The Role of the Terrestrial Alga in Nature 197 



ical surface forms. The species of Chlorella, Coccomyxa, Dac- 

 tylococcus, and Stichococcus found in the subterranean com- 

 munity, although not so universally present, probably belong 

 to the same element. Blue-green Algae (especially the species of 

 Phormidium) and the majority of the Diatoms should probably 

 also be included here.^ 



Second, there are found in the subterranean community a 

 small number of forms which, so far as present evidence goes, 

 are more or less characteristic of this habitat; among these 

 are Gongrosira terricola, and certain Yellow-green forms {Bo- 

 try diopsis, Pleurochloris, Monodus, Monocilia, Bumilleria, etc.), 

 as well as, perhaps, a few of the Diatoms. Divers of these forms 

 have not so far been found except in soil cultures or commonly, 

 at least, have been met with only in such cultures, suggesting 

 the subterranean regions of the soil as the usual habitat. 



The third element is what I should like to describe as the 

 casual one (Bristol,' p. 570), including species of Chlamydo- 

 monas, AnJ^istrodesmus, Vaucheria, etc., which are only of occa- 

 sional occurrence and probably owe their presence in the soil to 

 wind-borne resting stages which fall on the surface and after- 

 ward are washed down by rain (see below, p. 201; also 

 Fritsch"^). 



A problem of considerable interest concerns the distribution 

 of Algae in a soil. The first serious attempt to determine this 

 quantitatively was made by Bristol,^ who employed a method of 

 progressive dilution of a soil suspension, the number of organ- 

 isms in each gram of soil being then computed statistically.' She 

 arrived at the conclusion that there were no appreciable diflFer- 

 ences in the horizontal distribution of soil Algae in a uniform 

 soil, but that there were marked quantitative differences in the 

 vertical distribution. She found an abundant algal population 



