EFFECT OF DECOMPOSITION PRODUCTS 119 



(2) Symbiotic Organisms. In normal conditions leguminous plants 

 possesses nodules on their roots which contain numbers of bacteroids 

 living in association with the plant. This organism, Bacterium 

 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. 17). 



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

 associated with mycorrhiza, fungi which grow on their roots and aid 

 in the nutrition of the plant. These were first investigated by Frank * 

 and have received considerable attention from mycologists. 



I (J5). Organisms Bringing About Changes of Importance 



to the Plant. 



From time to time indications have been obtained that some of 

 the soil bacteria bring about changes harmful to the plant, but the 

 evidence is not sufficiently good to justify any detailed discussion 

 here. It has been supposed that plant toxins are produced (p. 113), 

 that soil nitrates are decomposed (p. 101), and that the food, air and 

 water which should otherwise be available to the plant are taken up 

 by the micro-organisms. A careful and critical examination of this 

 whole subject of bacterial interference is badly needed. 



The change that has been most frequently studied, and must there- 

 fore claim most of our attention, is the production of nitrates and 

 humus substances during the decomposition of the organic matter of 

 the soil. The initial material consists in the residues of roots, stems 

 and leaves shed by earlier generations of plants : material which is not 

 itself helpful to plant growth and indeed may be detrimental through 

 opening up the soil too much. Under the influence of earth-worms, 

 fungi, bacteria, and other organisms it breaks down to form humus and 

 nitrates, both of great importance for the plant. Agricultural chemists 

 have been so interested in these two substances that they have tended 

 to overlook the effect of any others that may be produced. Schreiner 

 and Skinner (252) deal with some of these, and show that nucleic acid, 

 hypoxanthine, xanthine, guanine and others which may well be 

 formed in the decomposition, are all beneficial to plants grown in 

 water cultures, while picoline carboxylic acid and guanidine were 

 harmful. It does not follow that these effects are produced in the 

 soil, but the investigation shows the necessity for a broader outlook- 



For the present, however, the lack of material leaves us with no option 

 \ 



1 Botan. Ztg. 1891, 9, 244-253, where references to his earlier papers are given. 



