuments (layers of protective sporophytic tissue that eventually develop into the seed coat).

Pollination, the transfer of pollen from anther to stigma, is the first step in a chain of events that can lead to fertilisation. This step is accomplished in various ways. In some angiosperms, including grasses and many trees, wind is a pollinating agent. In such plants, the release of enormous quantities of pollen compensates for the randomness of this dispersal mechanism. At certain times of the year, the air is loaded with pollen grains, as anyone plagued with pollen allergies can attest. Some aquatic plants rely on water to disperse pollen. Most angiosperms, however, depend on insects, birds, or other animals to transfer pollen directly to other flowers.