Anthophyta: Male

Flower Structure (review)

The flower is basically a strobilus. Here is a generic flower:

The flower above is bisexual (perfect, hermaphroditic) and the plant is monoecious. Of course not all flowers are this way; if flowers are unisexual (imperfect, pistillate/staminate), the plant can have flowers of both sexes on the same plant = monoecious or it can have flowers that are either pistillate or staminate = dioecious.

For today we focus on the stamen. Whether the stamen is a leaf on a short shoot or is a short shoot on its own is somewhat controversial among botanists. For now we will treat the stamen as fundamentally a microsporophyll.

This leaf (microsporophyll) of course will bear sporangia, namely microsporangia. These are the anther sacs. Inside the anther early in development will be microsporocytes. These diploid cells inside the sterile jacket will be dividing by meiosis to produce haploid microspores. We went through a protracted review of the process of meiosis, to dust off some rust.

The microspores are not shed from the sterile jacket. Rather they are held until mitotic division has produced an endosporic microgametophyte within the microspore wall. This is, of course, the pollen grain. The pollen grain is shed to the pollination vector (wind, animal, etc.) for delivery to the stigma. Below is a diagram some of the development of the stamen and pollen for which this color code key is needed:

Color Key

As you can see above, the microgametophyte is two-celled. The reduction of the microgametophyte is at its extreme. However the path which it must follow to reach the egg is extended! While the microgametophyte is reduced in evolution, the layers of coverage of the egg have been increased. In ferns, the gametophytes could be bisexual or at least growing side-by-side. In Selaginella they were produced by separate strobili but the gametophytes still grew side-by-side and sperm had to swim to the egg. In gymnosperms the megagametophyte was endosporangial and the microgametophyte was endosporic. The microgametophyte (pollen) was deposited in the micropyle. This meant that the pollen tube had to digest a path through the megasporangium (nucellus) but no further. Here, in the angiosperms, we are dealing with hidden ovules: they are folded inside a carpel. The pollen grain has no means to get itself in the micropyle. The tube cell must digest its way from stigma, through style, into the locule of the ovary, find the micropyle, and penetrate to the synergids of the embryo sac. This is a much greater distance to digest for the tiny tube cell to penetrate!

The tube cell is the larger of the two in the microgametophyte; note this is another example of unequal cytokinesis. Upon landing on the stigma of the flower, the tube cell cracks open the microspore (pollen) wall and begins to grow into the tissues of the stigma. In many species the stigma produces a sugar-laden liquid for the germination and growth of the pollen tube. The tube orients its growth toward the ovule. Precisely how this is accomplished is not clear. Pollen tubes grow toward sources of Ca2+, B+1, and IAA (Indole-Acetic-Acid). It may be a gradient of these elements or molecules exists in the style and that the pollen tube grows toward the chemical signal (chemotropism). By whatever means does occur, the pollen tube arrives at the micropyle.

The generative cell divides mitotically to produce two sperm cells. These two cells may be produced while the generative cell is inside the microspore wall (trinucleate pollen), or they may be produced as the generative cell follows the tube cell down the pathway to the micropyle. In any case, the two sperm consist of nucleus and a tiny bit of cytoplasm but no means for motility. Lacking any motile stage in the life cycle, it is no surprise that angiosperms lack centrioles! Both of these sperm will participate in a syngamy event; syngamy is double in angiosperms.

As we shall see in female development, one sperm will join with the egg to make the zygote. The other sperm will join with the two polar nuclei in the central cell to form the endosperm cell. The polyploid endosperm is a unique apomorphism (an autapomorphism!) in angiosperms; there is nothing genetically like it in lower plants. Functionally it will nourish the sporophyte embryo, a function plesiomorphically accomplished by the megagametophyte.