2o8 ^ E. Fritsch 



preliminary steps in the colonization of burnt heath in England.^ 

 In both places what appears first is in large part a covering of 

 gelatinous palmelloid forms that in wet weather constitutes a 

 thin but practically continuous mucilaginous covering on the 

 otherwise bare surface. This must aid materially in the establish- 

 ment and early germination of the seeds and spores of the col- 

 onizing plants. It is probable that a more extensive study of 

 primary colonization of newly exposed soil will show that mem- 

 bers of the surface algal community are almost always the first 

 to appear. To what extent such forms may also be active in the 

 disintegration of the particles at the soil surface and immediately 

 below it, is at present unknown. 



The Aerial Community 



Turning last to what I have at the outset called the aerial com- 

 munity, I think we should include here all algal growths that do 

 not occur on the soil proper, that is, not only those which are 

 found on vertical surfaces (rocks, tree trunks) above the soil 

 level, but also those inhabiting more or less horizontal surfaces 

 of a similar nature. In all these the substratum will furnish rela- 

 tively Htde water and the growth of Algae must be in large 

 measure dependent on moisture obtained direcdy from the at- 

 mosphere. Among the most important of the rock-growing 

 Algae are the Myxophyceae,*' "^^ "' '^' "' *^ the terrestrial members 

 of which are indeed seen at their best in such a habitat; species of 

 Trentepohlia also thrive but are of secondary importance, and 

 other Green Algae and Xanthophyceae are usually quite sub- 

 sidiary except apparently on some kinds of sandstone." On trees 

 the most important forms are Chlorococcales, Pleurococcus, and 

 species of Trentepohlia, the last-named especially in the Tropics 

 or in very humid regions such as parts of the Monterey Penin- 



