30 PRESENT DISEASE IN THE POTATOE. 



to a sultry sun, when they will wither and die. In this 

 case I have observed a most striking difference between 

 the plants in the morning and evening, and in two or three 

 days I have seen the whole crop die down and rot away. 



(110.) Previously, however, to the collar of the plant 

 actually exhibiting the gangrene, it swells, and is much 

 distended with water, and if scratched with a nail at this 

 time the preternatural wetness is more apparent. This 

 fact is of very general occurrence ; and it is not only 

 shown in the potatoe, but in all other plants affected by this 

 disease, which I shall hereafter have to describe. 



(111.) I am not quite certain whether the over-distended 

 cells burst, or whether they die in consequence of the ex- 

 cess of the aqueous material ; but I am confident that this 

 distension is speedily followed by the destruction of the 

 part. 



(112.) When, however, the malady does attack the 

 plant in this vital situation, it is not necessarily so destruc- 

 tive, for a portion only of the under-ground stem may 

 perish.' In this case the effect upon the leaves and above- 

 ground stems is more limited, and the leaves will partially 

 die. 



We have thus seen that the disease has two general 

 modes of proceeding ; in one of which the disease proceeds 

 from above downwards, in the other the plant is affected 

 from below upwards. 



(113.) In either case the tube-bearing stems eventually 

 take on the gangrene, and the tubers become isolated from 

 the roots, the leaves, and other parts of the plant. 



(114.) The tubers themselves at last become more or 

 less diseased, and sometimes the disease may be seen ex- 

 tending from the tuberose stem to the tuber. In some 



