— 104 — 



In short careful periodic observations in nature do not give 

 any reason for departing from tlie resuit of Klebs' investiga- 

 tions, which afîorded such brilliant proof of reproduction in 

 Algae being in tlie main a consermence of the stimulus of certain 

 outside conditions (*>. 



Bcnecke (1908) has attacked the problem of periodicity in 

 Spiroggra in another wav and cornes to the conclusion, on tlie 

 basis of experiments with S. communis, that conjugation in 

 this species is due to a deficiencv of nitrogen-salts in the water, 

 conséquent on tlie rapid increase of the Angiospermous aquatics 

 with the advent of spring. Cultures on this basis undertaken 

 by Danfoith (1910) with other species of Spiroggra did not 

 lend gênerai support to Benecke's conclusions, which Danforth 

 thinks are « probablv not of gênerai application, or are applic- 

 able only under very spécial conditions » (loc. cit., p. 58). 

 Klebs' work lias shownthatmany conditions that are not realis- 

 ed in nature can give the stimulus to reproduction in Algae, 

 and before Benecke's results can be taken to apply to Spirogyra 

 in nature, it would be necessary to establish an actual deficiencv 

 in nitrogen-supply at the time of conjugation as compared with 

 the preceding period, and also to show that an addition of suit- 

 able nitrogen-salts to the water at the right time inhibited the 

 conjugation of Spiroggra under natural surroundings. It 

 would be difficult on Benecke's view to explain the irregularities 

 in reproduction of Spiroggra, mentioned in the last paragraph 

 on p. 103. 



Although as a gênerai rule the sexual process does not set in un lil 

 the maximum is reached,it has occasionally been our expérience 

 that a forai commences to exhibit sexual reproduction while 

 still relatively scarce,andthen subsequently increases to its maxi- 

 mum, a few sexual organs being apparently présent ail the time. 

 This may indicate the existence of outside conditions favourable 

 both to végétative growth and to the formation of sexual 



(1) We do not mean to say that internai conditions are altogether negligihle 

 or tliattliey may not even explain occasional irregularities in reproduction. Klebs 

 showed in m an y cases that an Alga as a resuit ol previous conditions may get 

 into such a state that the ordinary stimuli no longer serve to bring about sexual 

 reproduction. But \\& feel convinced that outside l'actors are as a gênerai rule 

 the main déterminants l'or the formation of sexual organs on the part of Algae 

 in nature. 



