DISEASES OF PLANTS ILLUSTKATED BY THE POTATO MURKAIX. 179 



General attention was first attracted to the subject in particular 

 countries in 1840, which extended in 184o to the whole of P^urope. 

 Since the former date disease has been more or less prevalent, 

 thouo-h the climatic conditions have been as various as possible. 

 No especial variety has been entirelj^ exempt, or indeed has uni- 

 versally exhibited the disease in a milder form. No single kind of 

 soil has been entirely free, or even peculiarly privileged. In the 

 nei"'hbourhood of Jena, for instance, cases have occurred wliere 

 potatoes have succeeded extremely well in ill-drained moors, 

 Avhile the crops in tlie neighbouring sands have been wortliless. 

 No particular situation seemed to make any difference, with the 

 single exception, and tliat of some importance, of the whole ex- 

 tent of coast on the Nortii Sea. as far as the immediate influence 

 of the sea-fog extended. Whether the crop was raised from seed, 

 from entire tubers, from sets or eyes, the result was just the 

 same ; or perhaps those crops which were immediately derived 

 from seed exhibited the worst form of the disease. Moreover, 

 the crops suffered indifferently, whatever peculiar place in the 

 usual rotation they might occupy, except indeed that the murrain 

 appeared the latest, and was first ameliorated, where potatoes, as 

 in Thuringia, are never planted, on a large scale at least, in fresh 

 manure. The first symptoms of disease appeared in the tubers 

 as soon as in the haulm, or even sooner. It is at least indubitable 

 that highly diseased tubers proceeded from perfectly sound 

 haulm, in which therefore the cause of the disease could not 

 reside, nor could it be propagated from the green parts to the 

 tubers. On the same plant sound and highly diseased potatoes 

 frequently occur, and in the same heap during winter sound 

 tubers are found amongst those which are quite putrid, from 

 whence I conclude that the disease is not connuunicated by con- 

 tagion. 



2. The first symptom of the disease consists of brownish dis- 

 coloured specks which extend from the superficial cells inwards. 

 The nitrogenous lining of the cells, which in sound tubers is 

 colourless and uniform, assumes the appearance of a pale yellow 

 membrane. As the disease increases, this membrane is finely 

 granulated, giving off reticulated threads in which a distinct 

 circulation is often visible. The nucleus at the same time be- 

 comes sharply defined ; the nitrogenous coat assumes a darker 

 tint, the starch-grains are absorbed exactly as in a sprouting 

 potato ; in short the phenomena are the same as occur generally 

 in germination, a circumstance which agrees remarkably with the 

 fact that diseased potatoes, when they do not pass too rapidly 

 into a state of putrescence, have a surprising tendency to germi- 

 nate, in consequence of which, after three or four weeks, they are 

 often covered with young tubers. 



VOL. vii. o 



