SOIL CONDITIONS AFFECTING PLANT GROWTH 61 



centration of nitrogenous food in the soil, but also increases 

 the amount of root, i.e. of absorbing surface, and of leaf, i.e. 

 assimilating surface. The process thus resembles autocatalysis, 

 where one of the products of the reaction acts as a catalyser 

 and hastens the reaction. The increase does not go on in- 

 definitely because some limiting factor steps in. 



The effect of nitrogen supply on the grain is very marked. 

 In Table XV. it is seen that the grain formed, when nitro- 

 genous food is wholly withheld, is only two-thirds of the normal 

 weight per individual. The first addition of nitrate causes a 

 marked rise in the weight per grain and the proportion of 

 grain to total produce, but successive additions cause no further 

 rise. Indeed other experiments prove that excess of nitrogen- 

 ous food causes the proportion of grain to fall off somewhat. 

 The leaf and the general character of growth are affected to a 

 much greater extent. Nitrogen starvation causes yellowing of 

 the leaf, especially in cold spring weather, absence of growth, 

 and a poor starved appearance generally : a moderate supply 

 of nitrogen leads to more rapid growth, very useful in cold 

 weather or in case of attacks by insect pests. Abundance of 

 nitrogen, on the other hand, leads to the development of large 

 dark green leaves which are often crinkled, and usually soft, 

 sappy, and liable to insect and fungoid pests (apparently 

 because of the thinning of the walls and some change in com- 

 position of the sap) and to retarded ripening: the effects 

 resemble those produced by abundant water supply. A series 

 of plants receiving varying amounts of 'nitrate are thus at 

 somewhat different stages of their development at any given 

 time, even though they were all sown on the same day, those 

 supplied with large quantities of nitrate being less advanced 

 than the rest. If they could all be kept under constant con- 

 ditions till they had ripened this difference might finally 

 disappear, but in crop production it is not possible much to 

 delay the harvest owing to the fear of damage by autumn 

 frosts, so that the retardation is of great practical importance. 

 Seed crops like barley that are cut dead ripe are not supplied 

 with much nitrate, but oats, which are cut before being quite 



