This disease is accompanied by different symptoms, according to the species of the tree which it infects. In some of those whose true sap contains a considerable quantity of free acid, as in the genus Pyrus, it is rarely accompanied by any discharge. To this dry form of the disease it would be well to confine the term canker, and to give it the scientific name of gangraena sicca. In other trees, whose sap is characterized by abounding in astringent or mucilaginous constituents, it is usually attended by a sanious discharge. In such instances it might strictly be designated ulcer, or gangraena saniosa. This disease has a considerable resemblance to the tendency to ossification, which appears in most aged animals, arising from their marked appetency to secrete the calcareous saline compounds that chiefly constitute their skeletons. The consequence is, an enlargement of the joints, and ossification of the circulatory vessels and other parts, phenomena very analogous to those attending the cankering of trees.

As in animals, this tendency is generally throughout their system, but as is observed by Mr. Knight, "like the mortification in the limbs of elderly people, it may be determined as to its point of attack by the irritability of that part of the system".

This disease commences with an enlargement of the vessels of the bark of a branch or of the stem. This swelling invariably attends the disease when it attacks the apple tree. In the pear the enlargement is less, yet it is always present. In the elm and the oak sometimes no swelling occurs; and in the peach I do not recollect to have seen any. I have never observed the disease in the cherry-tree, nor in any of the pine tribe. The swelling is soon communicated to the wood, which if laid open to view on its first appearance by the removal of the bark, exhibits no marks of disease beyond the mere unnatural enlargement. In the course of a few years, less in number in proportion to the advanced age of the tree, and the unfavourable circumstances under which it is vegetating, the swelling is greatly increased in size, and the alburnum has become extensively dead; the superincumbent bark cracks, rises in discoloured scales, and decays even more rapidly than the wood beneath. If the caries is upon a moderately-sized branch, the decay soon completely en-, circles it, extending through the whole alburnum and bark. The circulation of the sap being thus entirely prevented, all the parts above the disease of necessity perish.

In the apple and the pear, the disease is accompanied by i scarcely any discharge; but in the elm this is very abundant. The only chemists who have examined these morbid products, are Sir H. Davy and Vauque-lin; the former's observations being confined to the fact, that he often found carbonate of lime on the edges of the canker in apple trees.

Vauquelin has examined the sanies discharged from the canker of an elm with much more precision. He found this liquor nearly as transparent as water, sometimes slightly coloured, at other times a blackish-brown, but always tasting acrid and saline. From this liquor a soft matter insoluble in water is deposited upon the sides of the ulcer. The bark over which the transparent sanies flows, attains the appearance of chalk, becoming white, friable, crystalline, alkaline, and effervescent with acids. A magnifier exhibits the crystals in the forms of rhomboids and four-sided prisms. When the liquid is dark-coloured, the bark appears black-ish, and seems as if coated with varnish. It sometimes is discharged in such quantities as to hang from the bark like stalactites. The matter of which these are composed is alkaline soluble in water, and with acids effervesces. The analysis of this dark slimy matter shows it to be compounded of carbonate of potass and ulmin, a product peculiar to the elm. The white matter deposited round the canker was composed of Vegetable matter .... 605 Carbonate of potass . . . 342 Carbonate of lime .... 50 Carbonate of magnesia . . 3 Vauquelin calculated from the quantitv of this white matter that was found about the canker of an elm, that 500 pounds weight of its wood must have been destroyed.

There is no doubt that such a discharge is deeply injurious to the tree; but the above learned chemist appears to have largely erred, for he calculated from a knowledge of the amount of the saline constituents in the healthy sap, whereas in the diseased state these are much and unnaturally increased. I once was of opinion that the disease does not arise from a general diseased state of the tree, but that it is brought on by some bruise or injury, exasperated by an unhealthy sap consequent to an unfavourable soil, situation, and culture; but more extensive and more accurate examinations convince me, that the disease is in the tree's system; that its juices are vitiated, and that disease will continue to break out independent of any external injury so long as these juices continue peccant and unaltered. This conclusion will be justified, I think, by the preceding facts, as well as by those distributed through the following pages.

The disease is not strictly confined to any particular period of the tree's age. I have repeatedly noticed it in some of our lately introduced varieties that have not been grafted more than five or six years; and a writer in the Gardener's Magazine, vol. v., p. 3, states, that the trees in his orchard, though " only of four years' growth, are sadly troubled with the canker."Although young trees are liable to this disease, yet their old age is the period of existence most obnoxious to its attacks. It must be remembered that that is not consequently a young tree which is lately grafted. If the tree from which the scion was taken be an old variety, it is only the multiplication of an aged individual. The scion may for a few years exhibit signs of increased vigour, owing to the extra stimulus of the more abundant supply of healthy sap supplied by the stock; but the vessels of the scion will, after the lapse of that period, gradually become as decrepid as the parent tree. The unanimous experience of naturalists agrees in testifying that every organized creature has its limit of existence.

In plants it varies from the scanty period of a few months to the long expanse of as many centuries; but of all the days are numbered; and although the gardener's, like the physician's skill, may retard the onward pace of death, he will not be permanently delayed. In the last periods of life they show every symptom that accompanies organization in its old age, not only a cessation of growth, but a decay of former development, a languid circulation and diseased organs.