154 PLANT DISEASE 



percentage of sugar. Readers familiar with 

 locusts will say they are very fond of sugar, 

 and they may be, but sugar is a chemical com- 

 pound governed by the iron and nitrogen of 

 the plant. We have the testimony of Mr. 

 Francis that locusts did not injure crops 

 growing on soils rich in iron. Then we have 

 the fact that locusts would not eat pea plants 

 when devouring everything on the surround- 

 ing country. 



Peas are said to extract nitrogen from the 

 air, and they may be more potent for extract- 

 ing inert iron out of the soil ; in any case, the 

 fact remains that the plant with a greater ten- 

 dency to normality than its neighbour remained 

 immune to this insect. 



So also sweet potato vines which have the 

 power of producing more sugar than the 

 average plant, and consequently likely to be 

 richer in chlorophyll products, have a tendency 

 to remain immune to this pest. 



It seems clear, then, that as the locust attacks 

 by preference those plants which are less likely 

 to be rich in chlorophyll, the best protection 

 against their inroads lies in the provision of a 

 normal soil capable of providing the plant 



