with moisture, and the remainder was dispersed, in a light and constant shower, over the seeds in the vertical wheel, and on others placed to vegetate at rest in different parts of the box. Every seed on the horizontal wheel, though moving with great rapidity, necessarily retained the same position, relative to the attraction of the earth; and, therefore the operation of gravitation could not be suspended, though it might be counteracted, in a very considerable degree, by centrifugal force; and the difference I had anticipated, between the effects of rapid, vertical, and horizontal motion, soon became sufficiently obvious. The radicles pointed downwards about ten degrees below, and the germens as many degrees above, the horizontal line of the wheels' motion; centrifugal force having made both to deviate 80 degrees from the perpendicular direction each would have taken, had it vege tated at rest. Gradually diminishing the rapidity of the motion of the horizontal wheel, the radicles descended more perpendicularly, and the germens grew more upright; and when it did not perform more than 80 revolutions in a minute, the radicle pointed about 45 degrees below, and the germen as much above, the horizontal line, the one always receding from, and the other approaching to, the axis of the wheely I would not, however, be understood to assert that the velocity of 250, or of 80 horizontal revolutions in a minute, will always give accurately the degrees of depression and elevation of the radicle and germen, which I have mentioned; for the rapidity of the motion of my wheels VOL. XLVIII. was sometimes diminished by the collection of fibres of conferva against the wire grate; which obstructed in some degree the passage of the water; and the machinery, having been the workmanship of myself and my gardener, cannot be supposed to have moved with all the regularity it might have done, had it been made by a professional mechanic. But I conceive myself to have fully proved that the radicles of germinating seeds are made to descend, and their germens to ascend, by some external cause, and not by any power inherent in vegetable life: and I see little reason to doubt that gravitation is the principal, if not the only agent employed, in this case, by nature. I shall, therefore, endeavour to point out the means by which I conceive the same agent may produce effects so diametrically opposite to each other. The radicle of a germinating seed (as many naturalists have observed) is increased in length only by new parts successively added to its apex or point, and not at all by any general extension of parts already formed; and the new matter, which is thus successively added, unquestionably descends in a fluid state from the cotyledons. On this fluid, and on the vegetable fibres and vessels whilst soft and flexible, and whilst the matter which composes them is changing from a fluid to a solid state, gravitation, I conceive, would operate sufficiently to give an inclination downwards to the point of the radicle; and as the radicle has been proved to be obedient to centrifugal force, it can scarcely be contended that its direction would remain uninfluenced by gravitation. I have stated that the radicle is increased in length only by parts 30 successively successively added to its point: the germen, on the contrary, elongates by a general extension of its parts previously organized; and its vessels and fibres appear to extend themseves in proportion to the quantity of nutriment they receive. If the motion and consequent distribution of the true sap be infuenced by gravitation, it follows, that when the germen at its first emission, or subsequently, deviates from a perpendicular direction, the sap must accumulate on its under side; and I have found, in a great variety of experiments on the seeds of the horse chesnut, the bean, and other plants, when vegetating at rest, that the vessels and fibres on the under side of the germen, invariably elongate much more rapidly than those on its upper side; and thence it follows, that the point of the germen must always turn upwards; and it has been proved that a similar increase of growth takes place on the external side of the germen, when the sap is impelled there by centrifugal force, as it is attracted by gravita tion to its under side, when the seed germinates a rest. This increased elongation of the fibres and vessels of the under side, is not confined to the germens, nor even to the annual shoots of trees, but occurs and produces the most extensive effects in the subsequent growth of their trunks and branches. The immediate effect of gravitation is certamly to occasion the further depression of every branch which extends horizontally from the trunk of the tree, and, when a young tree inclines to either side, to increase that inclination; but at the same time attracts the sap to the under side, and thus occasions an increased longitudinal extension of the substance of the new wood on that side. The depression of the lateral branch is thus prevented, and it is even enabled to raise itself above its natural level, when the branches above it are removed; and the young tree, by the same means, becomes