SOIL ALGAE 



301 



depend on the heaviness of the rainfall, the state of the soil, i.e. 

 whether dry and cracked, and the nature of the algae, i.e. whether 

 or not they possess a mucous sheath. Passage through the soil is 

 facilitated in the filamentous algae either by fragmentation or else 

 by the formation of zoospores, the factors that are responsible for the 

 former process appearing to differ for the various species. Many 



6-0 



5'5 



S 5-0 



o 



6 



u 

 a 

 a 



'a 

 o 



3 



to 3-5 

 o 



4-5 



4-0 



3-0 



2 4 6 8 10 DAYS 



Fig. 191. Rate of growth of Scenedesmus in a solution of mineral salts under 

 different light intensities, (After Roach.) 



green algae are known to form zoospores when put into water 

 after a period of dryness, and hence one may presume that a shower 

 of rain will also induce zoospore formation. Petersen (1935) has 

 demonstrated experimentally that rain can carry algae down 

 efficiently to a depth of 20 cm., but that the process is facilitated by 

 the presence of earth-worms, although these animals probably only 

 operate indirectly in that they loosen the earth. Farmers in the 



