C/N RATIO 207 



adsorptive properties of colloids, the depressing action of 

 some examples may be due to their adsorbing mineral salts 

 and thus rendering them unavailable for the use of the plant. 

 Further, in considering the action of inorganic compounds 

 on growth, certain important aspects are to be remembered : 

 the plant has a specific and a varietal physiology; a good crop 

 of nettles indicates a high nitrogen content in the soil and 

 varieties of barley differently respond to various types of 

 manuring ; * the inter-relationship of the various compounds 

 concerned, carbohydrate and nitrogen for instance ; and the 

 fact that conditions which favour vegetative growth are not 

 necessarily those for reproductive activity. An abundance 

 of inorganic salts in the soil promotes vegetative activity, 

 whilst a relatively small salt supply induces reproduction ; 

 but this only results provided that other conditions are 

 satisfactory. Thus increase in growth is impossible for the 

 green plant if carbon assimilation be of a low order of in- 

 tensity, since carbohydrate is required for many purposes, 

 structural, respirative, and as raw material for the elaboration 

 of other compounds such as proteins. And for this last purpose 

 nitrogen also is necessary ; wherefore intense carbon assimi- 

 lation in the absence of nitrogen-containing substances cannot 

 lead to a growth commensurable with the intensity of carbo- 

 hydrate formation. In fact, there is between nitrogen and 

 carbohydrate a correlation, and growth is affected according 

 to their ratio. Thus if carbon assimilation be increased by- 

 growing plants in an atmosphere enriched by the addition 

 of carbon dioxide, whilst the nitrogen-containing salts of the 

 soil are not increased, the ratio C/N is high and the reproductive 

 phase is induced ; if, on the other hand, the ratio C/N is low, 

 the vegetative activity is intensified.f These results may, 

 however, not occur if the ratio mentioned runs to extremes. 

 Kraus and Kraybill % in their experimental work on the tomato 

 found that a very high C/N ratio results in but little vegetative 

 growth and but poor reproduction ; a medium ratio gives 



* Gregory and Crowther : " Ann. Bot.," 1928, 42, 757. 



f Fischer : " Gartenflora," 1916, 65, 232. 



\ Kraus and Kraybill: " Oregon Agric. Exp. Sta.," 1918, Bull. 149. 



