From this short account, we may perceive, that, while Trees retain full vigour, that is, while they continue in a rapidly progressive state, they may be made alternately to assume or lay aside those properties, which best fit them for Removal. Moreover we see, that, as vegetation is always more active in shelter, than in exposure, the properties just now mentioned, that is, the Protecting Properties, are far more slowly obtained or reassumed, than the Non-protecting. From such facts and experiments, therefore, as well as from analogy, we are warranted to conclude, that the doctrine held forth in the text is fully confirmed, namely, that, "By the Law of Nature, Shelter and Exposure, that is, Heat and Cold have the power alike of diminishing or encreasing, of bestowing or taking away, what may be called the Protecting Properties."

At page 92, I have endeavoured to apologize, to the scientific reader, for the use of these two terms, as being by no means "philosophical," but applied solely for the purpose of accommodating my New Theory to practice, in the Removal of Wood. "Protecting and Non-protecting properties," I am aware, are acquired by Trees, solely in consequence of differences in their situation; and, for that reason, it certainly would have been better, if terms had been found, having a reference to what the Tree is, and to the conditions that have made it so, rather than to certain future uses, which the character, thus acquired, is considered to serve. Perhaps, however, they may be defended in some degree, on the same grounds, as the terms "Conducting," and "Non-conducting," as applied to certain substances, capable of receiving and transmitting the Electric fluid, which were first invented by Desaguliers, and have been since admitted into the philosophic nomenclature.

SECTION IV.

NOTE I. P. 107.

He

MALPIGHI was born An. 1628. He was a native and physician of Bologna, and professor of Medicine in the University of that city. For his discoveries in Anatomy, he has been justly celebrated, in conjunction with the well-known Borelli, and for having thrown a new light on the diseases of the Liver. was the first writer, who gave to the world a system of the true Anatomy of Plants, of which one of the most important doctrines is the theory of the Circulation of the Sap, its ascent in the wood, and its descent in the bark. His work seems to have appeared in 1671. In 1669, he was admitted a Fellow of the Royal Society of London; and he kept up a regular correspondence with several of its members, till his death.

Dr Nehemiah Grew, the father of English Phytology, and one of the most eminent physicians of his time, was a cotemporary of Malpighi's. He published, about the same period, his “ Anatomy of Plants," wherein he advanced, on similar principles, the doctrine of the Circulation of the Sap. The second edition bears date London, 1688; so that, as they investigated and wrote, in different countries, and without communication with each other, on this obscure subject, so they justly divide the honour of systematizing the conjectures of the Greek Naturalists. Notwithstanding the importance of later researches, their works are held in high esteem, down to the present period.

NOTE II. Page 108.

It was extremely natural for phytologists, after the discovery of the Circulation of the Blood in animals, to extend the analogy

to the vegetable kingdom. They had, in the latter, no visible organs, corresponding to the stomach, the intestines, or the lacteals, and, above all, to the heart, the main-spring and centre of the circulation of the blood; but these wants were readily supplied. The Root was supposed to correspond to both the Mouth and the Stomach, and to effect such a change on the fluid which it absorbed, as fitted it for the nourishment of the plant. It was supposed also to have the power of propelling the digested fluid, when impregnated with the principles of nutrition, growth, and developement, to the summit of the leaf. From thence it was again returned to the root; where, mingling with the newly-digested fluid, it was again propelled to the summit, as before; and, in that way, a regular circulation was maintained. In this process, these propelling vessels were said to be Arteries, and the returning vessels were considered as Veins. Such is the theory of the circulation of the Sap, held forth by the earlier phytologists; and, as it was found to rest on a very slender basis, they did not fail to prop and bolster it up, with a multitude of ingenious arguments.

Of late years, the doctrine has been revived, as mentioned in the text, and supported by some of the most distinguished modern phytologists; but it has been improved by patient investigation, and accurate experiment, and cleared of all ill-founded analogy to animal life. Hedwig declared himself to be of opinion, that plants possess a circulation of the fluids, in some sort similar to that of animals. Costi united in the same opinion, and is said to have found it exemplified in the stem of the Chara, and other plants. Professor Willdenow, in his Principles of Botany, has also introduced the subject, and defended the doctrine. (See Eng. Translation, p. 85.) He confidently asserts, that he believes a circulation to exist; because it would be utterly impossible for the leafless Tree to resist the cold, if there were no circulation of the fluids. This, as Mr Keith observes, "is no argument, and therefore merits no reply;"-yet, we must admit, that it is a presumption, of which the force is more easily evaded than invalidated.

