330 Plant Life and Evolution 



and the clearly differentiated eggs and sperms of 

 the lower Metazoa there are no intermediate stages. 

 With the increasing individuality shown by the 

 higher animal types, the sexual elements become 

 more and more sharply segregated from the somatic 

 tissues, and the direct derivation cf the generative 

 tissues of one generation from those of the pre- 

 ceding one looks very plausible, and the theory of 

 a definite germ plasm handed on from one genera- 

 tion to another has a basis of actual observation. 



Sex has Arisen Repeatedly in Plants. Sex in 

 plants has evidently arisen over and over again. 

 In some half a dozen quite unrelated groups among 

 living plants, every stage of development of the 

 gametes can still be found, from nearly or quite 

 similar isogametes, to perfectly differentiated eggs 

 and sperms. It is also clear that the simpler types 

 of gametes are derived from non-sexual zoospores, 

 or from non-sexual individuals in the case of uni- 

 cellular plants. The slight difference between the 

 sexual and non-sexual cells is shown by the ease 

 with which some of the lower plants may be forced 

 to produce one or the other as the result of differ- 

 ent stimuli. Klebs' experiments upon various algse 

 and fungi are especially striking as showing how 

 readily the character of the reproductive cells may 

 be controlled. 



While the mechanism of fertilization may be very 

 much the same in plants and animals, the results 

 are usually very different. The fusion of the egg- 



