An introduction to physiological and systematical botany/Chapter 8

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Much contrariety of opinion has existed among physiologists concerning the vascular system of plants, and the nature of the propulsion of the sap through their stems and branches. Indeed it is a subject upon which, till lately, very erroneous ideas have prevailed.

That the whole vegetable body is an assemblage of tubes and vessels is evident to the most careless observer; and those who are conversant with the microscope, and books relating to it, have frequent opportunities of observing how curiously these vessels are arranged, and how different species of plants, especially trees, differ from each other in the structure and disposition of them. Such observations, however, if pursued no further, lead but a little way towards a knowledge of the wonderful physiology of vegetables.

In our 2d chapter, mention is made of the general cellular and vascular texture of plants; we must now be a little more particular in our inquiries.

That plants contain various substances, as sugar, gum, acids, odoriferous fluids and others, to which their various flavours and qualities are owing, is familiar to every one; and a little reflection will satisfy us that such substances must each be lodged in proper cells and vessels to be kept distinct from each other. They are extracted, or secreted, from the common juice of the plant, and called its peculiar or secreted fluids. Various experiments and observations, to be hereafter enlarged upon, prove also that air exists in the vegetable body, and must likewise be contained in appropriate vessels. Besides these, we know that plants are nourished and invigorated by water, which they readily absorb, and which is quickly conveyed through their stalks and leaves, no doubt by tubes or vessels on purpose. Finally, it is observable that all plants, as far as any experiment has been made, contain a common fluid, which at certain seasons of the year is to be obtained in great quantity, as from vine branches by wounding them in the spring before the leaves appear, and this is properly called the sap. It is really the blood of the plant, by which its whole body is nourished, and from which the peculiar secretions are made.

The great difficulty has been to ascertain the vessels in which the sap runs. Two of the most distinguished inquirers into the subject, Malpighi and Grew, believed the woody fibres, which make so large a part of the vegetable body, and give it consistence and strength, to be the sap-vessels, analogous to the blood-vessels of animals, and their opinion was adopted by Du Hamel. In support of this theory it was justly observed that these fibres are very numerous and strong, running longitudinally, often situated with great uniformity (an argument for their great importance), and found in all parts of a plant, although in some they are so delicate as to be scarcely discernible. But philosophers sought in vain for any perforation, any thing like a tubular structure, in the woody fibres to countenance this hypothesis, for they are divisible almost without end, like the muscular fibre. This difficulty was overlooked, because of the necessity of believing the existence of sap-vessels somewhere; for it is evident that the nutrimental fluids of a plant must be carried with force towards certain parts and in certain directions, and that this can be accomplished by regular vessels only, not, as Tournefort supposed, by capillary attraction through a simple spongy or cottony substance.

I received the first hint of what I now believe to be the true sap-vessels from the 2d section of Dr. Darwin's Phytologia, where it is suggested that what have been taken for air-vessels are really absorbents destined to nourish the plant, or, in other words, sap-vessels. The same idea has been adopted, confirmed by experiments, and carried to much greater perfection by Mr. Knight, whose papers in the Philosophical Transactions for 1801, 1804 and 1805 throw the most brilliant light upon it, and, I think, establish no less than an entirely new theory of vegetation, by which the real use and functions of the principal organs of plants are now for the first time satisfactorily explained.

In a young branch of a tree or shrub, or in the stem of an herbaceous plant, are found, ranged round the centre or pith, a number of longitudinal tubes or vessels, of a much more firm texture than the adjacent parts, and when examined minutely, these vessels often appear to be constructed with a spiral coat. This may be seen in the young twigs and leaf-stalks of Elder, Syringa, and many other shrubs, as well as in numerous herbaceous plants, as the Peony, and more especially many of the Lily tribe. If a branch or stalk of any of these plants be partly cut through or gently broken, and its divided portions slowly drawn asunder, the spiral coats of their Vessels will unroll, exhibiting a curious spectacle even to the naked eye. In other cases, though the spiral structure exists, its convolutions are scarcely separable at all, or so indeterminate as to be only marked by an interrupted line of perforations or slits, as shown by M. Mirbel. Indeed the very same branches which exhibit these spiral vessels when young, show no signs of them at a more advanced period of growth, when their parts are become more woody, firm and rigid. No such spiral-coated vessels have been detected in the bark at any period of its growth.

