I / 



8 



J 



10Z?/3KS 



Fig. 204 Rate of growth of Scenedesmus in a solution of mineral 

 salts under different light intensities. (After Roach.) 



1/4 to full sunlight. Under full light the growth curves (cf. Fig. 204) 

 rise to an optimum by means of photosynthesis alone, but at lower 

 intensities the optimum is only approached if additional nutriment, 

 in the form of glucose, is present as well. There is no adequate 

 evidence that such organic media are present in the soil layers so 

 that it is very doubtful whether growth in the dark can occur in 

 nature, but it has been shown, however, that Nostoc punctiforme 

 from the leaves of Gunner a and also a species of Euglena are capable 

 of growth in the dark. 



Even if the algae cannot grow in the lower layers of the soil be- 

 cause of the darkness, we must still enquire into the process re- 

 sponsible for their appearance in those layers. The possible 

 agencies are (i) cultivation, (2) animals, (3) water seepage and (4) 

 self-motiUty. Mechanical resistance and lack of hght are said to 

 prevent the Myxophyceae from moving down under their own 

 locomotion, and whilst it is possible that algae may move down 



378 



