28 PLANT DISEASES 



The former occurs in tropical and South Africa, the latter 

 in Natal. The same parasite also occurs on leaves of an 

 unknown plant from Natal. This parasite again is so close 

 to that of the coffee leaf disease, that it is almost certain, 

 opportunity afforded, the parasite would pass from one to 

 the other. To start a coffee plantation in the neighbour- 

 hood of the Vangueria would in all probability mean 

 disaster. Allied plants usually require the same food con- 

 stituents, whereas plants belonging to different Orders in 

 many instances do not rob each other of the same kind of 

 food derived from the soil, but actually mutually benefit 

 each other. The value of rotation of crops turns mainly 

 on this feature in plant life. It has been known for 

 centuries that the fertility of the soil could be increased 

 by the growth of leguminous plants, and Dr. Watt — Pests 

 and Blights of the Tea Plant, p. 169 — has shown that 

 leguminous trees, as Acacia dealbata, Dalbergia assamica, 

 Albizzia stipulata {sau tree), and others belonging to the 

 same family, proved highly beneficial to the tea plant by 

 enriching the soil, in addition to serving the function of a 

 shade tree. 



Finally, success in eradicating plant diseases depends 

 almost entirely on universal co-operation. It is compara- 

 tively waste time and money attempting to hold a particular 

 disease in check, especially in those instances where spores 

 are carried by wind, if your neighbour, whose plants are 

 suffering from the same disease, does not exercise ordinary 

 precautions. 



