50 MISSOURI BOTANICAL GARDEN 





and while his data as to the time of liberating the sexual prod- 

 ucts do not exactly coincide with those of Williams, the gen- 

 eral features of the cycle are the same and he too was able to 

 demonstrate a periodicity in aquarium plants that agreed 

 closely with that shown under natural conditions, even 

 though the gametangia were born on parts that had devel- 

 oped entirely in the laboratory and had consequently never 

 been subjected to the usual periodic variations in the environ- 

 ment. He also obtained some evidence to the effect that 

 plants growing in seas where there are almost no tides like- 

 wise show a similar periodicity but, as the author remarks, 

 the evidence on this point is perhaps not sufficient. In short 

 the results of both these investigators indicate that in Dic- 

 tyota dichotoma a periodicity in the production and libera- 

 tion of the sexual products, while seemingly influenced to a 

 marked degree by environmental fluctuations, is primarily 

 due to an inherent periodic tendency on the part of the plant 



itself. 



Whether Spirogyra is similar to Dictyota in respect to its 

 periodicity has not been definitely settled. Fritsch and Rich 



07) made 



came 



probably 



presum 



ent for the different species. These observers point out that 

 the same species in the same body of water conjugates at 

 varying times in different seasons and that different species 

 under similar conditions do not behave in like manner 

 during the same season. Brown ('08) likewise made nu- 



observations on Spirogyra and othe 

 I was led to conclude that these algae 



become 



adverse. Spirogyra conjugates when conditions become 

 "hard enough." Copeland ('09), on the other hand, kept 

 a large number of cultures in aquaria and supplemented a 

 study of them 



with observations on Spirogyra in nature. His 

 results indicate that Spirogyra has definite periods of growth 



n«l ho finds "overwhelming evidence. . . .that 



