320 
Nature and Cause of the Potato Disease. 
one. The sound stem originated in a germ somewhat lower in 
the tuber. On dissecting the old tuber or set I discovered that 
the two diseased stems were the offspring of diseased germs, and 
that the sound stem sprang from a sound germ. Previously to 
dissecting the old tuber or set I carefully examined it, but could 
discover no trace of disease from its external character. The 
cuticle was clean and healthy, and no external examination could 
possibly have detected the disease existing among the germs. 
The state of the set indicated the disease to have existed from 
the last season, for the infected parts were deep seated ; and as 
no abrasion of the cuticle could be found in any part of the tuber, 
its origin could not proceed from any external or outward influ- 
ence acting on the set. Another tuber or set to which two stems 
were attached, contained one sound and one diseased stem, which 
I as carefully and microscopically examined as the former one, 
and with the same results. The diseased stem sprung from a 
diseased germ, and the sound stem from a sound germ. This 
cause and effect so plainly revealed in these instances, induced 
3ne, notwithstanding the labour, to continue the examination with 
the remainder of the plants, the stems of which were all visibly 
diseased, and I met with the same unvarying results ; for the 
whole of the diseased stems were the offspring of diseased germs. 
The sets from which these plants grew were all of them out- 
wardly sound, and from their appearance I judge they had been 
selected with great care for the purpose of seed. The stolons on 
the diseased stems were dead, and the growth of the tubers con- 
sequently stopped ; but to the sound stems sound stolons existed, 
to which were attached tubers: these I examined, both from the 
sound and diseased stolons, but could discover no trace of fungi, 
insect, or any indication whatever of disease. 
In the last season's plant fungi were generally discovered by 
several authorities ; among whom may be mentioned the Rev. 
J. M. Berkley, Prof. Morren, and M. Payen. These gentlemen 
found the botrites fungi invariably preceding the disease, in the 
specimens they examined. But in many plants that I examined 
I failed to detect them, although the tubers were much diseased. 
In the examination of this year's plants I discovered the bolritis 
in some of the stems, and externally on some of the leaves, while 
in others no trace was perceivable. I have said that the cause of 
the diseased state of the heaves is referable, and may be traced to 
the stem, and that the stem, so far as the affection of the leaves 
is concerned, is the seat of the disease. Let us now see what 
evidence we have to support this view of the case. In all the 
examinations that I have made of the superior parts of a diseased 
plant I have invariably found the medulla of the stem to be the most 
seriously affected ; and in many instances where the leaves pre- 