Malpighi asserts that these vessels are always found to contain air only, no other fluid; while Grew reports that he sometimes met with a quantity of moisture in them. Both judged them to be air-vessels, or, as it were, the lungs of plants, communicating, as these philosophers presumed, with certain vessels of the leaves and flowers, of an oval or globular form, but destitute of a spiral coat. These latter do really contain air, but it rather appears from experiment that they have no direct communication with the former. Thus the tubes in question have always been called air-vessels, till Darwin suggested their real nature and use[1]. He is perhaps too decisive when he asserts that none of them are air-vessels because they exist in the root, which is not exposed to the atmosphere. We know that air acts upon the plant under ground, because seeds will not vegetate in earth under the exhausted receiver of an air-pump. Phil. Trans. No. 23. I do not however mean to contend that any of these spiral vessels are air-vessels, nor do I see reason to believe that plants have any system of longitudinal air-vessels at all, though they must be presumed to abound in such as are transverse or horizontal.

Dr. Darwin and Mr. Knight have, by the most simple and satisfactory experiment, proved these spiral vessels to be the channel through which the sap is conveyed. The former placed leafy twigs of a common Fig-tree about an inch deep in a decoction of madder, and others in one of logwood. After some hours, on cutting the branches across, the coloured liquors were found to have ascended into each branch by these vessels, which exhibited a circle of red dots round the pith, surrounded by an external circle of vessels containing the white milky juice, or secreted fluid, so remarkable in the fig-tree. Mr. Knight, in a similar manner, inserted the lower ends of some cuttings of the Apple-tree and Horse-chesnut into an infusion of the skins of a very black grape in water, an excellent liquor for the purpose. The result was similar. But Mr. Knight pursued his observations much further than Dr. Darwin had done; for he traced the coloured liquid even into the leaves, "but it had neither coloured the bark nor the sap between it and the wood; and the medulla was not affected, or at most was very slightly tinged at its edges." Phil. Trans, for 1801, p. 335.

The result of all Mr. Knight's experiments and remarks seems to be, that the fluids destined to nourish a plant, being absorbed by the root and become sap, are carried up into the leaves by these vessels, called by him central vessels, from their situation near the pith. A particular set of them, appropriated to each leaf, branches off, a few inches below the leaf to which they belong, from the main channels that pass along the alburnum, and extend from the fibres of the root to the extremity of each annual shoot of the plant. As they approach the leaf to which they are destined, the central vessels become more numerous, or subdivided. "To these vessels," says Mr. Knight, "the spiral tubes are every where appendages." p. 336. By this expression, and by a passage in the following page[2], 337, this writer might seem to consider the spiral line, which forms the coats of these vessels, as itself a pervious tube, or else that he was speaking of other tubes with a spiral coat, companions of the sap-vessels; but the plate which accompanies his dissertation, and the perspicuous mode in which he treats the subject throughout, prevent our mistaking him on the last point. In order to conceive how the sap can be so powerfully conveyed as it is through the vessels in which it flows, from the root of a tall tree to its highest branches, we must take into consideration the action of heat. We all know that this is necessary to the growth and health of plants; and that it requires to be nicely adjusted in degree, in order to suit the constitutions of different tribes of plants destined for different parts of the globe. It cannot but act as a stimulus to the living principle, and is one of the most powerful agents of Nature upon the vegetable as well as animal constitution. Besides this, however, various mechanical causes may be supposed to have their effect; as the frequently spiral or screw-like form of the vessels, in some of which, when separated from the plant, Malpighi tells us he once saw a very beautiful undulating motion that appeared spontaneous. This indeed has not been seen by any other person, nor can it be supposed that parts so delicate can, in general, be removed from their natural situation, without the destruction of that fine irritability on which such a motion must depend. We may also take into consideration the agitation of the vegetable body by winds, which is known by experience to be so wholesome to it[3], and must serve powerfully to propel the fluids of lofty trees; the passage, and evolution perhaps, of air in other parts or vessels, surrounding and compressing these; and lastly the action, so ingeniously supposed by Mr. Knight, of those thin shining plates called the silver grain, visible in oak wood, which pressing upon the sap-vessels, and being apparently susceptible of quick changes from variations in heat or other causes, may have a powerful effect. "Their restless temper," says Mr. Knight, "after the tree has ceased to live, inclines me to believe that they are not made to be idle whilst it continues alive." Phil. Trans, for 1801, p. 344. These plates are presumed by the author just quoted to be peculiarly useful in assisting the ascent of the sap through the alburnum of the trunk or chief branches, where indeed the spiral coats of the vessels are either wanting, or less elastic than in the leaf-stalks and summits of the more tender shoots.

