On reading the communication on leaf-blight, by Mr. Hooker, I was reminded of a paper on a similar subject by the late Andrew Knight. On referring to it, I find Mr. Knight's observations so nearly coincide with those recorded by Mr. Hooker, that it is probable the diseases observed by them are identical. The attack of a parasitic fungus (and as will probably hereafter be found of the same species,) seems to have been the immediate cause of disease in both cases. As so little seems to have been written hitherto by horticulturists on this important subject, I send you extracts from Mr. Knight's paper, considering that a statement of the facts observed by him, and of the experiments he made with a view to determine how the disease is communicated from one plant to another, and whether by any expedient it could be prevented or cured, might possibly prove interesting to Mr. Hooker and to the growers of pears generally. I may mention that Mr. Knight's paper was read before the Horticultural Society in 1815, and that the fungus which attacked his pear trees was known to botanists at that time by the name of Lycoperdon cancellatum.

"I observed, about seven years ago, a disease upon a few of the leaves of one of the pear trees in my garden at Downton. Bright yellow spots, from which a small quantity of liquid exuded, appeared upon the upper surface of the leaves in June; and subsequently, several conic processes, about one.third of an inch in length, were protruded from the same parts, but from the opposite surface of each leaf; and from these a large quantity of brown impalpable powder, consisting of very minute globular bodies, was discharged in August and September. These minute globular bodies I concluded to be seeds of a species of fungus; but as a few only of the leaves of my trees were affected, and no very injurious effects were visible, I did not take any measures to prevent their dispersion over my garden.

"I did not, however, long remain ignorant of the formidable nature of my new enemy: for within two years, every pear tree in my garden became in some degree diseased. The leaves only, at first, appeared to be injured; but the disease soon extended itself to the annual branches in many protuberant yellow spots, beneath which the bark was found to have acquired a bright yellow color; and as far as this color extended, the bark and the wood beneath it invariably perished, either in the same or following season, leaving wounds similar to those inflicted by canker, but less curable. The fruit also became diseased and worthless, and almost all the young shoots, when once attacked, perished in the following winter. These effects were not confined to my garden, but extended to the pear trees in an orchard which was two hundred yards distant, and I cannot entertain a doubt, but that the disease was communicated to these by seeds which had been conveyed by the prevalent west winds. I endeavored during the summer of 1813 and 1814, to check its progress in my garden, by picking off every diseased leaf; but I found all my efforts nearly abortive, and I have been obliged to destroy the greater part of my pear trees: those which remain have become annually more diseased, and I fear never can be ultimately preserved, unless a remedy for the disease can be discovered."

Mr. Knight tried the effect of sprinkling the leaves th quick-lime and fresh wood ashes, but he had not an opportunity of observing the te effect of these substances, for almost all the leaves fell off prematurely.

"Those of a single small standard pear-tree, on our of sulphur had been sprinkled, remained alive till late in the autumn; and upon these I did not observe the sulphur to operate in any degree, till the period at which the conic processes above mentioned would have appeared; but the yellow spots then became black, and perished, without affording seeds; whence I have reason to hope, that flour of sulphur will prevent, in some measure at least, the rapid extension of this disease.

"As the existence of this species of fungus appeared, three years ago, to be confined to my garden and a few pear trees in its vicinity, and to the hawthorn in an adjoining hedge (for it attacks the hawthorn as well as the pear tree,) I then thought it would be practicable to ascertain decisively the means by which it transfers itself from one tree to another: and this appeared to me to be an important object, because the habits of the Lycoper-don cancellation, and of the fungus which forms the rust or mildew of wheat, are, in many respects, very similar.

"I had so often tried, without success, to transfer the mildew of wheat, and other plants, from a diseased to a healthy subject, in the same season, that I had not any expectation of succeeding in an attempt of that kind; but I thought it not improbable that I might succeed in communicating this disease to seedling plants of the pear tree, having long ago satisfied myself that the species of fungus, which forms the mildew of wheat, always rises from the root of the plant.

"I have many years been in the habit of raising annually pear trees from seeds, with the hope and expectation of obtaining new and hardy varieties; and as the means I employ to obtain seeds well calculated for my purpose, necessarily cost me a good deal of time and labor, I have always planted them in pots, and in the kind of mould which long experience has pointed out to me as the best. This I have always obtained, at the period of sowing the seeds, in January or February, from the banks of a river at some distance from my garden; and in this mould my seedling pear trees always sprang up, and remained during the first season perfectly free from disease. In the spring of 1813, a portion of this mould, which I did not want, was intentionally placed very near some hawthorns and pear trees upon which the Lycoperdon cancellatum abounded, where it remained till the spring of 1814, when it was put into pots, and new seeds deposited in it. These sprang up as usual, and remained in perfect health till the end of May or beginning of June; when the fungus presented itself upon almost all the first true leaves of the plants, which leaves had composed the plumules of the seeds.

"That the fungus in this case rose from the ground, will, I think, scarcely be questioned; but it is necessary to state, that the seeds were all taken from trees which were not quite free from disease, and that I saw in the last spring some diseased plants, in a case where every precaution, except that of using new pots, (which had been my previous custom,) had been taken; and therefore whilst so little is known respecting the habits of plants of this tribe, the preceding facts are not sufficient to support a decision, that the source of the disease might not have been in the seeds themselves. For as the fructification is probably every thing which is seen of this, and many other parasitical fungus plants, the plant may extend in minute filaments through the whole body of the tree which supports it; and it appears in this view of the subject possible, that these slender filamen's may extend into the seeds. The following circumstances, however, militate strongly in opposition to this conclusion. A great number of seedling pear trees, which were very much diseased, were removed in the last spring, from my garden to a distant situation, after having had their roots and stems carefully and repeatedly washed and brushed, so as to remove from them every particle of the mould in which they had previously grown; formed of parts of diseased trees, have in all cases produced perfectly healthy foliage, even when inserted into the branches of other diseased trees; which circumstance I think interesting, because it tends to point out a further apparent similarity in the habits of this species of fungus, and that which forms the mildew of wheat, which ceases to vegetate as soon as the straw is severed from its roots, though that remains for some time green and living: whence arises the advantage of cutting mildewed crops of wheat in an immature state.

Further experience can, however, alone decide these points: and the only inference I wish to draw from the facts I have stated is, that the Lycoperdon cancellatum is capable under certain circumstances, of being transferred from one plant to another in its vicinity by means of its seeds."

I had iutended to have added a few remarks on the conditions required for the growth and increase of parasitic fungi, together with suggestions for an extended and systematic inquiry with a view to determine whether any means are known, or can be devised, to prevent an attack, or diminish the power of these parasites, but as I find they would make this article much too long, I reserve them for a separate communication.

Respectfully yours, John Townley.

Moundville, Marquette county, Wisconsin,