110 OUTLINES OF EVOLUTIONARY BIOLOGY 



embryo sac at first represent the female gametophyte, which dis- 

 appears as the zygote develops into the embryo of another 

 sporophyte. In this way a very intimate relation is established 

 between each sporophyte generation and the one which precedes 

 it, and the gametophyte is crushed out of existence, as an inde- 

 pendent generation, between the two. 



Accompany ing this almost total suppression of the gametophyte 

 we find a delegation of certain responsibilities connected with the 

 sexual function to the asexual sporophyte, and the development 

 by the latter of what may be termed vicarious sexual characters. 



These characters find 

 their expression in that 

 most remarkable feature 

 of all the flowering 

 plants, the flower itself. 

 Thus the use of the 

 terms male and female 

 may be extended in the 

 first instance to the sta- 

 mens and pistil, though 

 these are really merely 

 the spore-bearing leaves 

 of the asexual genera- 

 tion. Similarly the 

 transference of the pol- 

 len grains from stamens 



A. ripe seed split in half; B. seed germinating. ri -\ 



cot., cotyledons or seed leaves ; pi., plumule or shoot of to stigma IS often spoken 



embryo; rad., radicle or root of embryo; test., Q f ag faQ fertilization of 



testa or seed coat. 



the flower, though it is 



obviously not the true process of fertilization but only a necessary 

 preliminary to the conjugation of the gametes, and is therefore 

 more accurately spoken of as pollination. 



It will have been observed from the foregoing description that 

 not only is the male gametophyte of the flowering plant reduced 

 almost to the point of disappearance, but the male gamete itself 

 has apparently suffered great degeneration. It is no longer an 

 active spermatozoon, swimming about by means of flagella or 

 cilia, as in Eudorina or in the ferns, but a shapeless nucleated 

 mass of protoplasm which has at the most a sort of amoeboid 

 power of locomotion. By far the greater part of the travelling 

 which it has to accomplish in order to reach the ovum is 



rad. 



FIG. 56. Garden Pea (Pimm sativum}. 



