Albert Howard 389 



may entirely alter its value to the plant. This in turn would influence 

 the cell-sap throughout the shoot-system. How such alterations affect 

 the struggle between the protoplasm, on the one hand, and the hyphae 

 of an invading fungus on the other and why insects like Aphides thrive 

 on the juices of a Quetta almond tree grown in soil consolidated by over- 

 irrigation the previous winter and disregard it altogether under a different 

 system of soil management are interesting problems for the vegetable 

 pathologist eager to break new ground and to carry his science beyond 

 the beaten track. 



The examples quoted suggest another direction in which a knowledge 

 of the root-system is desirable, namely, in the determination of the 

 factors on which the disease-resistance of a unit species depends. A 

 collection of unit species grown under any particular set of soil conditions 

 generally exhibit among themselves marked differences in disease- 

 resistance. At Pusa, it has been found in several crops that an investiga- 

 tion of the root-system throws a considerable amount of light on this 

 point. Both in the rains and in the cold weather, deep-rooted varieties 

 yield less and are more liable to disease than shallow-rooted types. 

 Soil-aeration and its consequences will probably be found to be an im- 

 portant factor in this case. Many more examples of disease-resistance 

 in other parts of ^the world must, however, be examined before we can 

 say how far immunity depends on morphological root-fitness for the 

 environment and how far it is inherent in the natural resistance of the 

 protoplasm to the invasion of a parasite. 



