THE VIRUS DISEASES OF PLANTS 431 



stage. In this case the insect would correspond to the aecidial 

 host where the latter has ceased to be essential to the fungus, 

 as with black rust of wheat in India. The similarity to typhus 

 fever in many points is also worthy of note. 



These diseases may be hereditary, both in the plant (trans- 

 mission by seed in bean mosaic and the like) and in the insect 

 (spinach blight). 



The injurious effect of these diseases varies from almost nil 

 to total destruction : sugar-cane mosaic in Louisiana produces 

 such slight effects, even in cases of 100 per cent, infection, that 

 no reduction in yield of sugar has been proved, while spike 

 disease of sandalwood trees is invariably fatal and is exterminat- 

 ing this valuable tree throughout a large area. The disorder is 

 clearly a nutritional one in the first instance and the profound 

 alterations in the enzymic activity of diseased plants have been 

 chiefly responsible for the so-called enzymic theory of mosaic 

 disease. In a case like the potato, where Quanjer has recently 

 distinguished no less than four distinct diseases of the infectious 

 mosaic type, besides true leaf roll and a second leaf roll not yet 

 known to be infectious, it is evident that an immense amount of 

 work is still required before the effects on the metabolism of the 

 host can be clearly understood. 



The increasing attention being paid to this class of disease 

 by plant pathologists is not entirely the result (though in great 

 part) of improvement in diagnosis. In certain cases there is a 

 real extension going on, as in the sugar-cane mosaic, which has 

 appeared recently in several important cane-growing countries. 

 It is not suggested that any of them are of recent origin, as the 

 facts can be sufficiently explained by the great activity of 

 recent years in the introduction and testing of new varieties of 

 plants from all parts of the world. Sugar-cane mosaic has been 

 present in the Dutch East Indies for many years, and there is 

 little doubt that it has reached the New World with eastern 

 sugar-cane varieties introduced in considerable quantity in 

 recent years. As a class, the diseases mentioned in this paper 

 rank at present amongst the most destructive diseases of plants 

 known. 



