to repeat it here; but it is particularly deserving of the attention of the reader. I have myself made several experiments in order to compare the different progress of trees, both young and old, that had their tops lightened with that of others in which the tops had been left untouched, and the results have been similar to those experienced by Miller; only, in the latter, the results were more striking, on account of the more advanced age of the trees. But I feel peculiar satisfaction in being able to strengthen my own opinion by the authority of so eminent a phytologist, whose great work cannot be too frequently recommended to the young planter's notice. It is most particularly valuable in the edition of the late Professor Martyn of Cambridge; who, besides nearly doubling the whole matter contained in the original work, has added some new and valuable articles, and brought the history of the plants enumerated down to the present times.

NOTE VIII. Page 72.

If the reasonings in the foregoing part of this Section be well founded, the proposition in question here must necessarily be true in respect to trees removed from exposed to sheltered situations, as well as its converse; but probably there is no one who has verified it by experiment.

In 1818, I transferred some Beeches, Oaks, Witch Elms, Limes, and Sycamores, from an exposed situation, in order to form a close screen of some size in conjunction with Underwood, which screen or plantation was accordingly executed. These trees possessed, in a very considerable degree, what has been called in the text the protecting properties, so that they might with great advantage have been set out in the open park. In 1826, at the distance of eight years, it was quite visible that these properties had greatly disappeared, and that the non-protecting were about to be superinduced in their stead. In the spring of the year last mentioned, I removed to an exposed situation in the park, a few of the Oaks and Beeches from the centre of the wood, where the warmth was the greatest, and where they had begun to be drawn up; and I am persuaded that, in ten or twelve years more, the former properties will return, and be as fully developed as they were in the beginning.

In 1809, I took two fine Sycamores about five-and-twenty feet high, amply provided with the protecting properties, and fitted for situations of the greatest exposure, and removed them into the centre of a close wood. Being well supplied with roots, they were soon established in the ground, and began to push vigorously towards the

light. Their stems were speedily elongated; their bark became smoother; their side branches more slender, and thinner in spray and foliage; and by 1816-that is, after seven years-they could scarcely be recognised as the same plants. Soon after the fall of that season, I once more transferred them to the open field. Here, although they carried a good leaf, they appeared for some time altogether stationary in their progress, as was to be expected. In the absence of the shelter and warmth which they had so long experienced, they could not at once generate provisions to enable them to resist the cold but in consonance to that law of nature by which "plants, as well as animals, accommodate themselves to the circumstances in which they are placed," they began gradually but slowly to generate them; so that it was only in 1824 that I observed the trees to display any decided symptoms of induration of bark, increase of roots, stoutness of stem, and closeness of ramification, which constitute such provisions—and it is evident that it will require some years more to effect a complete renovation of their former character.

From this short account we may perceive, that while trees retain their 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 greatly 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 nonprotecting. 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 increasing, of bestowing or taking away, what may be called the protecting properties."

SECTION IV.

NOTE I. Page 87.

MALPIGHI was born A. D. 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 light on the diseases of the liver. He 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 contemporary 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 realising 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 88.

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 development, 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 English 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,

66 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."-Physiolog. Botany, vol. 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 conclusion, to which late writers have come, as it is one of the most obscure, though important processes, in the whole of vegetable economy.

NOTE III. Page 92.

Although trees, as is said in the text, have no organs analogous to the mouths of animals for receiving their food, yet perhaps it may be said, that animals sometimes take in their food like trees. Men, for example, have been known to become so debilitated by age or disease that they could receive no food by the ordinary organ of the mouth. The consequence has been, that they were immersed in milk and vealbroth baths, and fairly subsisted by means of absorption. Thus, every one of their pores became like leaves for the introsusception of food. Some few years since an instance occurred in a noble Duke of sporting notoriety, who was so supported during the last months of his life.

NOTE IV. Page. 95.

Opinions quite opposite to these are entertained by Dr Yule, and also by Sang, who is a nurseryman and a planter of some experience; but they are not borne out by facts. The author of the Encyclopædia of