THE REQUIREMENTS OF PLANTS 33 



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- 

 duce too much straw if the nitrate supply is excessive, and the straw 

 does not commonly stand up well, but is beaten down or " lodged " by 

 wind and rain. Swede and potato crops also produce more leaf, but not 

 proportionately more root or tuber, as the nitrogen supply increases ; 

 no doubt the increased root would follow, but the whole process is 

 sooner or later stopped by the advancing season the increased root 

 does in fact follow in the case of the late-growing mangold. Toma- 

 toes, again, produce too much leaf and too little fruit if they receive 

 excess of nitrate. On the other hand, crops grown solely for the sake 

 of their leaves are wholly improved by increased nitrate supply : 

 growers of cabbages have learned that they can not only improve the 

 size of their crops by judicious applications of nitrates, but they can 

 also impart the tenderness and bright green colour desired by pur- 

 chasers. Unfortunately the softness of the tissues prevents the cabbage 

 standing the rough handling of the market. 



Three cases are illustrated in Table XL : wheat shows increases in 

 straw greater than those in grain as the nitrogen supply is increased ; 



