ASSOCIATIVE GROWTH OF GREEN PLANTS AND MICROBES 195 



organic acids, peptones, amino acids, and purine bases. In fact, 

 certain plants, as many of the Orchidaceae, cannot initiate growth 

 from the seeds without having access to certain assimilable organic 

 substances furnished either artificially or by means of their sym- 

 biotic fungus associates. 



Long periods of field experiments argue very favorably for the 

 use of a certain amount of organic fertilizers, or at least of organic 

 residues in the rotation. While in the first few years crop growth 

 may be fully as abundant with inorganic fertilizers alone, plants 

 respond more favorably to organic manures over long periods of 

 time. It is recognized that organic substances greatly improve 

 the physical condition of the soil, but it is likely that certain other 

 properties of the organic materials also contribute to the bene- 

 ficial effects upon plant growth, and that these effects are not 

 apparent with the use of inorganic substances alone. It has been 

 suggested, for example, that animal manures when used as fer- 

 tilizers result in the production of seeds which are much superior 

 to those of plants grown on soils either receiving no fertilization 

 at all or merely mineral fertilizers. Such seeds produced from 

 soils receiving animal manures were found to grow plants bearing 

 a much greater abundance of seeds. These results, of course, need 

 further confirmation and more extensive study. 



Under certain conditions, organic substances elaborated by 

 microorganisms, formerly referred to as auximones, appear to have 

 a stimulating effect on plant growth; the hypothesis has been 

 brought forth that certain vitamines, similar to those which are so 

 prevalent in higher plants, are formed by microorganisms and 

 absorbed by the plants. Because of the fact that information 

 on this subject is extremely limited this hypothesis is little more 

 than suggestive. Organic substances other than those contained 

 within plant seeds are apparently not essential to plant growth, 

 but they may prove beneficial as buffering agents and may in some 

 case even act directly as plant nutrients. 



In general, microorganisms in the plant rhizosphere may exert 

 effects similar to those produced by microbes at a distance from 

 the roots, but their effects may become more quickly apparent by 

 reason of the fact that the action occurs nearer to the organ 

 affected. 



Associative Growth of Green Plants and Microbes. — 

 There are certain much more intimate associations between micro- 