However its conveyance may be accomplished, it is certain that the sap does reach the parts above mentioned, and there can surely be now as little doubt of the vessels in which it runs. That these vessels have been thought to contain air only, is well accounted for by Dr. Darwin, on the principle of their not collapsing when emptied of their sap; which is owing to their rigidity, and the elastic nature of their coats. When a portion of a stem or branch is cut off, the sap soon exhales from it, or rather is pushed out by the action of the vessels themselves: hence they are found empty; and for the same reason the arteries of animals were formerly thought to contain air only. When the sap-vessels have parted with their natural contents, air and even quicksilver will readily pass through them, as is shown by various experiments. Arguments in support of any theory must be very cautiously deduced from such experiments, or from any other observations not made on vegetables in their most natural state and condition; and, above all, that great agent the vital principle must always be kept in view, in preference to mere mechanical considerations.

These to which I give the common name of sap-vessels, comprehending the common tubes of the alburnum, and the central vessels, of Mr. Knight, may be considered as analogous to the arteries of animals; or rather they are the stomach, lacteals and arteries all in one, for I conceive it to be a great error in Dr. Darwin to call by this name the vessels which contain the peculiar secretions of the plant[4]. These sap-vessels, no doubt, absorb the nutritious fluids afforded by the soil, in which possibly, as they pass through the root, some change analogous to digestion may take place; for there is evidently a great difference, in many cases, between the fluids of the root, at least the secreted ones, and those of the rest of the plant; and this leads us to presume that some considerable alteration may be wrought in the sap in its course through that important organ. The stem, which it next enters, is by no means an essential part, for we see many plants whose leaves and flowers grow directly from the root.

Part of the sap is conveyed into the flowers and fruit, where various fine and essential secretions are made from it, of which we shall speak hereafter. By far the greater portion of the sap is carried into the leaves, of the great importance and utility of which to the plant itself Mr. Knight's theory is the only one that gives us any adequate or satisfactory notion. In those organs the sap is exposed to the action of light, air and moisture, three powerful agents, by which it is enabled to form various secretions, at the same time that much superfluous matter passes off by perspiration. These secretions not only give peculiar flavours and qualities to the leaf itself, but are returned by another set of vessels, as Mr. Knight has demonstrated, into the new layer of bark, which they nourish and bring to perfection, and which they enable in its turn to secrete matter for a new layer of alburnum the ensuing year. It is presumed that one set of the returning vessels of trees may probably be more particularly destined to this latter office, and another to the secretion of peculiar fluids in the bark. See Phil. Trans, for 1801, p. 337. In the bark principally, if I mistake not, the peculiar secretions of the plant are perfected, as gum, resin, &c., each undoubtedly in an appropriate set of vessels. From what has just been said of the office of leaves, we readily perceive why all the part of a branch above a leaf or leaf-bud dies when cut, as each portion receives nourishment, and the means of increase, from the leaf above it.

By the above view of the vegetable œconomy, it appears that the vascular system of plants is strictly annual. This, of course, is admitted in herbaceous plants, the existence of whose stems, and often of the whole individual, is limited to one season; but it is no less true with regard to trees. The layer of alburnum on the one hand is added to the wood, and the liber, or inner layer of the bark, is on the other annexed to the layers formed in preceding seasons, and neither have any share in the process of vegetation for the year ensuing. Still, as they continue for a long time to be living bodies, and help to perfect, if not to form, secretions, they must receive some portion of nourishment from those more active parts which have taken up their late functions.