more upright, in direct opposition to the immediate action of gravitation; nature, as usual, executing the most important operations by the most simple means. To enable I could adduce many more facts in support of the preceding deduc tions, but those I have stated, ! conceive to be sufficiently conclu sive. It has, however, been objected by Du Hamel (and the greatest deference is always due to his opinions) that gravitation could have little influence on the direction of the ger men, were it in the first instance protruded, or were it subsequently inverted, and made to point perpen dicularly downwards. myself to answer this objection,! made many experiments on seeds of the horse chesnut, and of the bean, in the box I have already described, and as the seeds there were suspend ed out of the earth, I could regularly watch the progress of every e fort made by the radicle and get. men, to change their positions. The extremity of the radicle of the bean, when made to point perpendicularly upwards, generally formed a cons derable curvature within three or four hours, when the weather was warm. The germen was more slaggish; but it rarely or never failed to change its direction in the course of twenty-four hours; and all my e forts to make it grow downwards, by slightly changing its direction, were invariably abortive. Another, and apparently a more weighty, objection, to the preceding hypothesis, hypothesis, (if applied to the subsequent growth and forms of trees) arises from the facts that few of their branches rise perpendicularly upwards, and that their roots always spread horizontally; but this objection I think may be readily answered. The luxuriant shoots of trees, which abound in sap, in whatever direction they are first protruded, almost uniformly turn upwards, and endeavour to acquire a perpendicular direction; and to this their points will immediately return, if they are bent downwards during any period of their growth; their curvature upwards being occasioned by an increased extension of the fibres and vessels of their under sides, as in the = elongated germens of seeds. The more feeble and slender shoots of the same trees will, on the contrary, grow in almost every direction, probably because their fibres, being more dry, and their vessels less am-ply supplied with sap, they are less affected by gravitation. Their points, however, generally shew an inclination to turn upwards; but the operation of light, in this case, had been proved by Bonnet, to be very considerable. The radicle tapers rapidly, as it descends into the earth, and its lower part is much compressed by the greater solidity of the mould into which it penetrates. The true sap continues to descend from the cotyledons and leaves, and occasions a continued increase of the growth, and is subsequently augmented by the effects of the motion, when the germen lies above the ground. The true sap is, therefore, necessarily obstructed in its descent; numerous lateral roots are generated, into which a portion of the descending sap enters. The substance of these roots, like that of the slender horizontal branches, is much less succulent than that of the radicle first emitted, and they are in consequence less obedient to gravitation; and, therefore, meeting less resistance from the superficial soil, than from that beneath it, they extend hori zontally in every direction, growing with most rapidity, and producing the greatest number of ramifications, wherever they find most warmth, and a soil best adapted to nourish the tree. As these horizontal or lateral roots surround the base of the tree on every side, the true sap descending down its bark, enters almost exclusively into them, and the first perpendicular root, having executed its office of securing moisture to the plant, whilst young, is thus deprived of proper nutriment, and, ceasing almost wholly to grow, becomes of no importance to the tree. The tap root of the oak, about which so much has been written, will possibly be adduced as an exception; but having attentively examined at least 20,000 trees of this species, many of which had grown in some of the deepest and most favourable soils of England, and never having found a single tree possessing a tap root, I must be allowed to doubt that one ever existed. As trees possess the power to turn the upper surfaces of their leaves, and the points of their shoots to the light, and their tendrils i any direction to attach themselves to contiguous objects, it may be sus pected that their lateral roots are by some means directed to any soil in their vicinity, which is best calcu lated to nourish the plant to which they belong; and it is well known that much the greater part of the roots of an aquatic plant, which 302 has grown in a dry soil, on the margin of T. A. Knight. Elton, Nov. 22, 1805. On the inverted Action of the Albur- My dear sir, favourable result, I am not in possession of a single fact which is not perfectly consistent with the theory I have advanced. There is, however, one circumstance stated by Hales and Du Hamel, which appears strongly to militate against my hypothesis; and as that circumstance probably Dduced Hales to deny altogether the