The Role of the Terrestrial Alga in Nature 201 



terricola, Monocilia, etc.), it is possible that some of these are 

 specially equipped, in a way at present not recognized, for life 

 beneath the surface. However, it is equally probable that they, 

 too, are actual surface forms, rarer than the others, and not so 

 far discovered in their normal habitat. An intensive study of the 

 surface of the soil has as yet been undertaken only in Denmark 

 and Iceland by Petersen,^' ^^ whose investigations showed that a 

 far larger number of surface-growing Algae exist than had 

 previously been recognized. The same investigator^ records two 

 species of Bumilla'ia, hitherto mainly observed in cultures of sub- 

 terranean soil-Algae (especially B. exilis), from the surface of 

 soils of Hammer Bakker, and Botrydiopsis minor from the sur- 

 face of Iceland soils.^* These, as well as certain of the Chloro- 

 phyceae recorded, were not macroscopically visible. 



It is noticeable that a considerable number of the forms so far 

 mainly recorded from beneath the surface of the soil are Xantho- 

 phyceae (Heterokontae). Certain members of this group are 

 known to be intolerant of strong illumination, and it is possible 

 that the soil forms do not inhabit the actual surface, but live in 

 the feebler light just beneath (cf. also below). This would ac- 

 count for their not having been observed, so far, in the study of 

 actual surface growth. 



The resting stages of aquatic Algae are known to be to a large 

 degree wind-dispersed. Only a small proportion of such wind- 

 borne spores will be deposited in other adjacent waters. Large 

 numbers of them will fall on the soil surface and become im- 

 mersed in the soil during showers of rain (Fritsch,^ p. 254) . Soil 

 animals may also be instrumental in their burial. No doubt many 

 such resting stages will perish without further development, but 

 some may survive and, in my opinion, will amply account for the 

 occurrence of the casual element among the soil Algae (species 



