CYTOLOGY OF REPRODUCTION IN ANGIOSPERMS 137 



The many families of angiosperms show almost innumerable variations 

 in floral type. Such characteristics as the size, arrangement, and struc- 

 ture of parts often determine the suitability of a species for cytological 

 oi- cytogenetical investigation. Of special importance is the type of sex 

 differentiation present. Some angiosperms, such as willows, the red 

 campion, and the date palm, are heterophytic (dioecious), the flowers 

 on one plant having only pistils or only stamens. Others are homophytic : 

 of these, some are monoecious, having pistillate flowers and staminate 

 flowers on the same plant, as in maize, while others are hermaphroditic, 

 having pistils and stamens in the same flower, as in buttercups, tulips, 

 and apple trees. Both unisexual and bisexual flowers ma}^ be borne by 

 the same plant (some maples), and various other arrangements are known. 

 In some flowers bearing both pistils and stamens the two maj^ not be 

 functional at the same time, so that cross-pollination is favored or made 

 necessary even though the plant is structurally bisexual. Furthermore, 

 some bisexual plants are self-sterile, producing seeds only after cross- 

 pollination. The mode of pollination, i.e., whether b}^ \^•ind or by insects, 

 is often a matter of importance in designing experimental procedures. 



Microsporogenesis and the Male Gametophyte (Figs. 101, 103). — 

 The anther commonlj" differentiates internally into three regions: an 

 outer wall consisting of several layers of cells, a nutritive tapetum of 

 one layer, and a central mass of sporogenous cells. The sporogenous 

 cells eventually enlarge as microsporocytes, round up from one another, 

 and lie in a fluid filling the enlarged anther. Each microsporocyte then 

 divides into four microspores. The two nuclear divisions here are 

 meiotic, each nucleus of the resulting quartet containing the reduced 

 chromosome number. In some plants, cytokinesis occurs after division / 

 and again after division II, but in most species it does not take place 

 until after //, the spherical cell then dividing simultaneously into four 

 tetrahedral spores (quadripartition) . The shape of the spores in the 

 quartet often reveals which mode of cytokinesis has occurred. The walls 

 of the microspores become greatly thickened, the characteristic patterns 

 formed often being useful in the identification of species. In many 

 plants this wall thickening involves the activity of the tapetum. This 

 tissue sometimes breaks down into a tapetal Plasmodium which flows 

 in among the young spores and deposits materials upon them. The wall 

 consists typically of two distinct layers: the thickened exine and AAathin 

 this an inline. 



The male gametophyte of angiosperms is structurally very simple. 

 Its development begins mth the division of the microspore into a small 

 generative cell and a large tube cell. The generative cell may lie against 

 the spore wall at one side, or it may be completely enclosed by the 

 cytoplasm of the tube cell. The generative cell divides into two male 



