388 GAMETOPHYTE OF ANGIOSPERMS 



progressed steadily through the various changes outHned above 

 and that consequently in the following lessons we can begin with 

 the most primitive type and proceed by regular steps to the 

 highest forms. The various orders of angiosperms have doubt- 

 less been derived from several distinct stocks and they have 

 not only varied in different degrees but especially will it be noted 

 that some orders have a tendency to emphasize certain forms of 

 these variations, while in other alliances, the variations will 

 proceed along quite different lines. These various modifications 

 have been retained because they were of advantage to the plant 

 (p. 144). The cause of the variations is unknown. Similar 

 lines of variations also appear among the insects, and singularly 

 these modifications of the flower and insect have been mutually 

 beneficial the one to the other. In the simpler forms of flowers 

 the microspores are carried by the wind, as in the gymnosperms. 

 In the higher types odor and nectar glands appear and bright 

 colors which serve to attract insects. This type of flower becomes 

 modified so that it is adapted to special types of insects — all 

 others being excluded. In this way the microspores are carried 

 with greater certainty from one flower to another of the same 

 kind. 



125. The Gametophyte of the Angiospermae. — The angio- 

 sperms have developed along quite distinct lines, but they show 

 such a remarkable uniformity in the development and character 

 of the gametophyte generation that this feature of their life 

 history may be considered at this point, as it applies to all forms. 

 The megasporangium, also called the ovule, originates in the 

 cavity of the ovary at various points known as the placenta (Fig. 

 269, A). The structure of the sporangium and the formation 

 of the megaspore is very similar to that of the gymnosperms. 

 More often two integuments are formed and the sporangium or 

 the stalk which supports it becomes curved so that it very fre- 

 quently is turned completely over (Fig. 269, B). The germina- 

 tion of the megaspore results in a female gametophyte which is 

 very much more reduced than in the case of the gymnosperms. 

 As the megaspore enlarges, disorganizing the cells of the sporan- 

 gium, also called the nucellus, its nucleus divides and the daugh- 



