194 HIGHER PLANTS AND SOIL MICROORGANISMS 



matter relatively low in nitrogen appear to be more closely asso- 

 ciated with nitrogen starvation than toxic excretions by plants. 



Among the excretions of roots, phosphatides and certain muci- 

 laginous substances appear to be of particular importance in certain 

 cases. It has been noted that such compounds act as stimulants 

 to the growth of certain microorganisms, increasing growth to a 

 greater degree than would seem to be likely from the energy which 

 the substances might furnish in the concentrations in which they 

 occurred. Such substances may be of importance in stimulating 

 growth of microorganisms in the rhizosphere, and may even lead to 

 a penetration of the root cells by these organisms. 



That the numerous root excretions enhance development of 

 microorganisms is quite well defined, but the nature of the organ- 

 isms affected is unknown except in a few instances such as mycor- 

 rhiza fungi. With the common cereals and root crops, there 

 appears to be as varied an assortment about the roots as in the 

 soil distant from the roots; this assortment includes bacteria, 

 actinomyces and filamentous fungi of a great variety. Whether 

 any transformations of the organisms in the rhizophere may be of 

 particular importance beyond the ways mentioned above is not 

 known. The limitation of our present knowledge leads to the 

 conclusion that the general abundance of the population is greater 

 here and the activity of the cells is greater, as indicated by forma- 

 tion of carbon dioxide. 



Conditions Favoring Nitrogen Fixation. — The following 

 conditions favor fixation of nitrogen: the presence of available 

 non-nitrogenous organic matter and necessary inorganic nutrients, 

 a favorable reaction, and the presence of nitrogen-fixing organisms. 

 These conditions appear to exist about the roots of many develop- 

 ing plants, and it is quite likely that the soil becomes enriched in 

 nitrogen, because of non-symbiotic development of nitrogen-fixing 

 organisms about plant roots. Although such nitrogen might not 

 be available to the crop which was responsible for its fixation, 

 nevertheless it would become available as decomposition of these 

 fixed nitrogen compounds progressed. 



Absorption of Organic Compounds by Plants. — Not only 

 inorganic substances but also many organic compounds may be 

 absorbed by plant roots. Some may have pronounced beneficial 

 effects upon root development. The organic substances which 

 may be utilized by green plants include carbohydrates, alcohols, 



