The Role of the Terrestrial Alga in Nature 203 



growth. It is very probable that some of these forms Uve beneath 

 the actual surface in a region where photosynthesis is still pos- 

 sible, but where better conditions in regard to moisture supply 

 and protection from desiccation exist; also, some of these forms 

 may be shade-loving species (see the endolithic forms referred 

 to below). Moreover, the more specifically saprophytic forms 

 may be influenced in their distribution by the availability of 

 organic matter. A study of the surface community, therefore, 

 should include not only the forms visible at the surface, but also 

 all those encountered at depths to which light is found to pene- 

 trate. It is not improbable that a microstratification of the surface- 

 living soil Algae may be found to obtain, depending on their 

 immediate light requirements and on their relative degree of 

 susceptibility to desiccation. 



No data are at present available concerning the depth to which 

 light can penetrate beneath the surface of a compact soil, but it 

 is doubtful whether there is anything but darkness below the top 

 few centimeters. Forms such as Chlorococcum and Hormidium 

 appear to grow on the actual surface. How far there is direct (as 

 opposed to passive) penetration from here into the illuminated 

 layers of the soil has not as yet been ascertained. The forms 

 growing at the surface are of course exposed to considerable 

 desiccation during dry periods, and many of them become con- 

 spicuous only in wet weather. Then, however, they often appear 

 in remarkable numbers. On peaty and sandy soils Zygogonium 

 ericetorum may form an intermittent covering over acres of 

 ground, whereas Prasiola and Hormidium often occur exten- 

 sively on heavier soils, the former especially where there is or- 

 ganic pollution. These forms are conspicuous by virtue of the 

 wefts which they produce. Unicellular types are more difficult 

 to recognize unless they occur in abundance, but on damp sur- 



