"conic, and tipped with a tuft of "cartilaginous hairs: the clypeus " is only depressed, and the jugular "triangle wider : the eyes are "large, and very evident, those of "the male black, though in a certain light appearing greenish ; but those of the female are like 66 pearls, or as if they were covered with a crystalline membrane : "the angles of the brim of the "socket are small and rounded at "the top, and the hinder one lower than the eye. The pivots of the "antennæ are not so discernible as "in the former species, being like the surrounding parts in colour : "the under joint is without any ❝hairy papilla or wart: the upper "joint or clava is of the size of the "head, quite globular, and resem❝bles an inflated bladder, being "almost pellucid, and of a light

flesh-colour: the keel is nothing <6 more than a raised line, finish

66

ing on the vertex in only one ❝chesnut-brown tubercle covered "with cartilaginous hairs: behind "there is a little conical shining "hook, of the same colour and with "the same sort of hairs bending outwardly, being of equal length "with the horn on the head, but 66 narrower the pedicle is short, "straight and cylindrical. The in❝terior palpi, furnished with very "visible hinges, are a little thicker "towards the top, but look, in "some directions, as if they were "filiform: the mandibles have large hinges, and the superior sheath "almost as long as the inferior one, "and nearly cylindrical. The tho66 rax is of the same breadth with "the head, and not very uneven, "the two parts being separated by

a furrow only on the sides and "underneath, the foremost above

"and on the sides, resembling an "annular segment, and the hinder "one impressed in the middle with "a mark somewhat like two small "diverging wings, of a blackish "silvery colour. The elytra are "shorter than the abdomen, and "minutely punctated: the under "wings are of a shining and change"able violaceous colour, and not very dark the abdomen has the "terminal segment a little convex, and in the female more so than in "the male: underneath, the third "and last segments are darker than "the others, the legs are all of equal "length; the tarsi longer than "those of the pausus microcepha"lus, and have both the joints and "the claws much more distinct."

Account of some Experiments on the Descent of the Sap in Trees. In a Letter from Thomas Andrew Knight, Esq. to the Right Hon. Sir Joseph Banks, Bart.

My dear sir,

In a memoir which I had the ho nour to present to you two years ago, I related some experiments on trees, from which I inferred, that their sap, having been absorbed by the bark of the root, the trunk and the branches; that it passes through what are there called the central vessels, into the succulent part of the annual shoot, the leaf-stalk, and the leaf; and that it returns to the bark, through the returning vessels of the leaf-stalk. The principal object of this paper is, to point out the causes of the descent of the sap through the bark, and the consequent formation of wood.

These causes appear to be gravitation, motion, communicated by

winds, or other agents, capillary attraction, and, probably, something in the conformation of the vessels themselves, which renders them better calculated to carry fluids in one direction than in another. I shall begin with a few observations on the leaf, from which all the descending fluids in the tree appear to be derived. This organ has much engaged the attention of naturalists, particularly of M. Bonnet, but their experiments have chiefly been made on leaves severed from the tree; and, therefore, whatever conclusions have been drawn, stand on very questionable ground. The efforts which plants always make to turn the upper surfaces of their leaves to the light, have with reason induced naturalists to conclude, that each surface has a total distinct of fice; and the following experiments tend strongly to support that conclusion.

I placed a small piece of plate glass under a large vine leaf, with its surface nearly parallel with that of the leaf, and as soon as the glass had acquired the temperature of the house in which the vine grew, I brought the under surface of the leaf into contact with it, by means of a silk thread and a small wire, adapted to its form and size. Hav. ing retained the leaf in this position one minute, I removed it, and found the surface of the glass covered with a strong dew, which had evidently exhaled from the leaf. I again brought the leaf into contact with the glass, and, at the end of half an hour, found so much water discharged from the leaf, that it ran off the glass when held obliquely. I then inverted the position of the leaf, and placed its upper surface in contact with the glass: not the

slightest portion of moisture now appeared, though the leaf was ex, posed to the full influence of the meridian sun. These experiments were repeated on many different leaves, and the result was in every instance precisely the same. It seems, therefore, that in the vine the perspiratory vessels are confined to the under surface of the leaf; and these, like the cutaneous lym phatics of the animal economy, are probably capable of absorbing moisture, when the plant is in a state to require it. The upper surface seems, from the position it always assumes, either formed to absorb light, or to operate by the influence of that body; and, if any thing exhale from it, it is probably vital air, or some other permanently elastic fluid. It nevertheless appears evident in the experiments of Bonnet,

that this surface of the leaves of many, when detached from the tree, readily absorbs moisture.

