frequently before the tree can overcome the resistance of the bark, it is materially injured in its health, and when it fails altogether, dies. It may be observed, from the straps on wall trees, that a very slight stricture checks the growth of a tree or branch, and produces canker.

And it may be observed, that fruit trees seldom bear well till they burst the bark, and then they are soon after infested by the vermin; so that they never are, at best but a few years, in what can be called a good bearing state.

A question has been started, whether the vermin is the cause or effect of the canker? The vermin certainly is not the general cause of the canker, though I believe they may sometimes produce it topically; nor is

the canker the exciting, but only the predisposing cause of the vermin; as is the rending of the bark, wounds, injuries, &c. and I do not find that these produce even topical canker, in a tree not bark-bound. So wounds on the animal body in good habit, heal up without any bad consequence; whilst, on a bad habit, the slightest scratch often becomes serious, and sometimes fatal.

The stricture of the bark, however, must be viewed in a very different light; for it is undoubtedly the general cause of the canker, by not allowing the juices to disperse freely over the tree, which bursting out at any weak part, form with the bark, this ragged substance called canker. For we know, that all stagnant animal and vegetable juices, when exposed to the air, are much given to putrefaction, and communi

cate it to, and promote it in, other bodies. Accordingly, we observe the sap of the tree first breaking out like sweat upon the surface of the bark, soon corroding the outer bark, next communicating the disease to the periligneum and the wood.

Sometimes contractions, indurations, and rotting of the bark, are produced without this ragged appearance, which are equally dangerous, as they are effects of the same

cause.

And it will be found that the disposition of the trees to canker, is, ceteris paribus, in proportion to the transverse strength or power of the bark, i. e. the aggregate power of the transverse bark, and the transverse cohesion of the longitudinal, taken together. So the disposition to canker will be,

as the transverse power of the respective barks to the force exerted by the growth of the tree. This force it will be difficult to ascertain; but it cannot be thought irrational to suppose it, under similar circumstances, nearly the same. This seems to be supported by what follows.

The relative powers of the different barks can be ascertained pretty accurately, when it will be found that the trees most given to canker, and those which continue longest in an apparent healthy state, without bearing, have the strongest bark transversely, and that those which have the weakest bark transversely, are the greatest bearers, and least given to canker.

This both accounts for the canker, and "how full-grown fruit trees, especially

some of the finer sorts of French pears, which, though apparently in a very healthy and luxuriant condition, are yet in a state of almost total barrenness." For the very cause which, at last, destroys them altogether, preserves them longer in the apparent healthy state, viz. the strength and firmer cohesion of the bark, which continues to stretch longer without bursting, and forming residence for the vermin, than those of weaker cohesion; yet they are really and truly bark-bound, which prevents them receiving that nourishment fit for bearing fruit. And it may be observed, that these trees, though they may continue for a number of years in this apparent healthy state, at last break out into one universal canker.

As the stricture of the bark is the cause