Nature and Cause of the Potato Disease. 
331 
The upper leaves retained their colour belter than the lower, 
for at this period the lower leaves were dead, and a weeping and 
exudation was observable at the leaf-slalks. At the end of the 
fourth week I removed the shade and examined the external fungi, 
and stem and leaves also. The external fungus was the botritis, 
and it had spread over all parts of the stem and leaves. I dis- 
sected the stem and found an abundance of boleti and radiated 
fungi in the cellular tissue of the medulla, but could detect no 
trace of the botritis internally, or animalcule of any kind, putres- 
cence having hardly set in. Now, in this case we have the leaves 
and stem of a plant apparently healthy generating fungi, or at all 
events rendering them sensible to us. Had this plant been grow- 
ing in a proper soil and atmosphere would fungi have appeared ? 
I answer no ; because I had the fellow plant with several others 
still growing healthily without any appearance of fungi. From 
whence, then, came the fungi if we repudiate equivocal genera- 
tion ? We have found the seed of the botritis in the earth, and 
we now find an apparent generation of them, with that of other 
fungi as well ; and we further find that they appear according to 
their character in certain kinds of matter, having regard to the 
state or condition of the matter they appear in. This question 
of apparent generation can only be answered by supposing that 
the germs of the various fungi existed in the plant, and that they 
act only when its vitality ceases. And thus being ever present 
they are ready to begin their duties when the condition of matter 
in which they appear is fitted for them ; hence their appearance 
in certain states of matter. This view is more in accordance with- 
the facts than any other, for we have seen that the various kinds 
are readily generated, and therefore we must either suppose their 
germs present or adopt the doctrine of equivocal generation. The 
precise view we take is, however, of little moment, seeing that the 
facts are always consistent with each other ; and as these facts 
sufficiently bespeak the nature of fungi, and inform us that they 
are secret and solitary in their habits, and that they cannot bear 
the light of the sun, we may infer that they have no power to 
destroy healthful vegetation, seeing they can only act conditionally, 
and consequently that they are not the cause but an effect of the 
disease. 
With regard to the use of the tubers for seed as being the 
cause of the disease, I would observe that the plants grown from 
seed were as much diseased as those groAvn from tubers ; but were 
this fact not so, it is incorrect to view the tuber as a part of the 
plant, as if it were a mere slip or cutting grafted on a foreign 
stock ; and thus suppose that, its powers decline as the parent 
from which it sprung grows old. 
The tuber contains distinct germs, and these germs are as much 
VOL. VII. 2 A 
