teral branches to be produced, or will pass into those already existing. The forms of such branches will be similar to that of the trunk; and the growth of the insulated tree on the mountain, will be, as we always find it, low and sturdy, and well calculated to resist the heavy gales to which its situation constantly exposes it. Let another tree of the same kind be surrounded, whilst young, by others, and it will assume a very different form. It will now be deprived of a part of its motion, and another cause will operate: the leaves on the lateral branches will be partly deprived of light, and, as I have remarked in the last paper I had the honour to address to you, little alburnum will then be generated in those branches. Their vigour of course becomes impaired, and less sap is required to support their diminished growth; more, in consequence, remains for the leading shoots; these, therefore, exert themselves with increased energy, and the trees seem to vie with each other for superiority, as if endued with all the passions and propensities of animal life. An insulated tree, in a sheltered valley, will assume, from the foregoing causes, a form distinct from either of the preceding*; and its growth will be more or less aspiring, in proportion to the degree of pro tection it receives from winds, and its contiguity to elevated objects, by which its lower branches during any part of the day are shaded. When a tree is wholly deprived of motion, by being trained to a wall, or, when a large tree has been deprived of its branches to be regrafted, it often becomes unhealthy, and not unfrequently perishes, apparently owing to the stagnation of the descending sap, under the rigid cincture of the lifeless external bark. I have, in the last two years, pared off this bark from some very old pear and apple trees, which had been re-grafted with cuttings from young seedling trees; and the effect produced has been very extraordinary. More new wood has been generated in the old trunks, within the last two years, than in the preceding twenty years; and I attribute this to the facility of communication which has been restored between the leaves and the roots, through the inner bark. I have had frequent occasion to observe, that wherever the bark has been most reduced, the greatest quantity of wood has been deposited. Other causes of the descent of the sap towards the root, I have supposed to be capillary attraction, and something in the conformation of the vessels of the bark. The alburnum also appears, in my former experiment, to expand and contract very Not only the external form of the tree, but the internal character of the wood, will be affected by the situation in which the tree grows; and hence, oak timber which grew in crowded forests, appears to have been mistaken, in old buildings, for Spanish chesnut. But I have found the internal organization of the oak and Spanish chesnut to be very essentially different. The silver grain, and general character of the oak and Spanish chesnut, are also so extremely dissimilar, that the two kinds of wood can only be mistaken for each other, by very careless observers. Many pieces of wood found in the old buildings of London, and supposed to be Spanish chesnut, have been put into my hands, but they were all most certainly forest oak. very freely, under changes of temperature and of moisture; and the motion thus produced must be in some degree communicated to the bark, should the latter substance be in itself wholly inactive. I however consider gravitation as the most extensive and active cause of motion in the descending fluids of trees; and I believe that from this agent vegetable bodies, like unorganized matter, generally derive, in a greater or less degree, the forms they assume; and probably it is necessary to the existence of trees, that it should be so. For if the sap passed and returned as freely in the hori zontal and pendant, as in the perpendicular branch, the growth of each would be equally rapid, or nearly so: the horizontal branch would then soon extend too far from its point of suspension, at the trunk of the tree, and thence must inevitably perish, by the compound ratio in which the powers of destruction, compared with those of preservation, would increase. The principal office of the hori. zontal branch, in the greatest number of trees, is to nourish and support the blossoms and the fruit or seed; and as these give back little or nothing to the parent tree, very feeble powers alone are wanted in the returning system. No power at all had been fatal; and powers sufficienty strong, wholly to counteract the effects of gravitation, had probably been in a high degree destructive. And it appears to me by no means improbable, that the formation of blossoms may, in many instances, arise from the diminished action of the returning system in the horizontal or pendant branch. I have long been disposed to be lieve the ascending fluids in the alburnum and central vessels, where ever found, to be every where the same; and that the leaf-stalk, the tendril of the vine, the fruit-stalt, and the succulent point of the an nual shoot, might in some measure be substituted for each other; and experiment has proved my conjer. ture, in many instances, to be wel founded. Leaves succeeded, and continued to perform their office, when grafted on the fruit-stalk, the