them, I hardly know a more admirable spectacle in the microscope; it requires but triffing powers to show it well. The last process, and completion of the leaf, is the form- Formation of ing of the pores. Whether it is, that the young leaf being the pores, thicker and more hairy than it is afterward, the pores are obscured and hidden, or that the upper net grows last, I cannot say; but in the many hundred forming leaves I have exposed to the solar microscope, I have never once been able to view the pores, as I have often done after the leaves had com pletely quitted the bud. I must not forget to mention, that Two sorts of there are two sorts of pores in the leaf; the large ones are those which receive the dew drops and rain, the smaller are those which appear in the day to give out the oxigen, and at night to inhale the carbonic gas. I mentioned, that I sus pected these smaller pores of yielding a sort of insensible perspiration; as I find, when out of doors, a scurf only to be seen with a microscope; and under a glass this seems to rise as water, to bedew the glass. But to place an object in Unnatural sian unnatural situation, in order to judge of its secretions, is tuations may something like putting a human being into a warm bath, to natural secre judge how fast the blood flows. We know not what un- tions. natural secretions we may cause in that confined air, or how much it may alter the nature of the plant, as I shall show at a future time with respect to melons and grapes. occasion un The two cuticles of leaves differ in most plants: for in Upper and unthe under one I have hardly ever found the large pores into der cuticle, which the dew or rain enters; and but little oxigen is given out also from the under part of most leaves; while this part has a number of very small apertures, formed I suppose for the reception of the carbonic gas. I cannot but notice here how strange is the contradictory ac- Contradiction count of the leaves now generally received. They are sup- notions. in the received posed to perspire 17 times more than a man: water must therefore be yielded from each pore. They at the same time give out oxigen, and receive carbonic gas. Is this credible, or is it not contradictory? That they give out oxigen in the day, and inhale carbonic gas in the night, I am convinced, and I think it requires but the simple experiment of keeping a plant in the window, and examining it with a microscope 8 or 10 times in a day, to convince a person, that there Completion of the edge of the leaf. Two sorts of Motive ves sels Use of the hairs on leaves Why leaves turn from the wind & to the sun. there is no perspiration worthy being so called. But I tes turn to my subject. While the upper and under cuticles are growing, the edge of the leaf is completing; the bubbles generally divide, and partly dry up, and horny points appear in their stead. When this is complete, the leaves burst from the bud; but there are few that will not show for a long time the manner of their formation; the planes for more than a month remain covered with the ends of vessels, some attached to the leaf, some loose: and most leaves have a bunch of vessels fastened at the outside to the corner of each side rib. The vessels of the leaves (I mean those confined within the midribs and side ribs of the leaf) are of two sorts; the spiral and nourishing vessels. The spiral vessel is that corkscrew wire, that surrounds the two last rows of the sap vessels (as I shall show when I describe the division into which the stem should, I conceive be separated). The nourishing vessels are the only part formed of the wood, and convey the sap necessary for the support of the leaf, and run on each side of the spiral ones; which are generally divided into little bundles of 3, 5, or 7 sets. It is impossible for any delineation to be more exact, than that given in the Phil.Trans. by Mr. Knight, of the entrance of these vessels into the midrib of the leaf. That these spirals vessels are the cause of motion in leaves, and that they are perfectly solid and incapable of carrying moisture, I hope to prove in my next letter. Many leaves have a number of hairs fastened to the under cuticle of the leaf, and some to the upper. On the latter they appear designed to divide the rain drop to the size of the pore it is fitted for, and those at the back of the leaf seem intended to guard it from moisture, that the wet might not prevent the entrance of the carbonic gas at night; which it probably would do, without this precaution, by resting on the apertures. But it is watching nature in her natural state, that her laws are to be understood. When the wind blows with violence, the leaves turn their backs to the wind; and when the sun shines, they present their face to it: guarding by the first means the oxigen from dispersing, and in the latter case procuring a greater quantity, from the heat of the sun shining on the leaves. When the leaves are very young, they are pressed together, their backs exposed to the heat; probably to dry them, and clear the pores for the reception of the carbonic gas; and as young leaves give out hardly any oxigen, the shade in which the other side is immersed is of little consequence. To prove, that in forming the leaf I have given it no features, Description of but what it really possesses, I shall finish by showing all the the leaf. parts of a full grown leaf. The colour of leaves is not to be found in their substance, but in the liquid with which it is filled. The darkest green leaf that cau be taken, has a perfect white cuticle, both above and below it. In this cuticle are the pores. It is rather a thicker net below than above; but not enough to account for the difference of the tints; but the under one lies not near so close to the pabulum of the leaf as the upper one; which may account for the colour not piercing so much through. When these two nets are taken off, the pabulum of the leaf appears. It is formed of little bladders, filled with a dark green liquid, and interlaced with vessels. Take this off, and a bed of larger vessels presents itself; then a collection of bladders, which is followed by the larger lines of the leaf; and then a bed of bladders repeated, which the under cuticle covers. Though the bladders differ in size and colour in different leaves, and in thickness also, yet the general arrangement is the same. I mean not however to include either the firs, the grasses, or those grassy leaves of early spring, the iris, crocus, snowdrop, &c., which are all of a different nature, as I shall show hereafter. of hairs. I cannot quit the subject without adverting to the differ- Different sorts ent sorts of hairs, that are found on the back and face of the leaf. I have before mentioned some on the former part, intended to preserve the dryness; but on the face of the leaf there appear often many filled with moisture, as a kind of reservoir for the cuticle, and these are replete or not, according to the dryness of the atmosphere. There is also an innumerable multitude of things, that Microscopic are truly parasite plants, that grow on leaves, forming groves parasitical plants. and orchards for the various tribes of insects, that live and breed under them. As I do not wish to mix the different subjects, I shall conclude this letter, but mean to trouble you with another on the division of the stem of plants, without which I cannot well explain the discoveries I think I have made with respect to the motion and general formation of plants, or the effect that grafting and budding of every kind have on trees; a study which is now occupying every moment of my time, and from which I hope to draw many useful hints. The mistake made by my directing my letters to be sent to Mr. I. has led you into an errour. It is Mrs. Agnes IbLetson, who has the honour of being your correspondent. Dear Sir, Belleveu, June 8. Your obliged servant, A. IBBETSON. Explanation of the plate. • Explanation of the Figures. Plate VII, figs. 1, 2, 3, 4. Commencement of the growth of leaves, exhibited in different stages. a, a, a, a, the midrib. b, b, b, the young vessels appearing like cotton. c, c, the spiral nerves. d, the smaller vessels crossing each other. Fig. 4. The formation of the pabulum. e, e, the fine vessels growing up each side of the midrib. f, the pabu, lum. Fig. 5. Leaf-bud of the limetree. Fig. 6. Leaf-bud of the horse chestnut about January. Figs. 7, 8, and, 9, with some others, belong to two papers, which will appear next month. Canal in the spinal marrow XVI. A Letter on a Canal in the Medulla Spinalis of some Quadru peds. In a Letter from Mr. WILLIAM SEWELL, to EVERARD HOME, Esq. F.R.S.* SIR, ACCORDING to your request, I send you an account of the facts I have ascertained, respecting a canal I disco Philos. Trans. for 1809, Part I. p. 146. |