SOIL ALGAE 299 



shown that the algal flora is mainly confined to the top 12 in. of 

 soil with a maximum abundance at about 3-6 in. below the surface. 

 With increasing depth the number of algae decrease regularly, the 

 maximum depth at which they have been recorded being two 

 metres; there is, how^ever, really no conclusive evidence which 

 shows that algae can grow in the deeper layers where there is no 

 light, and it is very probable that they are only present in these 

 layers in a resting phase. > The number of reproductive bodies in the 

 surface layers reaches a maximum in spring, but in the lower levels 

 it remains constant throughout the year. In Denmark the quality 

 of the soil is apparently decisive in determining the luxuriance of 

 the flora irrespective of whether the ground has been disturbed or 

 not. In Greenland soil algae have been found down to a depth of 

 40 cm., and their presence there can only be satisfactorily explained 

 by the action of water trickling down the cracks because burrowing 

 animals are absent. A study of soils from all over the world has 

 emphasized the existence of a widely distributed algal formation in 

 cultivated soils. This flora consists of about twenty species of 

 diatoms, twenty-four of Cyanophyceae and twenty species of green 

 algae, among which Hantzschia amphroxys, Trochiscia aspera, 

 Chlorococcum humicolum, Bumilleria exilis and Ulothrix suhtilis var. 

 variahilis are the most frequent. 



The growth of these soil algae has been a source of interest and 

 experiment for a number of years. Roach (1926) has found that 

 ordinary growth in Scenedesmus costulatus var. chlorelloides is best 

 in a glucose medium but that xylose is toxic, the factors controlling 

 the normal growth rate being light, temperature and aeration of 

 the medium (cf. fig. 190). The same alga has been used for growth 

 experiments in the dark in order to determine how far such algae 

 can grow when they are below the soil surface (cf. fig. 191). This 

 and four other species can be made to grow in the dark provided an 

 organic medium is present, but they all react differently to the 

 various conditions and also they vary in the amount of growth that 

 occurs. At constant temperature, increasing the light intensity 

 from ^ to J has a far greater effect than increasing the intensity 

 from J to full sunlight. Under full light the growth curves (cf. fig. 

 191) rise to an optimum by means of photosynthesis alone, but at 

 lower intensities the optimum is only approached if additional 

 nutriment, in the form of glucose, is present as well. There is no 



