Let us now consider the diseases of plants, as a study progressive to those of man; and as their physiology has so extensively served us, we may possibly also find in their pathology much material for instruction; not that it will be attempted to shew that the same diseases affect both kingdoms, but that diseases, though dissimilar in effects, may have similar sources. Unfortunately, there are not many men in this country, who need go further than their own gardens to find abundance of disease among their fruit trees and vegetables. The vine, the apple and the potato, common to most gardens, will furnish specimens. It is an error of a serious kind to suppose, that the parasites which infest plants are not essentially the cause, or, perhaps, more properly speaking, the elements of disease. I confine myself here to disease of parasitic origin, as that is the subject of which I am chiefly treating. That parasitic growths are the elements of disease in some instances, is now beyond dispute. The experiments of Mr. Hassall, detailed in Part II. of the Transactions of the Microscopical Society of London, are most conclusive; and they are of that simple nature, that any one may convince himself of their accuracy, by a repetition of them from the directions there laid down. He says, the decay is communicable at will "to any fruits of the apple and peach kind, no matter {149} how strong their vital energies may be, by the simple act of inoculation of the sound fruit with a portion of decayed matter, containing filaments of the fungi. We may use with success the sporules of such fungi; but in this case the decomposition does not set in so quickly; in the one case, the smaller filaments of the fungi have advanced several stages in their growth; while in the other, the sporules have yet to pass through the several stages of their development." Mr. Hassan, however, seems to speak doubtfully as to the mode in which the disease becomes naturally introduced;[60] how the spores enter the fruit, "is not very clear--though probably, it is by insinuating themselves between the cells of which the cuticle is composed, or perhaps by means of the stomata, where they are present. I may here state that the experiments were made on fruit, while living, and attached to the tree." But why should there be a doubt as to the parts by which the sporules of minute fungi enter the plant, when it is clear, that not only can they enter {150} by the spongioles, but by the stomata of the leaves, and mingle with the sap. It is true, that they make their appearance and grow upon the leaves and the fruit; but these are the situations most adapted for their fructification. I have seen the spores of the fungi which attack the cucumber and vegetable-marrow, in the cells of the hairs, and even their filamentous prolongations; these appropriate the fluids conveyed to the cells of the hair, rupture them, and at length fructify.