Brown : Algal periodicity 233 



became rather abundant during November and December, then, 



increasing by cell 



j 



During February and March, the plant began to look somewhat 

 unhealthy and was only rather abundant. During April and the 

 first part of May, it again became healthy, grew rapidly, and be- 

 came very abundant. Later in May, it began to be rare again. 

 Following this, it became very rare and disappeared in June. In 

 pond no. 3, this form was found during the greater part of the 

 year but never in abundance. There was some fluctuation in 

 amount, becoming rather abundant in November, then rare (figure 



1 — chart, pond no. 3). 



In the upper part of stream no. I, C/osterium accrositm was 

 noticed September 24. It was rather abundant, growing among 

 filaments of Spirogyra. It became less plentiful during October 

 and disappeared during November. In every case £ accrositm 

 was found growing with Spirogyra, either among the filaments or 

 on the bottom. Just what caused the fluctuation in the amount of 

 the plant, is not easy to explain. It was probably not due to tem- 

 perature, for in one instance when most abundant, in January, the 

 temperature was low, 5 C. and in another, in April, it was rather 

 high, i8°C. ; it became rare in February under a low tempera- 

 ture, i° C, and rare in May under a temperature of 26 C. It 

 is doubtful if the animal life of the pond had any bearing on the 

 matter. There were few small animals in the pond that would 

 likely feed on the desmids. The periods of greatest abundance 

 seemed to follow seasons when the water was lower and more im- 

 pure. This doubtless had some influence on the growth, for in 

 some experiments, to be mentioned later, it was found that if the 

 liquid in which the desmids were growing became more dense, 



cell-division would be more frequent. 



A few specimens of Clostcrium Ehrcnbcrgii were obtained from 

 pond no. I during November and also during May. On May 14 

 a good quantity of this species was found growing among filaments 

 of Cladophora in a large glass jar in the University greenhouse. 

 This material was so rich in desmids that thousands of them could 

 be obtained free from impurities by rinsing a small mass of the 

 Cladophora in a small vessel of water a moment or two. After 

 obtaining the desmids in this way, they were placed in a large 



