Canker, a disease to which trees are subject; it proceeds principally from the nature of the soil, and causes the bark to decay. If the canker be seated in a bough, and a large one, the general practice is to cut it off at some distance from the stem ; if a small one, close to it.

When the tree is thus open and exposed, it is liable to receive injury from the air, moisture, and insects. To prevent this, white lead and boiled oil, made into a kind of thick paint, with the addition of sublimate of mercury, has been recommended by Dr. Darwin, as an useful remedy, especially when applied to the wounds of those trees, the wood of which contains less acrimony, and is consequently more liable to be penetrated, and eaten by a large worm or maggot, that would otherwise consume the whole internal wood.

In the 13th vol. of the Transactions of the Society of Arts, etc. the ingenious Mr. Bucknall observes, that, in pruning, this medication ought never to be omitted, as experience has demonstrated, that mercury removes the noxious effects of canker in the more delicate fruit trees, so effectually as to influence the vegetation of plants, by affording both smoothness and a free growth to the bark.

He directs every stump, together with the decayed or blighted branches, and all those that cross the infected tree, or where the leaves curl, to be taken off smooth and even ; the gum is likewise to be pared down close to the bark, and rather a little within it, but not so as to destroy the rough coat; the fissures, out of which it oozes, are next to be opened to the bot-- torn, the blotches to be cut away, and the canker extirpated: all the wounds are then to be anointed with the medication, a little being smeared over that part of the canker which was not large enough to be cut. The tree must also be scored, and the moss rubbed off; but care should be taken to avoid breaking off a single branch, as this would be productive of dangerous consequences.

"A tree thus managed (says Mr. Bucknall), will, with its remaining free shoots, run large; which, as they require a great flow of sap, will keep the roots in constant employ, and thus necessarily 'establish it in permanent health." He also remarks that, where the sole object is to remove the canker, hog's lard will be found of consi-dcrable utility; but, if wet also is to be guarded against, it is by no means so beneficial as tar.

There is also another method of curing this disease, which has been tried with success; namely, where a branch of a valuable tree is likely to be destroyed by the canker, to inclose the affected part, and some inches above it, in a garden pot of earth, previously divided, supported by stakes, and tied together round the branch, which will then strike roots in the mould; and which, after some months, may be cut off, and planted in the ground : thus preserved, it will produce a new tree.

Canker. - The general opinion respe6ting the cause of this disease is, that it proceeds chiefly from the nature of the soil, as stated vol. ii. p. 432. Mr. Forsyth, however, proves from experience, that it originates from the following circumstances, namely : injudicious pruning; leaving the foot-stalks of fruit on trees after it has been gathered ; bruises arising from the use of ladders in collecting fruit; nailing trees against walls, with too tight trelisses ; wet autumns, which prevent the young wood from ripening, and are succeeded by severe frosts that kill the shoots; birds and insects devouring the buds; and, lastly, from carelessly leaving dead shoots on trees, throughout the summer.

From whatever cause the canker may arise, Mr. Forsyth directs all the diseased parts to be cut out, and the composition to be applied, in the manner described p. 238 of the present volume. Should any gum be observed to exude after such excision, Mr. F. states it to be a certain criterion, that the canker is not completely extirpated : it will, therefore, be necessary to repeat the operation as speedily as possible; for, if these defects be suffered to remain, the whole tree will be overspread with canker and gum; so that it must speedily perish.

Apple-trees are peculiarly liable to this distemper; in consequence of which their value, together with that of their fruit, is greatly diminished. To prevent the total loss of the trees, Dr. Darwin suggests the ingenious expedient of renovating the diseased bark, by paring its edges to the quick, and carefully adapting a piece of sound bark taken from a healthy tree of inferior value ; the whole being secured with a flannel roller, or other elastic bandage.