120 FRESH- WATER BIOLOGY 



of spores from the base of the carpogonium, surrounded by sterile 

 filaments; these together form the cystocarp. 



Just as in the study of the higher plants attention has been 

 turned largely from a purely systematic investigation to a physio- 

 logical study, so among the algae the most important work is done 

 along the line of physiology. The simplicity of their structure, 

 the ease with which many may be cultivated, the readiness with 

 which they respond to and adapt themselves to external condi- 

 tions make them a most valuable group with which to experiment. 

 It would seem that many of the physiological phenomena which 

 in the higher plants are rendered obscure, due to intricacy of 

 structure and complexity of environment, might be made plain in 

 these lower forms which lend themselves so readily to manipula- 

 tion. 



Most valuable results in the physiology of reproduction have 

 already been attained by Klebs who has taken the chief elements 

 in the environment and studied their effect on the organism. As a 

 result he has shown that reproduction, at least in the forms studied, 

 instead of being a phenomenon which, without any determining 

 cause, occurs simply as a stage of growth, is a phenomenon which is 

 dependent upon external conditions; and that as these are altered, 

 the one or the other form of reproduction may be originated, per- 

 fected, or altogether checked, according to the will of the investi- 

 gator. He has shown most conclusively that the sexual form of 

 reproduction does not of necessity alternate with the asexual repro- 

 duction. If the conditions be maintained, it is possible in certain 

 cases to suppress either form of reproduction indefinitely, or if de- 

 sired, to call forth the one to the entire exclusion of the other. An 

 example of this is cited by Klebs in Vaucheria, Protosiphon, and a 

 number of other forms. These facts would go to prove that an alter- 

 nation of the sexual and asexual form of reproduction does not exist 

 in the green algae, though West and others hold that it occurs in 

 a large number of the Chlorophyceae. The sporophyte generation, 

 they believe, is represented by the sexual spore which produces 

 asexual zoospores; each of these in turn, on germinating, ushers in 

 a gametophyte generation. 



In studying the algal flora of any region and the conditions under 



