THE EVOLUTION OF SEX IN PLANTS. 301 



This is important, for it shows that sex did not arise at one period only 

 and at a certain level of the plant kingdom, but, on the contrary, at a 

 number of totally distinct times and in connection with as many 

 independent lines of ascent. We can see no reason why zoospores 

 might not at any time take on the attributes of sex, for the latter 

 conditions are controlled and probably in large measure developed 

 by the chemical and physical environment of the plant. Although 

 sex has arisen a great many times in the plant kingdom and in groups 

 independent of one another, the steps in the process and the structure 

 of the primitive gametes are essentially the same in all cases. There 

 are good reasons for believing that even now certain groups of the 

 algae are developing sexuality, and that this process may be expected 

 to continue wherever the lower algae have the habit of reproducing by 

 zoospores. 



With the origin of sex in any group of plants there is immediately 

 presented the possibility of such further evolution as will give the 

 differentiated and highly specialized sexual cells, the eggs and sperms. 

 This evolutionary history is briefly expressed as the development of 

 heterogamy from isogamy or the oosporic type of reproduction from 

 the zygosporic. 



Isogamy is the term applied to conditions in which the sexual cells 

 are similar in form, morphologically identical. Heterogamy is the 

 condition in which the female gamete is a motionless cell without 

 cilia and the male gamete generally a ciliated sperm frequently highly 

 specialized in form. Our problem is to understand the steps in the 

 evolution of heterogamy from isogamy and, as far as possible, the 

 factors influential in the process. 



Isogamy is a condition very generally distributed among the 

 various groups of algae. We may find it in almost all lines of ascent 

 above the unicellular forms and it is not uncommon among these. 

 Heterogamy always appears at higher levels of development than iso- 

 gamy and in connection with general advances in vegetative complexity. 



It customary to speak of the forms expressing the highest develop- 

 ment of evolutionary lines as climax types. Climax types among the 

 algae are almost all heterogamous. There are a few lines in which 

 this level of sexual evolution has not been attained, as for example, 

 the pond scums ( Conjugates), the Hydrodictyaceae, Ulvaceae and 

 several smaller groups of the lower algae. On the other hand, there 

 are several heterogamous types standing quite by themselves as the 

 final expression of certain lines of evolution that we can only con- 

 jecture because the lower representatives have become extinct. Notable 

 illustrations of this character are the stoneworts (Charales) and the 

 Oedogoniaceae. 



