300 PHYSIOLOGY, SYMBIOSIS, ETC. 



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 Gunnera and also a species of 

 Euglena are capable of growth in the dark. 



s 



d 



u 



a 



Cifl 



o 

 3 



O 



6-0 



^ 5-5 



5-0 



4-5 



4-0 



3-5 



3-0 



iMinerA sslib ^ 1% 

 1 6Lucose -diffuse 





/Minerd sa.lts + 



/ / Diffuse d^yli§hi 



I / 

 / /■ 



/ 



10 

 Days 



15 



20 



Fig. 190. Growth of the soil alga, Scenedesmus, under different conditions of 

 nutrition and light. (After Roach.) 



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

 because of the darkness, we must still enquire into the process 

 responsible for their appearance in those layers. The possible 

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

 self-motility. Mechanical resistance and lack of light are said to 

 prevent the Cyanophyceae from moving down under their own 

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

 through their own motility, further experimental work on this 

 aspect is much to be desired. The effect of water seepage will 



