REPRODUCTION IN PLANTS 269 



grains that form within the anthers (the male gametophytes), and a 

 seven-celled structure (the female gametophyte) that develops inside the 

 ovule enclosed in the ovary. So reduced is the gametophyte generation 

 that it might never have been recognized for what it is without the clues 

 furnished by the lower plant groups. It has lost all importance save for 

 its essential role in reproduction. 



The stamens and pistils are the spore-bearing organs of the sporophyte 

 generation. They do not themselves produce male and female gametes. The 

 stamens produce asexual microspores, from which develop the male 

 gametophytes ; the pistils produce asexual megaspores, from which develop 

 the female gametophytes. 



Microspores and pollen grains. The anther that forms the apical 

 end of each microsporophyll or stamen is in reality a spore case, though it 

 is common to speak of it as the "male organ" of a flower. Within the 

 anther a large number of spore mother cells is formed. Each of these 

 undergoes a maturation process that results in the production of four 

 haploid microspores. If our plant were like a fern, each of these could 

 germinate and by cell division produce a small green gametophyte, which 

 would in turn produce haploid male gametes. In the flowering plants 

 this does not happen, but something comparable does. 



Each asexual microspore "germinates" within its own spore case, by 

 nuclear divisions unaccompanied by any cytoplasmic division. The first 

 nuclear division produces two nuclei, one called the tube nucleus, which 

 may be regarded as somatic, and the other called the generative nucleus, 

 which may be regarded as a germ cell nucleus. The generative nucleus 

 then proceeds to divide once more, producing two nuclei which function 

 as male gametes and which are called the sperm nuclei. All this takes place 

 within the original microspore case, 1 so that no external difference in 

 appearance is visible after the nuclear divisions. Yet at this stage the 

 whole structure has in effect become a three-celled male gametophyte, 

 consisting of one vegetative (somatic) cell and two germ cells, although 

 the cells are not cytoplasmically divided. This tiny three-celled gameto- 

 phyte is the mature pollen grain. All of its nuclei are, of course, haploid. 



Megaspores and the female gametophyte. The megasporophylls 

 that produce the megaspores are called carpels, and the pistil is a struc- 

 ture consisting of a single carpel or of two or more fused carpels. At the 

 base of the pistil is an enlargement called the ovary, within which each 

 carpel produces one or several ovoid spore-cases called ovules. Each ovule 

 is a thick-walled structure composed of several layers of cells and contains 

 a single large diploid spore mother cell. By a maturation process not unlike 

 that of the metazoan egg, this cell gives rise to four haploid cells; one of 



1 Sometimes the division of the generative nucleus does not occur until after forma- 

 tion of the pollen tube. 



