LA was Tee es patel fet Kagentys ULE: 
dry ; but in trees, such as the elms, whose natural 
sap abounds in astringent or mucilaginous con- 
stituents, it is usually attended with a sanious 
discharge. George W. Johnson, Hsq., proposes 
that the dry forms of the disease should be called 
Gangrena sicca, and that the discharging forms 
of it should be called ulcer or Gangrena saniosa. 
The swelling or enlargement of the vessels of 
the bark, which constitutes so conspicuous a 
symptom of some of the ordinary kinds of canker, 
| invariably and prominently attends it in the 
apple-tree, invariably but less prominently at- 
|, tends it in the pear-tree, frequently but not al- 
ways attends it in the elm and the oak, and very 
seldom, if ever, attends it in.the peach. “The 
swelling,” says G. W. Johnson, “is soon commu- 
| nicated 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 al- 
| burnum has become extensively dead ; the super- 
incumbent bark cracks, rises in discoloured scales, 
and decays even more rapidly than the wood be- 
neath. If the caries is upon a moderately-sized 
branch, the decay soon completely encircles it, 
extending through the whole alburnum and bark. 
The circulation of the sap being thus entirely 
prevented, all the parts above the disease of ne- 
cessity perish.” The first appearance of the dis- 
ease in the peach is so very slight, that an unex- 
perienced observer of it would suppose it to be 
of no consequence. Small brown circular spots 
constitute the whole of this appearance, and may 
easily be cut out with the knife, so as to let the 
subsequent vegetation be as vigorous as if they 
had never existed. But let the spots be forgotten 
_ for a few days; and, when the observer returns 
to examine them, they will be found to have 
spread far and eroded deeply,—perhaps to have 
surrounded the shoot, eaten down to its core, 
stopped its circulation, and extinguished its life. 
Canker was never observed by Mr. G. W. John- 
son—one of the best writers on the subject—in 
any tree of the pine tribe; and yet—though in a 
somewhat different form than in fruit-trees—it 
has been found to damage and devastate entire 
plantations of larch. In its first or latent stage 
in a larch-tree, small diseased-looking spots may 
be observed on removing the bark ; and if the 
tree be otherwise healthy, and its foliage luxuri- 
ant, the sap-vessels of the affected part will be 
found charged with inert and morbidly-secreted 
rosin. In amore advanced stage, the outer rind 
of the bark is slightly discoloured, the rosin ex- 
udes to the exterior of the bark, and the cortical 
layers become blistered and die. When the dis- 
ease is virulent, it encircles the branches, stops 
the circulation, and occasions the death of all the 
portions of the branches exterior to it from the 
~ CANKER. - 
stem; and, in some instances, it makes such fright- 
ful havoc upon all the branches of a tree, as to 
leave nothing but their stumps and the stem ; or 
eventually, it attacks the very stem itself, and 
extinguishes the organism of the whole plant. 
In its milder forms, however, it does not encircle 
the stem or branches, but permits the sap to arise 
and the cambium to ascend upon part of the cir- 
cumference ; and in these instances, the trees 
attacked by it, if otherwise healthy, continue to 
have a vigorous growth. “In some plantations,” 
says Mr. Drummond of Perth, “the disease may 
be seen on the main stem of individual trees, 
which continue to make rapid progress, and ap- 
pear to sustain no injury unless at the part af- 
fected. But in some places, such is now its 
malignancy that the whole bark becomes dis- 
eased, and many young plantations totally de- 
stroyed.” 
Sir Humphrey Davy found depositions of car- 
bonate of lime on the edges of canker on apple- 
trees; yet he made only very general observa- 
tions, and rather hastily inferred from them that 
the disease is occasioned by an excess of alkaline 
or earthy matter in the cambium. Depositions 
made by a sanious canker can hardly guide a 
judicious diagnosis of any instance of dry can- | 
ker; and, especially, depositions by a running 
canker in trees of a mucilaginons nature, cannot, 
with any propriety, be treated as indications of 
the nature of canker in trees with acidulous 
juices. Yet the copious sanies discharged by 
canker in the elm is—on its own account, if not 
in connexion with the disease at large—a sub- 
ject of very considerable interest ; and this was 
examined, with chemical precision, by Vauque- 
lin, This liquid is nearly as transparent as wa- 
ter; in some instances, very slightly coloured, 
and in others, of a blackish brown colour, but in 
all, having a saline and acrid taste. It deposits, 
on the sides of the ulcer, a soft substance, which 
is insoluble in water. The bark over which it 
flows acquires an appearance like that of chalk, 
becoming white, frangible, crystalline, alkaline, 
and effervescent with acids; and its crystals, 
when examined through a magnifier, are seen to 
be rhomboids and four-sided prisms. When the 
liquid is discharged in large quantities, the alka- 
line deposit assumes a somewhat stalactitic form ; 
and when it has a dark colour, the bark becomes 
blackish, and appears as if coated with varnish. 
The dark-coloured, slimy deposit is found, by 
analysis, to consist of carbonate of potash and 
ulmin,—the latter a proximate principle pecu- 
liar to the elm, yet very nearly identical with 
the humus which forms so large and important 
a constituent of fertile soil; and the white, 
erystalline, chalky-looking matter is found to 
consist of 60°5 per cent. of vegetable matter, 
342 of carbonate of potash, 5:0 of carbonate of 
lime, and 0°3 of carbonate of magnesia. 
The causes assigned for canker have been very 
various and conflicting, and the subject of much 
2a 
{— I. 
| 
