FRESH WATER ECOLOGY; SOIL ALGAE 375 



(b) Epiphloeophytes, algae growing on bark, e.g. Pleiiro- 

 coccus. 



(c) Epizoophytes, algae growing on animals. 



(d) Lithophytes, algae growing on rocks, e.g. Prasioluy 

 Trentepohlia. 



(2) Edaphophytes, or algae that grow on or in soil. These also 

 can be subdivided into : 



(i) True soil species : 



(a) Epiterranean, or lying in the surface layers of the 

 soil. 



(h) Subterranean, or lying in the lower layers of the soil. 



So far as is known at present there are no obhgate species 



of this class, 

 (ii) Casuals. 



The study of soil algae, as such, began seriously at the com- 

 mencement of the nineteenth century with the works of Vaucher, 

 Dillwyn, Agardh and Lyngbye, whilst towards the end of the 

 century monographs by Bornet and Flahault, Gomont, Wille and 

 the Wests, father and son, began to make their appearance. In 1895 

 Graebner, in a study of the heaths of North Germany, gave the 

 first account of soil algae as ecological constituents, and subse- 

 quently many ecologists have shown that soil algae are pioneers on 

 bare soil where they prepare the ground for the higher plants that 

 follow. In such cases the algal flora is generally richest when the 

 soil is primarily or secondarily naked, e.g. mud flats developing to 

 salt marsh, or ploughed grassland. A manured soil also has a very 

 rich flora, whilst the same species are to be found in unmanured 

 soils, though not in such numbers. The richness of the flora is also 

 influenced by the moisture conditions, damp soils having a more 

 varied and extensive collection of algae than dry soils. In the case 

 of the diatom component of the soil flora it has been found that it is 

 more abundant when the soil is rich in phosphates and nitrates, so 

 that the soil salts may also be of importance. The addition of arti- 

 ficial fertilizers to a soil can also alter the soil flora. In recent years 

 dilution cultures have been widely used in order to give a quanti- 

 tative aspect to the work, and the results of such studies have been 

 to show that there is probably a seasonal variation in numbers, but 

 that the behaviour depends on the depth and kind of soil. There 

 seems little doubt but that the soil flora, and also fauna, has an 



