The Role of the Terrestrial Alga in Nature 205 



to resist desiccation,"^" ^ and the surviving cells may be those that 

 have recently been w^ashed downward and have not yet lost the 

 equipment which enables them to withstand drought. In fact, 

 if they are plentifully stocked with food reserves as a result of 

 photosynthesis at the surface, after burial they may possibly carry 

 on a certain amount of multiplication until these reserves are 

 exhausted, the more so because moisture conditions will be more 

 favorable than they normally are at the surface. But it seems 

 highly probable that, in many instances, when the food reserves 

 are used up, the alga is doomed to gradual death unless it falls 

 a prey to the Protozoa present. The fact established by Johann- 

 son,'" that the production of carbon dioxide from soils is nearly 

 always greater during the day than during the night, would 

 seem to imply that even the surface-living Algae are insufficient 

 in quantity to affect materially the gas relations. 



Soil Algae have been supposed to play a role in increasing the 

 fertility of the soil but, clearly, if they grow and multiply at all 

 beneath the illuminated zones of the soil, they can do so only 

 at the expense of their own reserves or of the organic material 

 present. In the deeper layers, therefore, they cannot increase 

 fertility unless through their action they render certain of the 

 organic ingredients more readily accessible to the remaining 

 population of the soil. For this view, however, there is at present 

 no evidence. On the other hand, the carrying down of surface 

 forms to the deeper layers will probably add materially to the 

 organic content of the soil because these forms have built up their 

 bodies and no doubt have often stocked them with food reserves 

 in an autotrophic manner. The inclusion of their substance in the 

 soil, whether direct or indirect, must enrich it in organic material. 



If the previous conclusions are correct, the importance of Algae 

 in soil economy lies mainly in their growth at the surface, 



