TEE ORIGIN OF SEX IN PLANTS. 75 



understanding how such results are accomplished in nature or what 

 other factors may be concerned. 



It is certainly plain that the conditions surrounding sexual processes 

 are immensely complex, and as yet we only know them in part and for a 

 very few organisms. There is every reason to expect that investigation 

 will so add to these that the subject will consist of very complicated 

 problems in physics and chemistry. But it is something to know that 

 important factors exist outside of the organism controlling in great 

 part the sexual phase, and that some of them are so simple as light, 

 temperature and osmotic pressure. Much is gained for biology in the 

 understanding that sexual elements have arisen from asexual repro- 

 ductive cells under the stress of environmental influences; that sex- 

 uality is not inherent in life although presented in almost all higher 

 organisms, and that, however complicated the extreme conditions may 

 be, they have arisen through a process of gradual evolution. 



In another paper I shall hope to show the steps by which the highly 

 differentiated egg and sperm in various groups of plants developed from 

 the similar gametes presented at the dawning of sex. As stated in the 

 beginning of this paper, the topic is a chapter in itself and well deserves 

 separate treatment. 