existence of circulation in plants, and Du Hamel to speak less deci sively in favour of it, than he possibly might have done, I am anxious to reconcile the statements of these great naturalists (which I acknowledge to be perfectly correct) with the statements and opinions I have on former occasions communicated to you. Both Hales and Du Hamel have proved, that when two circular incisions through the bark, round the stem of a tree, are made at a small distance from each other, and when the bark between these incisions is wholly taken away, that portion ei the stem which is below the incisions through the bark, continues to live, and in some degree to increase is size, though much more slowly ob Read May 15th, 1806. served than the parts above the incisions. They have also observed that a small elevated ridge (bour velet) is formed round the lower p of the wound in the bark, with some slight advances to meet the bark and wood projected, in much larger quantity, from the opposite, or upper lip of the wound. I have endeavoured to prove, in several Memoirs which you have done me the honour to lay before the royal society, that the fluid by which the various parts (that are annually added to trees and herbaceous plants, whose organization is similar to that of trees) are generated, has previously circulated through their leaves, either in the same, or preceding season, and subsequently descended through their bark; and after having repeated every experiment that occurred to me, from which I suspected an un I have endeavoured, in a former Memoir, to explain the cause why some portion of growth takes place below incisions through the bark, by supposing that a small part of the true sap, descending from the leaves, escapes downwards through the porous substance of the alburnum Several facts stated by Hales, seem favourable favourable to this supposition; and the existence of a power in the alburnum, to carry the sap in different directions, is proved in the growth of inverted cuttings of different species of trees. But I have derived so many advantages, both as a gardener and farmer (particularly in the management of fruit and forest trees) from the experiments which have been the subject of my former Memoirs, that I am confident much public benefit might be derived from an intimate acquaintance with the use and office of the various organs of plants, and thence feel anxious to adduce facts, to prove that the conclusions I have drawn are not inconsistent with the facts stated by my great predecessors. It has been acknowledged, I believe, by every naturalist who has written on the subject (and the fact is, indeed, too obvious to be controverted) that the matter which enters into the composition of the radicles of germinating seeds, existed previously in their cotyledons; and as the radicles increase only in length by parts successively added to their apices, or points, most distant from their cotyledons, it follows of necessity, that the first motion of the true sap, at this period, is down wards; and as no alburnous tubes exist in the radicles of germinating seeds, during the earlier periods of their growth, the sap in its descent must either pass through the bark or the medulla. But the medulla does not apparently contain any vessels calculated to carry the descending sap, whilst the cortical vessels are, during this period, much distended, and full of moisture; and as the medulla certainly does not carry any fluid in stems or branches of more than one year old, it can scarcely be suspected that it, at any period, conveys the whole current of the descending sap. As the leaves grow, and enter on their office, cortical vessels, in every respect apparently similar to those which descended from the cotyledons, are found to descend from the bases of their leaves; and there appears no reason, with which I am acquainted, to suspect that both do not carry a similar fluid, and that the course of this fluid is, in the first instance, always towards the roots. The ascending sap, on the con. trary, rises wholly through the alburnum and central vessels; for the destruction of a portion of the bark, in a circle round the tree, does not immediately, in the slightest degree, check the growth of its leaves and branches; but the alburnous vessels appear, from the experiments I have related in a former paper, and from those I shall now proceed to relate, to be also capable of an inverted action, when that becomes necessary to preserve the existence of the plant. As soon as the leaves of the oak were nearly full grown in the last spring, I selected, in several instances, two poles of the same age, and springing from the same roots, in a coppice, which had been felled about six years preceding, and making two circular incisions, at the distance of three inches from each other, through the bark of one of the poles on each stool, I destroyed the bark between the incisions, and thus cut off the communication between the leaves and the lower parts of the stem and roots, through the bark; much growth, as usual, took place above the space from which the bark had been taken off, and very little below it. Examining the state of the expe. 303 riment |