THE ALGAE OF THE SOIL 



37 



strictly so than most aerobic bacteria. They are as a rule more 

 sensitive to changes in reaction and hve over a narrower range of 

 acidity and alkaUnity. They appear to make more extensive 

 development than the bacteria in soils of low moisture content, 

 but they develop quite well in fairly moist soil under aerobic 

 conditions. Fig. 24 shows a typical actinomyces myceHum as it 

 appears under the microscope, using low magnification. A much 

 more complicated structure is shown in Fig. 25; this schematically 

 represents highly magnified portions of growth. 



The Algae of the Soil. — The chlorophyll-bearing micro- 

 scopic plants, namely the algae, also form an extensive group of 

 soil organisms. They vary greatly in size and in shape; but the 

 soil forms are largely 

 microscopic species, 

 unicellular or filament- 

 ous in structure. Since 

 they contain chloro- 

 phyll and are capable 

 of utilizing the energy 

 of the sun, they are 

 independent of the 

 energy sources in the 

 soil so long as they 

 have free access to 

 light. They are capa- 

 ble of making an ex- 

 tensive growth, deriv- 

 ing from the soil the 

 required nitrogen and minerals. They live both on the surface 

 of the soil and at various depths beneath the surface. Below the 

 soil surface they act in a manner similar to fungi, that is, living 

 at the expense of the energy derived from the utilization of organic 

 materials. 



Soil algae are divided into three groups: (1) Cyanophyceae or 

 blue-greens; (2) Chlorophyceae or grass-greens; and (3) Diatomaceae 

 or diatoms. Representatives of these three groups are found in 

 the soil in considerable numbers, generally less abundantly, how- 

 ever, than the bacteria and the fungi. Their development in the 

 soil results in increasing the supply of organic matter and in 

 temporarily transforming soluble forms of nitrogen and minera 



Fig. 26. — Soil alga, Bumillaria exilis 

 (from Bristol). 



