130 OBSERVATIONS ON THE DISEASES OF PLANTS 



In process of time the intercellular passages, which normally 

 are filled with air, are injected with a yellow fluid. Tliis takes 

 place even in the vessels, when once tliey are attacked by the 

 disease. In tliis fluid sharply defined granules soon appear, and 

 at length mycelium arises both in the intercellular passages and 

 cells. The individual cells separate from each other, their walls 

 become attenuated, and at length disappear. In general the 

 starch-grains remain involved in fungal threads, and the wliole 

 dries up into a hard white mass, or if the decay is more rapid, 

 the amylum is destroyed, so that frequently nothing remains of 

 the whole potato except a thin sac filled with a fetid fluid. 

 Parasites, whether insects or fungi, are never original, but de- 

 veloped on the diseased tissues. 



3. I shall now offer some remarks on the chemical condition 

 of the infected tubers. 



The juice of sound potatoes is slightly acid, whereas the dis- 

 eased spots exhibit at times a weak alkaline reaction. As the 

 disease increases, sometimes a decidedly acid, sometimes an 

 equally distinct alkaline reaction takes place, M-ithout any assign- 

 able reason ; in the latter case the circumstance does not depend 

 alone on the presence of ammonia. The liighly fetid tubers react 

 clearly on ammonia, as is of course the case in the decomposition 

 of nitrogenous substances. Tlie walls of the diseased cells are at 

 first blue when treated with iodine and sulpliuric acid, then, as 

 the disease is progressing, successively greenish blue, green, and 

 greenish yellow, and finally dark golden yellow or brownish 

 yellow. The nitrogenous matters become darker when treated 

 with iodine in proportion as the disease advances, and, according 

 to Liebig, consist no longer, or at least not principally, of albu- 

 men, but of casein. 



The starch grains, in all potatoes which are strongly affected, 

 even in the cells which still remain sound, exhibit a peculiarity 

 which distinguishes them from the starch of sound tubers. As 

 soon as it is dry, a quantity of broad cracks are formed within 

 them. This is never the case with sound potatoes, and proves 

 that these bodies in diseased tubers are far more saturated with 

 water than in those which are sound. 



When boiled there is also a great diflPerence. Diseased tubers 

 boiled for four hours remain as hard as at first ; if they are then 

 quartered, and the cortical layer removed, they remain unchanged 

 though repeatedly boiled, while sound potatoes from the same 

 field, after three hours, burst and separate into individual cells. 

 The starch in the cells of diseased potatoes when boiled swells 

 out into little gelatinous sacs, but these remain single and un- 

 connected in the cells, and do not long fill them out. 



Observations on diseased potatoes as compared with sound, by 

 Payen, Schacht and Janssen, Petzholdt, &c., give the following: 



