202 
DISEASES AND AFFECTIONS OF PLANTS. 
white matter, if carefully removed by dissecting off a thin transparent portion of the 
cuticle, forms, under a powerful lens, a most beautiful object : there is the network, 
like a web, already alluded to ; but in addition, many series of delicate cellular stalks 
are seen ascending, as from a common centre, and some of these support at their 
points little oval spores. It is requisite to throw a strong light from a speculum, 
beneath, and by this aid these fibres, or stems, if moistened by the breath of the 
observer, are seen to be hygrometric, insomuch that they revolve, forming upon 
their centre of motion regular circumgyrations, during which movements several of 
the spores explode. 
It is not here necessary to investigate the causes of a malady which has occasioned, 
and still produces so much solicitude ; but as the phenomena appear to be altogether 
new, and so many hypotheses have been hazarded to account for what lies buried in 
mystery, it may not be irrelevant to make some allusion to them. Most writers 
who look upon the disease as the result of a fungus, have referred to the genus 
Botrytis, No. 2481 of Loudon's Catalogue, so called from the clusters of globular 
seed-vessels which are attached to the fibres. The question is one of extreme 
difficulty ; but whatever may be the genus of the parasite, if we admit that the 
potato disease depends upon such a cause, the activity of the poison is beyond all 
precedent. The suffusion of meal extends a few lines beyond a black spot, but is 
traced no further ; yet the plant is completely destroyed in a few hours, and becomes 
entirely decomposed, precisely the same in degree as if it had been sprinkled by a 
corrosive or boiling fluid, emitting, at an early period, the odour of frosted kidney- 
beans. Is there, then, upon record one solitary fact which tends to prove that any 
living vegetable has been destroyed by a process so deadly, as that which has now 
been going on during many weeks, under the ascribed agency of a fungus ? 
Mould — Mouldiness. This forms, or is deposited, it should seem, upon all 
organic matter. Organised substances, whether living and growing, or deprived, to 
all outward appearance, of the vital principle, are redolent of life ; and mould as 
certainly is deposited upon a piece of leather as upon and round the wounded peel of a 
plum, a pear, or an apple which has been pierced by an insect. Where shall we look 
for the exciting cause ? Do the vitalised atoms (spores) of Erycibe, Uredo, JEcidium, 
Puccinia, &c, &c, which fix and prey upon the vegetables of the garden, the cereal 
crops of the farm, and the leaves of ornamental plants, float in the air, and in due 
season attach themselves, each to its appropriate medium of support ? or does the 
altered condition of the plants, or other organic body, under the influence of the 
great natural agents, give birth to, or bring to light as educts, the several genera of 
this lowest order of the vegetable creation ? The inquiry may be useful : it may 
indeed reveal the proximate causes of phenomena which are as yet involved in mys- 
tery ; but in the present state of our knowledge we can only presume that mildew 
and other minute fungi originate in a changed or diseased condition, but that they 
are not in themselves the primary exciting causes of disease. 
Dr. Liebig has said, " The microscopical examination of vegetable and animal 
