382 
On the Potato Disease, 
powerful agent, under peculiar conditions of the atmosphere, in 
accelerating the decay of the potato-plants, but I am of ojiinion 
that it will not attack them except they are predisposed, nor 
spread itself except in a close humid atmosphere, for another 
plant kept all the time in tlie same house, but not inoculated, has 
not had a blotch ujx>n it. On the 19th of August I observed a 
tomato-apple with a blotch similar to those on the potatoes, and, 
having placed it in my book-case, on the 30th I perceived a thick 
crop of Botrytis infestans upon it in full fructification, the sporidia 
being so distinct that I could discern tlie internal structure of many 
of them, and even the points at their summits (Fig. 3. a.). The 
leaves, stalks, and fruit were blotched like those of ihe potato, bot!i 
this year and last ; but although they have a somewhat greyish 
appearance underneath, I have never yet found the botrytis upon 
the leaves, notwithstanding I have placed them in situations likely 
to promote its growtii, which seems also to negative the idea of 
the blotches themselves being produced by botrytis. On examin- 
ing the blotch on the fruit a few days later, I found a thick crop 
of another (Fig. 4.) species of fungus, with globose sporidia on 
short stems without branches. 
I have also found the Botrytis infestans growing on the inside (Fig. 
5 and 6) of the tomato from which I had cut a diseased portion. 
I have entered thus fully into particulars relating to the botrytis, 
as it has, I believe, been everywhere manifest on the potato-leaves 
this year, but I have seen nothing to shake my conviction that, 
although a powerful one, it is only a secondary agent in promoting 
their decomposition. 
Tlie mildew on the specks called the scab, of which 1 have 
figures, is also only occasionally developed — indeed it is difficult 
to find a specimen of it. The scab being a cutaneous afiection, 
the specks are merely the result of some improper accretion being 
thrown off, and I suspect that the small white spots observable 
upon tlie skins of some varieties (as upon apples) denote the posi- 
tion of glands provided for some important purpose, and the im- 
pure fluid which escapes at these points becomes the favourite 
pabulum of a little dainty parasite. 
I have, however, another species to notice even more minute 
than the Botrytis infestans, but (as it appears to me) jierforming an 
office on the underground stem at least as destructive as that species 
on the leaves. It has been supposed, from the fact that the under- 
ground stem has been frequently found in a scurvy unhealthy con- 
dition previous to tiie disease being seen in otiier parts of the plant, 
that it invariably breaks out tliere first. There are, however, many 
exceptions to this ; imleed, such are the variations of latitude, soil, 
heat, moisture, &c., under which the plant is grown, and by which 
it is undoubtedly aliected, that it would probably be as easy to 
