PERIODICITY IN SPIROGYRA. 57 



in one of these there were slight indications of tubes. But 

 in both cultures of the No. 4 solution there was evident con- 

 jugation. Thus one of the three species partially fulfilled 

 the expectations of Benecke's theory, while the other two 

 failed entirely to do so. A larger series gave conjugation in 

 eleven cases out of an expectation of fifteen with S. Grevil- 

 leana, but no slightest indication of it in parallel cultures of 

 longata which were treated in identically the same manner. 

 As was stated above, S. Grevilleana was conjugating in na- 

 ture at the time the observations were made, and consequent' 

 ly it cannot be predicted how it would act during a period 

 when conjugation is not normally occurring. Later in the 

 season, however, when this species had become scarce occa- 

 sional filaments sometimes occurred in my cultures associated 

 with other forms; at this time, however, they did not conju- 

 gate, as would have been expected, but grew vegetatively. 



On several occasions masses of actively conjugating S. 

 dubia were brought into the laboratory and placed in cultu- 

 ral solutions. After several days a few filaments presenting 

 the normal vegetative appearance could generally be found 

 in all the solutions. The origin of these filaments was not, 

 determined with certainty, and while they seemed to be de- 

 rived from filaments that had in part conjugated, it is not en- 

 tirely impossible that they were younger plants that would 

 have remained vegetative even in nature. Their subsequent 

 fate in the laboratory varied, but those in the more complex 

 salt solutions generally showed the most vigorous growth, 



No. 4 of the second series (which Benecke 

 zygospore formation) being about the most 



(1 



The germination of the zygospores in this species is of 



some 



a very brief time. Zygospores that were forming about April 

 15 and at that time placed in solution No. 4 of the second 

 series were germinating abundantly by May 3 when some of 

 them had already become three cells long. They were also 

 actively germinating after the same length of time in No. 

 4 of series ten (.02g. NaN0 3 in 150 cc. of water) and to a 

 much less extent in No. 3 of series two. In pond and tap 





