THE MICRO-ORGANIC POPULATION OF THE SOIL 253 



3. During the growth of the plant some at any rate of 

 the soil organisms appear to be adversely affected : some of the 

 activities of the growing plant seem to be detrimental to the 

 micro-organic population. 



4. While sharp distinctions cannot be drawn some group- 

 ings of the micro-organic population are more conducive to the 

 production of plant nutrients than others, and some organisms 

 are definitely harmful to the plant. 



Organisms Acting Directly on the Plant. 



(i) Parasitic and Disease Organisms. 



The study of these organisms has developed into a special 

 branch of Economic Biology, and we need therefore only briefly 

 refer to them here. The commonest are the eel-worms, the 

 myxomycete Plasmodiophora, some of the " wilts," and certain 

 organisms that attack potatoes. 



Of the numerous kinds of eel-worms occurring in the soil, 

 about six are known to attack and enter the plant, and do 

 considerable direct injury, besides opening the way for the 

 entrance of fungi, bacteria, etc. The commonest are Heterodera 

 radicicola, which causes swellings or " knots " on the roots of 

 tomatoes, cucumbers and other plants, and Tylenchus dipsaci 

 {syn. devastatrix) which attacks oats, causing tulip root, and 

 clover, bringing on one form of clover sickness. In some soils, 

 especially those short of lime, another pest is common : the 

 myxomycete Plasmodiophora, which enters the roots of swedes, 

 turnips and other plants of the Brassica tribe, causing the 

 disease known as finger-and-toe. 



(2) Symbiotic Organisms. — In normal conditions legu- 

 minous plants possess nodules on their roots which contain 

 numbers of bacteroids living in association with the plant. 

 This organism, Bacillus radicicola, enters the plant roots 

 at an early stage and, having brought about the formation 

 of the nodule, proceeds to manufacture nitrogen compounds 

 for the plant from the gaseous nitrogen of the air (see p. 204) 



Certain trees and shrubs (notably beech, heather, etc.) 

 become associated with mycorrhiza, fungi which grow on their 