It is impossible, in the narrow compass of a Note, to give a detail of Mr Knight's ingenious and valuable experiments, to

account for the conversion of the Alburnum into Wood; but the reader is referred for them to the Philosophical Transactions for 1805 and 1806. By these experiments, he will see, that it is rendered in the highest degree probable, if it be not altogether certain, that a circulation of the vegetable fluids actually exists: For, if it once be admitted, that the descending or Proper juice, forms not only a new Epidermis, where it is wanted, and a new layer of Liber and Alburnum, but that it also partly enters into the Alburnum of the preceding year, where it mingles, and is again carried up with the ascending Sap, it cannot well be denied, that a circulation is completed. That Mr Keith is pretty nearly of this opinion himself, may be gathered from the following concise summary of Mr Knight's hypothesis, by that acute and ingenious censor :—

"Although the doctrine of a circulation (says he), as maintained by Mr Knight, should be false, yet the account which he gives, of the progress and agency of the Sap, and proper juice, short of Circulation, may be true. The sum of the account is as follows. When the seed is deposited in the ground, under proper conditions, moisture is absorbed and modified by the cotyledons, and conducted directly to the radicle, which is, by consequence, first developed. But the fluid, which has been thus conducted to the radicle, mingling, no doubt, with the fluid, which is now also absorbed from the soil, ascends afterwards to the plumelet, through the medium of the tubes of the alburnum. The plumelet now expands, and gives the due preparation to the ascending sap, returning it also, in its elaborated state, to the tubes of the bark; through which it again descends to the extremity of the root, not only forming, in its progress, new bark, and new alburnum ; but mixing also, as Mr Knight thinks, with the alburnum of the former year, where such alburnum exists, and so completing the circulation." Veget. Botany, V. II. p. 244. See also, on the same subject, Kieser, Organ. des Plantes, pp. 258, 259, &c.

This Note has been extended to an unusual length. But I conceived, that it would be interesting to the young planter, to have a brief account of the principal theories, which have been formed of the Circulation of the Sap, and the ultimate conclu

sion, to which late writers have come, as it is one of the most obscure, though important processes, in the whole of vegetable economy. I may add, in conclusion, that there are various writers, besides the distinguished names given in the text, who may be advantageously consulted by the young student, on this, and other points of Vegetable Physiology, namely, Leeuvenhoek, Spalanzani, Van Marum, Guetard, Sprengel, Bernhardi, Cotta, Link, Hill, Tournfort, Saussure, Comparetti, Treviranus, Bauer, Ellis, &c. I regret, however, to observe, that the elaborate prize Essay, by Dr Kieser of the University of Jena, drawn up for the Teylerian Society at Haarlem, "on the Organization, Structure, and Functions of Plants," &c. should be so little known in this country, as to escape the notice of some of our late writers. It is composed in French, and printed in 4to, in 1812, with numerous plates well executed. Making allowance for some fancies peculiar to the author, it is a work of great merit, and certainly deserving of being translated into English.

NOTE III. Page 119.

As there is no process, in the whole range of Arboricultural economy, more important than Pruning, it may be worth while to say something on it, in this place. Pruning may be said to embrace the five following objects: First, to advance the growth and bulk of Trees; Secondly, to reduce or lessen their bulk ; Thirdly, to modify or alter their form; Fourthly, to renew their decayed parts; and Fifthly, to cure and eradicate the diseases to which they are subject.

Of these, one of the most important, and, till of late years, certainly the least attended to, is the First; as the ultimate value of the Wood in most cases depends upon it, and the actual weight of the timber produced. With all deciduous Trees cultivated for profit, the art is, to cut off the weak and superfluous lateral shoots, so that the portion of sap, employed in their nourishment, may be thrown into the strong ones; and, above all, to direct a proper portion of the ligneous matter of the Tree into the main stem or trunk, and thereby generate clean and