/Mineral salts + /% 

 / glucose -diffuse 

 daylight 



y-Mineral salts + 

 ,' /% glucose -dark 



Mineral salts - 

 diffuse daylight 



15 20 



Fig. 203 Growth of the soil alga, Scenedesmus, under different 

 conditions of nutrition and Hght. (After Roach.) 



maxanga in the Belgian Congo yielded a flora almost entirely com- 

 posed of Myxophyceae, though in true savanna algae were rare, 

 probably because of space and light competition. 



The growth of the soil algae has been a source of interest and ex - 

 periment for a nimiber 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 hght, temperature and aeration of the 

 medium (cf Fig. 203). 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. 204). This 

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

 an organic medium is present, but they all react diflferently to the 

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

 occurs. At constant temperature, increasing the hght intensity from 

 1/25 to 1/4 has a far greater effect than increasing the intensity from 



2B 377 



