298 PHYSIOLOGY, SYMBIOSIS, ETC. 



SOIL ALGAE 

 Terrestrial algae may be classified conveniently as follows : 



(i) Aero-terrestrial species found growing on plants. 

 (2) Eu-terrestrial, 



True soil species : 



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



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

 So far as is known at present there are no obligate 

 species of this class. 



(c) Hydroterrestrial, or occupying the soil of aquatic areas. 



(d) Casuals. 



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

 ment 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 sub- 

 sequently 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 recent 

 years dilution cultures have been widely used in order to give a 

 quantitative 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. 



Subterranean Algae 



There are great fluctuations in the numbers of the different 

 species that compose the flora, but there are no species in the lower 

 layers of the soil which do not also occur in the surface layers. 

 Dilution cultures, together with the counting of samples, have 