tendril, and succulent shoot, of the vine; and the leaf-stalk, the tendril, and the fruit-stalk, alike supplied a branch grafted upon them with nourishment. But I did not succeed in grafting a fruit-stalk of the vine, on the leaf-stalk, the tendril, or suc culent shoot. My ill success, how ever, I here attribute solely to want of proper management; and I hate little doubt of succeeding in future. The young shoots of the vine, when grafted on the leaf-stalk, often grew to the length of nine or tea feet; and the leaf-stalk itself, to some distance below its juncture with the graft, was found, in the autumn, to contain a considerable portion of wood, in every respect similar to the alburnum in other parts of the tree. The formation of alburnum, in the leaf-stalk, seemed to point out to me the means of ascertaining the manner in which it is generated in other instances; and to that point my attention was in consequence attracted. Having grafted a great many leaf-stalks with shoots of the vine, I examined, in transverse sections, the commencement and gra dual formation of the wood. Itappeared evidently to spring from the tubles, which, in my last paper, (to which which I must refer you,) I have called the returning vessels of the leaf-stalk; and to be deposited on the external sides of what I have there named the central vessels, and on the medulla. The latter substance appeared wholly inactive, and I could not discover any thing like the processes supposed to extend from it, in all cases, into the wood. The organization of the young shoot is extremely similar to that of the leaf-stalk, previous to the formation of wood within it. The same vessels extend through both; and therefore it appeared extremely probable, that the wood in cach would be generated in the same manner and subsequent observation soon removed all grounds of doubt. It is well known that, in the operation of budding, the bark of trees being taken off, readily unites itself to another of the same or kindred species. An examination of the manner in which this union takes place promised some further information in the last summer, there fore, I inserted a great number of buds, which I subsequently examined in every progressive stage of their union with the stock. A line of confused organization marks the place where the inserted bud first comes into contact with the wood of the stock; between which line and the bark of the inserted bud, new wood regularly organized is generated. This wood possesses all the characteristics of that from which the bud was taken, without any apparent mixture whatever with the character of the stock in which it is inserted. The substance which is called the medullary process is clearly seen to spring from the bark, and to terminate at the line of its first union with the stock. An examination of the manner in which wounds in trees become covered, (for, properly speaking, they never can be said to heal,) affords further proof, were it wanted, that the medullary processes, (as they are improperly named,) like every other part of the wood, are generated by the bark. Whenever the surface of the alburnum is exposed but for a few hours to the air, though no portion of it be destroyed, vegetation on that surface for ever ceases. But new bark is gradually protruded from the sides of the wound, and by this new wood is generated. In this wood the medullary processes are distinctly seen to take their origin from the bark, and to terminate on the lifeless surface of the old wood within the wound. These facts incontestibly prove that the medullary processes, which in my former paper I call the silver grain, do not diverge from the medulla, but that they are formed in lines, converging from the bark to the medulla, and that they have no connection whatever with the latter substance. And surely nothing but the fascinating love of a favourite system could have induced any naturalist to believe the hardest, the most solid, and most durable part of the wood, to be composed of the soft cellulas and perishable substance of the medulla. In my last paper I have supposed that the sap acquired the power to generate wood in the leaf, and I have subsequently found no reason to retract that opinion. But the experiment in which wood was generated in the leaf-stalk, apparently by the sap descended from the bark bark of the graft, induces me to believe, that the descending fluid undergoes some further changes in the bark, possibly by discharging some of its component parts through the pores described and figured by Malpighi, I also suspected, since my former paper was written, that the young bark, in common with the leaf, possessed a power in proportion to the surface it exposes to the air and light of preparing the sap to genc rate new wood; for I found that a very minute quantity of wood was deposited by the bark, where it had not any apparent connection with the leaves. Having made two incisions through the bark round annual shoots of the apple-tree, I entirely removed the bark between the incisions, and I repeated the same operation at a little distance below, leaving a small portion of bark unconnected with that above and beneath it. By this bark, a very minute quantity of wood in many instances appeared to be generated, at its lower extremity. The buds in the insulated bark were sometimes suffered to remain, and in