204 



E E. Fritsch 



faces evident growths of Chlorococcum may be found, and forms 

 such as Coccomyxa and Stichococcus, though more occasional, 

 sometimes occur in conspicuous strata; a common habitat is the 

 basal parts of tree trunks just above the soil level." Striking 

 sheets of Blue-green Algae are seen, especially in wet periods, on 

 cultivated soils and calcareous soils, and Petersen^^ records for 

 Iceland extensive sheets of Phormidium autumnale wherever 

 the ground is rich in organic nutriment (see also West," p. 421). 

 It remains to be discovered whether, at times when such growths 

 are not evident on the surface, they are persisting in the illumi- 

 nated subjacent layers, to aid in the resuscitation of a stratum in 

 wet weather. The numerous forms of soil Diatoms, character- 

 ized in general by their small dimensions and their capacity for 

 motility,^' ^^ are no doubt mainly forms living in the illuminated 

 region of the soil, though possibly only a few are present at the 

 actual surface. 



The occurrence of a rich surface algal flora, especially in wet 

 weather, affords, therefore, ample material from which the sub- 

 terranean element can be supplied, although it is evident that 

 the agencies bringing about downward transference operate 

 differently on the various members of the surface community 

 (p. 205) . Bristol' has shown that a large proportion of the Algae 

 in the soil are in a vegetative condition, or at least in a condi- 

 tion susceptible to the effects of desiccation. This is perhaps what 

 one would expect of an Alga carried down into darkness and 

 gradually losing its food reserves because probably its capacity 

 to withstand drought depends, partly at least, on a high osmotic 

 pressure and also apparently on the presence of fatlike sub- 

 stances."^' ^' ^' *°' *^ It is not necessary, however, to suppose that the 

 cells that survive are in the form of spores. The vegetative cells 

 of diverse terrestrial Algae are known to possess a great capacity 



