198 E E. Fritsch 



in the top inch* and an equal or even greater number in the 

 fourth inch, whereas samples from the two-inch and three-inch 

 depths and from depths below four inches contained far smaller 

 numbers. Such qualitative differences as were observed were 

 quite irregular and appeared to be of no general significance. 

 Moreover, by manurial treatment the proportion of the diverse 

 Algae present can be materially altered."" Bristol," p. 572, points 

 out that the four-inch depth is known to be that of the greatest 

 biological activity of most soil organisms, but it may be doubted 

 whether such a conclusion, which refers to Bacteria, Protozoa, 

 etc., can be legitimately extended to the pigmented Algae. 



Except in the immediate neighborhood of the soil surface, the 

 Algae inhabiting the soil must be in complete darkness, at cer- 

 tain times and, although in cultivated soils agricultural opera- 

 tions will tend to bring the deeper-living forms nearer to the 

 surface, there is litde that is apt to cause such a transference in 

 natural soils. The activities of earthworms and other animals are 

 perhaps more likely to lead to the destruction of the Algae than 

 to their transference (cf ., however, Esmarch,^ p. 253) . It is a well- 

 known fact that many of the simpler Algae are capable of living 

 in prolonged darkness and even of remaining green under these 

 circumstances, provided that suitable organic nourishment is 

 available. One of the most valuable studies in this respect is that 

 of BristoF on a species of Scenedesmus (S. costulatus Chod. var. 

 chlorelloides Bristol-Roach) isolated from the soil. She showed 

 that, under conditions permitting normal photosynthesis, this 

 alga carries out no heterotrophic nutrition, whereas in dimin- 

 ished light intensity an increasing amount of carbohydrates is 

 utilized and in darkness this of course is the only source of carbo- 

 hydrate nourishment. However, the capacity for growth is less; 



• It is not quite clear whether this included the actual surface layer. 



