BOTANICAL SOCIETY OF CANADA. 
143 
sites there must pre-exist in the objects of their attack an unhealthy condition of 
structure, resulting partly from being deprived of some chemical element essential 
to healthy growth, and partly to atmospheric changes which tend to foster a too 
rapid formation of cellular tissue, at the same time that they favor the rapid devel- 
opment of the parasite. The result of these changes in the plant is a lowered vi- 
tality, rendering it prone to the attacks of the fungus, which, once having found a 
habitat, spreads with prodigious rapidity, and by setting in motion chemical changes 
similar to those already spoken of, soon involves the whole plant in decay. Ex- 
amples of this will be familiar to you, as in the case of the potato disease, which not 
many years ago brought England to the verge of famine, and in Ireland, which de- 
pends almost solely on this crop, was the cause of untold misery and destitution. 
The failure of the vine crops in Spain and Portugal was owing to the ravages 
of another species, the Oidium Tuckeri ; and in some seasons the wheat crops in 
this country are to a great extent damaged or destroyed by another of these minute 
pests, which, under the name of mildew, often in the course of a single night, con- 
verts whole fields of waving corn into black useless rubbish. Dry-rot in timber is 
another example of the destructive power of this group. Nor are these the only 
commercial interests which thus suffer. The production of silk is often a complete 
lailure, owing to the silkworm being infested by a minute fungus, the Botrytis Bas- 
siana^ which, entering, probably by the spiracles or breathing apertures, insinuates 
itself into the blood-vessels and destroys the insect. Damp and want of cleanliness 
are found to be the causes of the attack. Other species again have been found in 
flies, beetles, eggs, in the air sacs of birds, on fishes, reptiles, and animals, the men- 
tion of which would encroach too much upon your time. A great part of those 
which have received distinct names, as well as nearly the whole of those from the 
human subject, I have proved to be mere initial or imperfect forms of one or two 
common species of mould which occur everywhere upon decaying organic matter, 
as cheese, apples, oranges, &c. The number of plants thus degraded from the rank 
of species is about thirty-four, and I doubt not that many others might be placed 
in the same category. 
The first discovery of a vegetable parasite on man was, as I have said, made 
by M. Schoenlein, of Berlin, while examining the crusts from the head of a person 
affected with favus (Porrigo lupinosa or scald head). The plant has been since 
known under the name of Oidium Schoenleinii. Another parasite was subsequently 
discovered in the hairs of persons affected with the disease termed plica polonica — 
also a similar one in ulcer was found by Mr. Robin. 
Others have been found in Tinea, Porrigo, Pityriasis, Lichen and Sycosis, &c., 
&c. Others again in the lungs and on the mucous surfaces of the body. Now the 
whole of these are referable to a common origin ; the characters which have caused 
them to be raised to the rank of species being due to the plant having been retained 
