Development of the Male Gametophyte

Once the anther's microsporangia have differentiated their four-layered wall and the central mass

of sporogenous tissue, the actual production of pollen -- microsporogenesis, followed by pollen

grain maturation -- proceeds through a precisely staged sequence that converts diploid tissue into

thousands of individual, haploid, gamete-bearing pollen grains.

The process begins when cells of the sporogenous tissue differentiate as microspore mother cells

(pollen mother cells, or PMCs). Each PMC is diploid (2n), with a large nucleus and dense cytoplasm,

and is competent to undergo meiosis. Because meiosis is a reductional division, each PMC that enters

meiosis produces, at its conclusion, four haploid (n) cells. These four cells remain grouped

together immediately after meiosis, forming what is called a microspore tetrad -- typically the

four cells are arranged tetrahedrally, though other arrangements occur.

As the anther continues to mature, the microspores that make up each tetrad separate from one

another, becoming free, individual microspores dispersed within the microsporangium (with the help

of an enzyme, callase, that dissolves the wall holding the tetrad together). Each of these

individual microspores now begins to differentiate into a pollen grain. The microspore first

develops its characteristic two-layered wall (the resistant, sporopollenin-based exine on the

outside and the thinner, cellulose-and-pectin-based intine on the inside, described further in the

following sections), and its single nucleus divides once, by mitosis. This division is markedly

unequal: it produces one large cell, the vegetative cell (or tube cell), with abundant food

reserves and a diffuse nucleus, and one much smaller cell, the generative cell, with dense

cytoplasm and a compact nucleus. At this two-celled stage, the pollen grain represents the immature

male gametophyte, and in many species pollen is shed from the anther in this condition.

The generative cell subsequently divides once more, by mitosis, to produce two non-motile male

gametes. In roughly a third of flowering plant species this second division happens before the

pollen is shed, so that a three-celled pollen grain (one vegetative cell plus two male gametes) is

released from the anther; in the remaining majority, the generative cell divides only later, after

the pollen grain has landed on a receptive stigma and its tube has begun to grow, so that the two

male gametes are formed inside the growing pollen tube itself. Either way, it is these two

non-motile male gametes -- and only these -- that will eventually be delivered, via the pollen

tube, to the embryo sac, where one will fuse with the egg cell and the other with the polar nuclei

of the central cell in the process of double fertilisation described later in this chapter. …