n8 SOIL CONDITIONS AND PLANT GROWTH 



plant growth : the existence of this group, however, is a matter of 

 inference only, it being impossible to establish the negative proposi- 

 tion that any particular organism is without effect on the plant. But 

 it is convenient to retain the group if only as a reminder that all soil 

 organisms are not necessarily engaged in doing work of importance to 

 plants and incidentally to ourselves. 



The groups may be further divided as follows, and they will be 

 discussed in this order : 



I. Organisms affecting plant growth. 



(A) By exerting a direct action on the plant. 



(1) Parasitic and disease organisms : Eel -worms, Plasmod- 



iophora, Wilts, etc. 



(2) Symbiotic forms the clover organism, Mycorrhiza. 



(B) By bringing about soil changes of importance to the plant. 



(3) Changes harmful to the plant : supposed production 



of toxins, removal of nitrates, competition. 



(4) Changes beneficial to the plant : production of humus 



substances, of ammonia and nitrates. 



II. Organisms not acting on the plant but on the organisms of 

 group I. 



Certain amoebae. 

 Certain flagellates. 



III. Organisms acting neither on the plant nor on groups I. and II. 

 None known with certainty. 



I (A). Organisms Acting Directly on the Plant. 



(l) 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 Plas- 

 modiophora, 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 root, 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 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 Plas- 

 modiophora, which enters the roots of swedes, turnips and other plants 

 of the Brassica tribe, causing the disease known as finger-and-toe. 



