130 GROWTH 



nutrient salts increases growth, but in high concentrations a 

 depression obtains. With regard to inorganic colloids, ferric 

 and aluminium hydroxides depress whilst colloidal silica in- 

 creases growth, doubtless because silica is much absorbed by 

 the cereal grasses and possibly in this form. In view of the 

 adsorption 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 physiology, a good crop of nettles 

 indicates a high nitrogen content in the soil ; the inter-rela- 

 tionship of the various compounds concerned, carbohydrate 

 and nitrogen for instance ; and the fact that conditions which 

 favour vegetative growth are not necessarily those for re- 

 productive activity. An abundance of inorganic salts in the 

 soil favours vegetative activity whilst a relatively small salt 

 supply is inductive to reproduction ; but this only obtains 

 provided that other conditions are satisfactory. Thus increase 

 in growth is impossible for the green plant if carbon assimila- 

 tion be of a low order of intensity, since carbohydrate is re- 

 quired 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 assimilation in the absence of nitrogen- 

 containing substances cannot lead to a growth commensurable 

 with the intensity of carbohydrate formation. In fact, there is 

 between nitrogen and carbohydrate a correlation, and growth 

 is affected according to their ratio. Thus if carbon assimila- 

 tion be increased by growing plants in an atmosphere enriched 

 by the addition of carbon dioxide whilst the nitrogen-contain- 

 ing 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.* 

 These results may, however, not obtain if the ratio mentioned 

 runs to extremes. Kraus and Kraybill f 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 



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



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