There is a tribe of plants called monocotyledones, characterized by having only one lobe to the seed, whose growth requires particular mention. To these belongs the natural order of Palms, which being the most lofty, and, in some instances, the most long-lived of plants, have justly acquired the name of trees. Yet, paradoxical as it may seem, they are rather perennial herbaceous plants, having nothing in common with the growth of trees in general. Their nature has been learnedly explained by M. Desfontaines, a celebrated French botanist, and by M. Mirbel in his Traité d'Anatomie et de Physiologie Végétales, vol. 1. p. 209, and Linnæus has long ago made remarks to the same purpose. The Palms are formed of successive circular crowns of leaves, which spring directly from the root. These leaves and their footstalks are furnished with bundles of large sap-vessels and returning vessels, like the leaves of our trees. When one circle of them has performed its office, another is formed within it, which being confined below, necessarily rises a little above the former. Thus successive circles grow one above the other, by which the vertical increase of the plant is almost without end. Each circle of leaves is independent of its predecessor, and has its own clusters of vessels, so that there can be no aggregation of woody circles; and yet in some of this tribe the spurious kind of stem, formed in the manner just described, when cut across shows something of a circular arrangement of fibres, arising from the original disposition of the leaves. The common orange lily, Lilium bulbiferum, Curt. Mag. t. 36, and white lily, L. candidum, t. 278, which belong to the same natural family of monocotyledones, serve to elucidate this subject. Their stems, though of only annual duration, are formed nearly on the same principle as that of a Palm, and are really congeries of leaves rising one above another, and united by their bases into an apparent stem. In these the spiral coats of the sap-vessels are very easily discernible.

To conclude this subject of the propulsion of the sap, it is necessary to say a few words on the power which the vessels of plants are reported to possess of conveying their appropriate fluids equally well in either direction; or, in other words, that it is indifferent whether a cutting of any kind be planted with its upper or lower end in the ground. On this subject also Mr. Knight has afforded us new information, by observing that, in cuttings so treated, the returning vessels retain so much of their original nature as to deposit new wood above the leaf-buds; that is, in the part of the cutting which, if planted in its natural position, would have been below them. It appears, however, that the sap-vessels must absorb and transmit their sap in a direction contrary to what is natural; and it is highly probable, that after some revolving seasons new returning vessels would be formed in that part of the stem which is now below the buds. I presume there can be no doubt that successive new branches would deposit their wood in the usual position. It is nevertheless by no means common for such inverted cuttings to succeed at all. An experiment to a similar purpose is recorded by Dr. Hales, Vegetable Staticks, p. 132, t. 11, of engrafting together three trees standing in a row, and then cutting off the communication between the central one and the earth, so that it became suspended in the air, and was nourished merely through its lateral branches. The same experiment was successfully practised by the late Dr. Hope at Edinburgh upon three Willows, and in the years 1781, 2, and 3, I repeatedly witnessed their health and vigour. It was observed that the central tree was several days later in coming into leaf than its supporters, but I know not that any other difference was to be perceived between them. The tree which wanted the support of the ground was, some years after, blown down, so that we have now no opportunity of examining the course of its vessels, or the mode in which successive layers of wood were deposited in its branches; but the experiment is easily repeated.

In the weeping variety of the Common Ash, now so frequent in gardens, the branches are completely inverted as to position, yet the returning fluids appear to run exactly in their natural direction, depositing new wood, as they are situated above the buds or leaves; and if the end of any branch be cut, all beyond (or below) the next bud dies; so that in this case gravitation, to which Mr. Knight attributes considerable power over the returning fluids, Phil. Trans, for 1804, does not counteract the ordinary course of nature.

  1. Du Hamel, indeed, once suspected that they contained "highly rarefied sap," but did not pursue the idea.
  2. "The whole of the fluid, which passes from the wood to the leaf, seems to me evidently to be conveyed through a single kind of vessel; for the spiral tubes will neither carry coloured infusions, nor in the smallest degree retard the withering of the leaf, when the central vessels are divided.' Knight.
  3. See Mr. Knight's experiments in confirmation of this in the Phil. Trans. for 1803, p. 280.
  4. Phytologia, sect. 2.