88 Occurrence of Microorganisms in Soil 



the forked or tuber type, or it may be a result of conditions unfavor- 

 able for optimum growth of the fungus. Melin also recognized 

 pseudomijcorrhiza, which are endotrophic in nature but are not 

 comparable to the true endotrophic forms in orchids; the hyphae 

 are not digested and the fungus is largely parasitic. 



The stimulating effect of fungi on the growth of Ericaceae is 

 believed to be due to inactivation, destruction, or absorption of toxic 

 substances in the rooting medium, rather than to the secretion of 

 substances stimulating to the higher plants. Rayner claimed that 

 Phoma radicis is capable of bringing about systemic infection and 

 results in an obligate mycorrhizal relationship with ericaceous plants. 

 This concept has not been confirmed. 



Numerous species of fungi, nearly all Basidiomycetes, largely 

 Agaricineae, are capable of forming mycorrhiza. Many of the my- 

 corrhiza fungi are especially adapted to certain trees, some are less 

 specific, and still others grow without association with the living 

 tree. When a forest is removed, the obligate mycorrhizal fungi dis- 

 appear from the soil and reappear only when a new crop begins to 

 develop. The spores of these fungi do not germinate on artificial 

 media, and the mycelium and the fruiting bodies do not develop 

 when not connected with living tree roots. 



Algae 



Algae are widely distributed in the soil. Although they are largely 

 confined to the surface layer and are controlled by the moisture con- 

 tent, they may also be found below the surface and even in fairly dry 

 soils. Since they depend on sunlight for their growth, the subter- 

 ranean forms must either lead heterotrophic existence or remain there 

 largely in an inactive state. 



The soil algae comprise the Myxophyceae, or the blue-greens; the 

 Chlorophyceae, or the grass-greens; and the Bacillariaceae, which in- 

 clude the diatoms. Some of the blue-greens are able to fix atmos- 

 pheric nitrogen. The grass-greens are very abundant in acid soils. 



Protozoa 



Protozoa are unicellular organisms, varying in size from a few 

 microns to 4-5 mm. Some protozoa are also able to form colonies 

 which consist of numerous individuals. The majority of species, par- 

 ticularly the soil forms, are microscopic and can be studied in detail 



