PLANT DISEASES AND FUNGI.
29
has the power, in greater or less degree, of destroying all the vege-
table parasites which infest the skin, such as the different forms of
ringworm. When applied to the 'thrush' of early infancy sulphur
rapidly effects a cure, as I have found in a number of cases, etc."
Now all these diseases of the human subject are attributed to the
action of external fungus parasites, and, in all, the same remedy as
that found beneficial for epiphytal fungi has been employed with
success.
The other class, or endophytal, are more numerous, insidious,
and less subject to external treatment. The disease is present in
the tissues long before it makes its appearance on the surface, or
even before it produces a sickly habit, and when at length the pres-
ence of the foe is unmistakable, all the mischief has been done,
and the infested plant is more or less permeated with the disease.
To this class belong the corn rust and mildew, the potato disease,
the apple scab, and a host of other destructive agents.
The modes by which these internal parasites perpetuate their
species are rather elaborate, and must be taken into account in all
attempts to combat them. The rotting moulds of the genus Pero-
nospora generally produce a speedy decay of the tissues, as, it may be
believed, by a kind of fermentation. The initial conidium, or spore,
is elliptical and uncoloured; the contents are at first granular, then
they become divided into four parts, each of which is gradually
surrounded by a thin membrane. When matured, the membrane
of the mother cell is ruptured, and the four daughter cells are set
free, each furnished with a pair of movable cilia at one extremity,
by means of which they are capable of active motion in a thin film
of moisture, such as would lie upon the surface of a leaf after a
shower. After a short period of activity these little zoospores come
to rest, the cilia are absorbed, and they commence germination by
the protrusion of a delicate thread, which enters one of the stomata
of the leaf, and becomes the first thread of mycelium, in the interior
of the tissues of the plant, which is thereby infected. Ultimately
this mycelium, being largely increased, sends up erect threads
through the openings of the leaf, such threads being usually much
branched towards the apex, and bearing at the top of every
branchlet an elliptical conidium or spore, just like the original
conidium with which the series commenced. Each of these spores
falls away when mature, either upon the supporting leaf or upon the
ground, and then the differentiation into zoospores, and all the suc-