The Role of the Terrestrial Alga in Nature 207 



The Surface Community 



In the few surface Algae that have been investigated in this con- 

 nection, there is a very marked degree of adaptation to the con- 

 ditions of desiccation to which they are exposed.^' ^' ^' *^' " It may 

 particularly be emphasized that they possess the faculty of with- 

 standing drought without appreciable change and without the 

 assumption of special resting stages. There is also evidence that 

 they readily absorb atmospheric vapor as well as liquid water, 

 and in this way probably are able, in large measure, to tide 

 over periods devoid of actual precipitation and to resume active 

 growth and multiplication as soon as wet weather sets in. The 

 special machinery that appears to be connected with the peculiar 

 properties of these forms has been adequately described else- 

 where^' ^' ^ and need not be dealt with here. There is some evi- 

 dence to show that the special properties of such Algae enable 

 them during periods of drought to compete successfully with the 

 higher plants (Piercy,*° p. 515). 



Apart from the role which such surface-growing soil forms 

 probably fulfill after being washed down into the soil, they must 

 often lead to the production of surface humus after death. This 

 must be especially true when their nature is such that they are 

 not carried down in a living condition. Attention has also been 

 directed (West and West,^ p. 303) to the importance of surface 

 wefts of Algae such as Porphyrosiphon Notarisii and Zygogo- 

 nium ericetorum in consolidating the surface, and in affording 

 in damp weather a seed bed on which smaller seeds, etc., find a 

 ready means of germination. The role of surface forms as pri- 

 mary colonizers of new ground has frequently been empha- 

 sized.^^' " In this connection we may note the descriptions of the 

 first stages in the recrudescence of growth on Krakatoa" and the 



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