THE REQUIREMENTS OF PLANTS 



33 



TABLE X. BROADBALK WHEATFIELD, AVERAGE YIELDS, FIFTY-SIX YEARS, 1852-1907. 



The increasing effects produced up to a certain point by successive 

 increments of nitrogen may be due to the circumstance that the ad- 

 ditional nitrate not only increases the concentration 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 re- 

 sembles autocatalysis, where one of the products of the reaction acts as 

 a catalyser and hastens the reaction. The increase does not go on 

 indefinitely because some limiting factor steps in. 



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

 Table IX. it is seen that the grain formed, when nitrogenous 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 show no further rise. Indeed other experiments prove that 

 excess of nitrogenous 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 : abundance of nitrogen, on the 

 other hand, leads to a bright green colour, to a copious growth of soft, 

 sappy tissue, liable to insect and fungoid pests (apparently because of 

 the thinning of the walls and some change in composition 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 conditions 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 ripe, can receive larger quantities. All cereal crops, however, pro- 



