196 E E. Fritsch 



posed to increasing liability of desiccation. Nor is it possible to 

 demarcate them rigidly from mud-inhabiting forms such as 

 Botrydium, Protosiphon, etc., whose normal vegetative phase 

 is, however, developed only in the presence of considerable 

 quantities of moisture.* A rather special community is that 

 which occurs amid mosses in diverse habitats and consists, in 

 large part, of Desmids"^ and Diatoms,"' ^ the latter diflFering rather 

 markedly according to the degree of dryness of the habitat. 



The Subterranean Community 



The subterranean community, which is found in the water con- 

 stituting the films around the soil particles and occupying the 

 smaller interspaces between them, comprises manifold species, 

 many of which have been recorded as occurring at considerable 

 depths.'" "' ^'' ^' " In the present state of our knowledge, this com- 

 munity appears to be composed of a number of distinct elements. 

 First, it includes certain common members of the surface 

 community. A recent investigation of naturally occuring British 

 soils from 2- and 8-inch depths, conducted in my laboratory by 

 Miss J. James, has shown the universal presence of Chlorococcum 

 humicolum and species of Hormidium,-\ both of which are typ- 



* Yet, West's "association of irrorated rocks" (West,*^ p. 422) can hardly be 

 regarded as terrestrial because the forms involved, although some are terrestrial, 

 are probably normally well supplied with liquid water. 



t Here should also be included moss protonema,^ which appears almost in- 

 variably in soil cultures. Bristol^ records Ulothrix subtilis var. variabilis as a com- 

 mon subterranean alga, although it has not been recorded by other investigators 

 and no species of Ulothrix appeared in Miss James's cultures. I am not clear as 

 to the nature of this species of Ulothrix, but strongly suspect that further study 

 will show it to be one of the terrestrial Hormidiums. Similarly, I am inclined to 

 think that the older records of the occurrence of species of Pleurococcus in the 

 subterranean flora are a result of confusion with such forms as Pleurastrum and 

 possibly with some of the unicellular Xanthophyceae (Pleurochloris, etc.), which 

 recent research has brought to our notice. I am even doubtful whether Pleuro- 

 coccus occurs as a member of the surface community. 



