CHAPTER VIII 



INTERRELATIONSHIPS BETWEEN HIGHER PLANTS 

 AND SOIL MICROORGANISMS 



Interdependence of Higher Plants and Microbes. — To 

 casual observation, the development of higher plants may appear 

 to be an isolated phenomenon. Each plant seems to be quite 

 capable of developing independently of other living things, trans- 

 forming sunlight into available food and drawing from the soil its 

 requirements of mineral nutrients. However, although one cannot 

 accurately estimate the absolute extent to which the activities 

 of other forms of life are concerned with this apparently isolated 

 development of the higher plants, careful observation of the con- 

 ditions about plant roots indicates that there is a striking depend- 

 ence of higher plants upon microorganisms. This is particularly 

 the case in the absorption by plants of mineral nutrients from the 

 soil, and in some cases also in the absorption of organic substances 

 from the soil. The carbon dioxide of the atmosphere, upon which 

 the plant depends for the synthesis of its body substance, also 

 results from the activities of microorganisms. These, however, 

 depend upon the plant and its residues for their energy sources 

 and other nutrients. 



Neither higher plants nor microorganisms can develop long in 

 nature in the absence of the other without showing certain definite 

 abnormalities. Consequently, the fact that plant growth con- 

 tinues in soil and that microorganisms are increasingly active in 

 this habitat are suggestive of the associative development of roots 

 and microorganisms in soil. 



Some of these effects are only indirect, while others are asso- 

 ciated with a very intimate development of the plant and the 

 microbe, even to the actual penetration of the plant by the microbe. 

 In fact, it is difficult to determine what activities of microorganisms 

 in soils do not exert certain effects on plant growth; it is equally 



181 



