The Role of the Terrestrial Alga in Nature 



By F. E. Fritsch 



THE ESSENTIALLY AQUATIC NATURE o£ the plants classcd as 

 Algae is likely to cause one to lose sight o£ the fact that 

 a very considerable number of these plants lead a ter- 

 restrial existence under conditions of life that differ very pro- 

 foundly from those obtaining in the aquatic habitat. The major- 

 ity of the terrestrial Algae are of relatively lowly organization, 

 many being unicellular, colonial, or exhibiting a simple filament- 

 ous habit, and it is only in Trentepohlia and in some of the Blue- 

 green Algae (Stigonemataceae) that indications of a higher 

 differentiation are seen. No terrestrial alga attains to the com- 

 plexity of structure and of reproductive methods met with in 

 the advanced Phaeophyceae and Rhodophyceae. The terrestrial 

 algal flora is essentially recruited from the classes Chlorophyceae, 

 Xanthophyceae (Heterokontae), Bacillariophyceae (Diatoms), 

 and Myxophyceae (Cyanophyceae), although some species of 

 Porphyridium (Bangiales) are terrestrial and Pascher^** has 

 described a terrestrial member of the Chrysophyceae {Geo- 

 chrysis) . 



Broadly speaking, three groups of terrestrial Algae can be 

 distinguished: (i) those which are found between the particles 

 beneath the surface of the soil (subterranean community ),'^ (2) 

 those which live on or just beneath the soil surface (surface 

 community), and (3) those which live on other terrestrial 

 substrata — rocks, tree trunks, etc. — in part well raised above the 

 soil level (aerial community). No sharp line can be drawn be- 

 tween these three communities, but on the whole they are ex- 



* Superior figures refer to items in the bibliography at the end of this essay. 



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