other instances were taken away; bat these, unless they vegetated, did not at all affect the result of the experiment. I could therefore account for the formation of wood, in this case, only by supposing the bark to possess in some degree, in common with the leaf, the power to produce the necessary changes in the descending sap; or that some matter, originally derived from the leaves, was previously deposited in the bark or that a portion of sap had passed the narrow space above, from which the bark had been removed, through the wood. Repeating the experiment, I left a much 3 greater length of bark between the intersections; but no more wood than in the former instances was generated. I therefore concluded that a small quantity of sap must have found its way through the wood, from the leaves above; and I found, that when the upper incisions were made at ten or twelve lines distance,instead of one or two, and the bark between them, as in the former experiments, was removed, no wood was generated by the insulated bark. I shall conclude my paper with a few remarks on the formation of buds in tuberous rooted plants beneath the ground. They must, if my theory be well founded, be formed of matter which has descended from the leaves through the bark. I shall confine my observations to the potatoc. Having raised some plants of this kind, in a situation well adapted to my purpose, I waited till the tubers were about half grown; and I then commenced my experiment by carefully intersecting, with a sharp knife, the runners which connect the tubers with the parent plant, and immersing each end of the runners, thus intersected, in a decoction of log-wood. At the end of twenty-four hours I examined the state of the experiment ; and I found that the decoction had passed along the runners in each direction; but I could not discover that it had entered any of the vessels of the pare it plant. This result I had anticipated; because I concluded, that the matter by which the growing tuber is fed must descend from the leaves through the bark; and experience had long before taught me that the bark would not absorb coloured infusions. I now endeavoured to trace the pro gress gress of the infusion, in the opposite direction; and my success here much exceeded my hopes. A section of the potatoe presents four distinct substances: the internal part, which, from the mode of its formation and subsequent office, I conceive to be allied to the alburnum of ligneous plants; the bark which surrounds this substance; the true skin of the plant; and the epidermis Making transverse sections of the tubers, which had been the subjects of the experiments, I found that the coloured infusion had passed through an elaborate series of vessels, between the cortical and alburnous substance, and that many minute ramifications of these vessels approached the external skin, at the base of the buds, to which, as to every other part of the growing tuber, I conclude they convey nourishment. Observations on the Structure of the Tongue; illustrated by Cases in which a Portion of that Organ has been removed by Ligature. By Everard Home, Esq. F. R. S. Physiological enquiries have ever been considered as deserving the attention of this learned society, and whenever medical practitioners, in the treatment of diseases, have met with any circumstance which threw light upon the natural structure or actions of any of the organs of the human body, or those of other animals, their communications have met with a favourable reception. The following observations derive their real importance from offering a safe and effectual means of removing a portion of the tongue, when that organ has taken on a diseased VOL. XLVII. action, the cure of which is not with. in the reach of medicine; and, as the tongue, like many other glandular structures, is liable to be affected by cancer, it becomes of no small importance that the fact should be generally known. In a physiological view, they tend to show that internal structure of the tongue is not of that delicate and sensible nature which, from its being the organ of taste, we should be led to imagine. The tongue is made up of fasiculi of muscular fibres, with an interme diate substance met with in no other part of the body, and a vast number of small glands; it has large nerves passing through it; and the tip possesses great sensibility, fitting it for the purpose of taste. Whether the sense of taste is confined entirely to the point of the tongue, and the other parts are made up of muscles fitted for giving it mo tion; or whether the whole tongue is to be considered the organ, and the soft matter which pervades its substance and fills the interstices be tween the fasiculi of muscular fibres, is to be considered as connected with sensation, has not, I believe, been ascertained. stance, has always been considered, The tongue, throughout its subby physiologists, as a very delicate organ; and it was believed, that any injury committed upon it would not only produce great local irritation, but also affect, in a violent degree, the general system of the body. This was my own opinion, till I met with the following case, the circumstances of which induced me to see this organ in a different point of view. A gentleman, by an accident which it is unnecessary to describe, had his tongue bitten with great vio 3 HI lence. |