202 E E. Fritsch 



of An\istrodesmus, C/jlamydomonas, Tribonema, etc.). In fact, 

 the occurrence of such aquatic types in soil cultures seems to me 

 to provide further evidence in support of the view that the sub- 

 terranean Algae are forms washed down from the surface. 



I will frankly agree that it is difficult, in this interpretation of 

 the subterranean Algae, to find an explanation for the fact, estab- 

 lished by Bristol and referred to above, that there is often a 

 marked increase in the population of soil Algae at the four-inch 

 depth. This depth may, however, mark a physical boundary of 

 some kind (for example, increase in compactness of the soil) 

 beneath which the downward passage of algal cells occurs more 

 slowly so that an accumulation takes place at this level. In this 

 connection it is significant that some (though not all) of the 

 highest numbers recorded for the four-inch depth by Bristol,' 

 pp. 572, 573, followed a period of heavy rainfall. 



The alternative to the view that the placing of Algae in the 

 soil is a result of downward-washing during rain is to suppose 

 that motile reproductive stages actively move downward in the 

 soil and find in certain layers (namely, the four-inch level) con- 

 ditions specially favorable for development and multiplication. 

 Once buried beneath the surface in darkness, there is no appar- 

 ent reason why they should not move downward as well as up- 

 ward. For such forms as the species of Chlorella and Sticho- 

 coccus, however, in which no motile reproductive stages occur, 

 the only conceivable method of transference in the soil is a 

 downward one caused by the washing effect of rain. Moreover, 

 it still remains to be shown that, in forms such as Chlorococcum, 

 zoospores are formed within the soil; it is possible that, if re- 

 production occurs there, it is by means of aplanospores. 



A scrutiny of the mere superficies of the soil, even in wet 

 weather, is hardly likely to disclose all the surface-living algal 