Selecting two young shoots of the vine, growing perpendicularly against the back wall of my vinery, I bent them downwards, nearly in a perpendicular line, and introduced their succulent ends, as layers, into two pots, without wounding the stems, or depriving them of any portion of their leaves. In this position the shoots, which were about four feet long, and sprang out of the principal stem about three feet from the ground, grew freely, and in the course of the summer reached the top of the house. As soon as their wood became sufficiently solid to allow me to perform the operation with safety, I made two circular incisions through the bark of the depending part of each shoot, at a small distance from each other, ncar the surface of the mould in the pots;

and

and I wholly removed the bark between the incisions, thus cutting off all communication through the bark, between the layers and the parent stems. Had the subjects of this experiment now retained their natural position, much new wood and bark would have been formed at the upper lip of the wounds, and none at all at the lower, as I have ascertained by frequent experiments. The case was now different; much new bark and wood was ncrated on the lower lip of the wounds, become uppermost by the inverted position of the branches; and I have no doubt but that the new matter, thus deposited, owed its formation to a portion of sap, which descended by gravitation, from the leaves growing between the wounded parts and the principal

stems.

ge

The result of this experiment appears to point out one of the causes why perpendicular shoots grow with much greater vigour than others: they have, probably, a more perfect and rapid circulation.

The effects of motion on the circulation of the sap, and the consequent formation of wood, I was best able to ascertain by the following expedient:-Early in the spring of 1801, I selected a number of young seedling apple-trees, whose stems were about an inch in diameter, and whose height between the roots and first branches, was between six and seven feet. These trees stood about eight feet from each other; and, of course, a free passage for the wind to act on each tree was afforded. By means of stakes and bandages of hay, not so tightly bound as to impede the progress of any fluid within the trees, I nearly deprived the roots and the lower parts of the

stems of several trees of all motion, to the height of three feet from the ground, leaving the upper parts of the stems and branches in their na tural state. In the succeeding summer much new wood accumulated in the parts which were kept in mo. tion by the wind, but the lower parts of the stems and roots in creased very little in size. Remov. ing the bandages from one of these trees in the following winter, I fixed a stake in the ground, about ten feet distant from the tree, on the east side of it, and I attached the tree to the stake, at the height of six feet, by means of a slender pole, about twelve feet long; thus leaving the tree at liberty to move towards the north and south, or, more properly, in the segment of a circle, of which the pole formed s radius, but in no other direction. Thus circumstanced, the diameter of the tree from north to south, in that part of its stem which was most exercised by the wind, exceeded that in the opposite direc tion, in the following autumn, in the proportion of thirteen to eleven,

These results appear to open an extensive and interesting field to our observation, where we shall find much to admire, in the means which nature employs to adapt the forms of its vegetable productions to every situation in which art or accident may deposit them. If a tree be placed in a high and exposed situation, where it is much kept in mo tion by winds, the new matter which it generates will be deposited chiefly in the roots and lower parts of the trunk; and the diameter of the latter will diminish rapidly in its ascent. The progress of the as cending sap will of course be inpeded, and it will thence cause la

teral

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 ex. poses 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, ex. ert 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 extraordiuary. 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. 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

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 horizontal 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-stalk, and the succulent point of the an nual shoot, might in some measure be substituted for each other; and experiment has proved my conjecture, in many instances, to be well 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 have 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 ten 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, iu transverse sec tions, the commencement and gra dual formation of the wood. It ap peared evidently to spring from the tubles, which, in my last paper, (to