70 POPULAR SCIENCE MONTHLY. 



hut finally gathering in small groups and sorting themselves in pairs. 

 The elements in such a pair begin to fuse together; the process is 

 called conjugation and represents the simplest form of sexuality. The 

 two sexual cells are called gametes, but they are nothing more than 

 zoospores so constituted that they must fuse with one another in order 

 to live. 



The gametes show their relationship to zoospores in various ways 

 and there is no doubt that they arose from the latter. In the first place 

 they have the same general structure and are developed in the same 

 sorts of cells on the mother plant. But the most important evidence 

 of affinity is exhibited by certain gametes that are so much like zoospores 

 that they will sometimes settle down and germinate without conjuga- 

 tion. Tliis means that their sexual characters are not strongly enough 

 developed to overcome the vegetative tendencies of their parents the 

 asexual zoospores. However, the sporelings that come from these 

 abortive or perhaps parthenogenetic gametes are weaker than the prod- 

 ucts of the ordinary or normal zoosopores and sometimes never reach 

 full development. As may be guessed, this curious intermediate condi- 

 tion between the zoospore and gamete furnishes a most important clue 

 to the fundamental distinctions that separate the one from the other. 

 These differences are evidently physiological rather than morphological 

 in character. 



It is only recently that botanists have in part understood and at- 

 tempted to define precisely the conditions that determine the develop- 

 ment on the one hand of zoospores and on the other of gametes. In a 

 general way it has been believed for a long time that the problem 

 was a physiological one and that various enviromnental conditions of 

 season, temperature or light were responsible for the results. But in 

 the past ten years there have been numerous studies, on various types 

 of the lower plants, attempting to establish as exactly as possible the 

 chemical and physical factors at work. In this field of research the 

 botanist, Klebs, has been especially active, and he, above all others, 

 deserves the credit of developing certain experimental methods of at- 

 tack. These have yielded important results and justify the belief that 

 we may in the future obtain much precise knowledge. 



Klebs treats the forms to as many well-defined conditions as he 

 can devise, various as to the food, the osmotic properties of the water, 

 the light and the temperature. The results have been very remark- 

 able considering the difficulties of the problems. We can not do better 

 than to follow his studies on one or two forms to illustrate the possi- 

 bilities of investigations in this difficult field. 



His studies on TJlothrix are interesting. This is a lowly type of 

 unbranched filamentous alga common in both fresh and salt water. 

 The zoospores (Figure 2,h) are formed in varying numbers, but usually 



