Part 46
By the bark the tree is fed with a continual supply of moisture, protected from external injuries, and defended from the excesses of heat and cold; for these purposes it is variously disposed in different trees. In the hardy and slow growing, as the oak and chesnut, it is thin; in the quick growing, as willow, poplar, and the like, it is thick. And what is more particularly to be attended to is, that in some its inner verge is radiated. There are some trees, and a great many herbaceous plants, in which this part is continued inward, in form of rays, through the blea into the wood, and seems to form so many green wedges, that split as it were the substance of both those parts;[135] a circumstance which accounts for the vegetation of some particular trees, which are known to live when deprived of the bark; because they have rays of the same substance within which answer the purpose, and this in a degree answering to the nature of their life.
[135] Hill’s Construction of Timber, p. 118. Ibid. p. 120,
The bark appears to be formed, first, of longitudinal fibres, which Du Hamel considers as so many lymphatic vessels; secondly, by a sort of a filmy cellular tissue, which has been considered as a kind of bladders by some, or as parenchymatous by others; thirdly, of the vasa propria interiora, or interior juice vessels.
The longitudinal fibres are disposed in strata, which lie one over the other. In that stratum which is next the rind, or rather the cellular coat, we perceive a net of longitudinal fibres, the meshes of which are large and easily distinguished, particularly when the cellular tissue that fills up the interstices is removed. To do this, the branches should be macerated for a considerable time; some require to be kept in this state for years. It will then be easy to separate first the rind, then the cellular coating, and afterwards this pulpy matter. It may sometimes be easily removed after the branches have been boiled.
The most exterior stratum, when examined by the naked eye, seems to be formed of simple fibres, which graft, solder, or inosculate one with the other; but when examined by a microscope, each of these fibres will be found to be a bundle of filaments, which may be easily separated from each other.
Grew says, that each filament, like the nerves in animals, consists of twenty or thirty small contiguous tubes, which run uniformly from the extremity of the root, without sending off any branches, or suffering any change in their size and shape. Hence the bark may be torn or divided lengthwise, with greater ease than in an horizontal direction; when macerated, they are capable of a very great degree of subdivision.
The filaments of a cortical vessel are to be looked on, agreeable to what we have already observed, as so many little bundles placed near together, and at first growing parallel to each other; but soon quitting this direction, the filaments of one fascicle parting from that to which they originally belonged, and inclining more or less obliquely towards another, sometimes uniting with it, at others, bending backwards, and uniting again with that from which it proceeded, or with some one that it meets with. In this manner new fascicles are often formed, while other parcels are increased or diminished by the additions of new filaments; by these means, a kind of irregular net is formed, and the fibres proceed in a serpentine line from the top to the bottom of the tree.
The thickness of the bark is entirely formed of strata of these longitudinal fibres, which lie one over the other; each of these strata is similar to the exterior one, only the meshes are smaller, and the fibres finer, in proportion as they are more interior, insomuch that at last the meshes are almost annihilated, and the fibres seem to lie quite parallel to each other.
There are some trees, however, where the meshes are not visible, and in which the fibres lie quite in a straight direction. There are many other circumstances in which they vary in different trees; in some the meshes of each stratum correspond with each other, diminishing gradually in size as they are more interior, and forming as it were so many conical cells.
We may, I think, conclude from what has been said, that the bark is composed of several thin membranes, which extend over the whole exterior surface of the tree. The most exterior membrane is the rind; under this is what Du Hamel calls the cellular coat; next to this the cortical stratum or true bark of the tree, which is formed of lymphatic vessels ranged more or less in a reticular form, and of the vasa propria interiora. The meshes are so constituted as to form large cavities next the rind, and small ones near the wood. These cavities are filled with a parenchymatous substance or the cellular tissue, which being continued from the wood to the rind, joins and unites the cortical stratum, and afterwards spreading on the outside thereof, forms what has been termed the cellular coat.
OF THE CELLULAR TISSUE.
We now proceed to give some account of the substance which fills up the vacant spaces that are left between the longitudinal fibres. It is called by Grew the parenchyma or pulp, by Malpighi, the vesicular tissue or web; both of them consider it as formed of small bladders or reticles, that are in contact with each other, lying in an horizontal position, or at right angles to the longitudinal fibres: they do not suppose them to be all of the same size, or even of the same figure: Grew compares it to the froth of beer or eggs. The flesh of fruits consists for the most part of this substance, very much filled with juice, though with considerable difference in its organization. Be this as it may, the nature of this substance, its form and structure, are at present but very little known. It is floccose, and varies in colour in different species.
OF THE VASA PROPRIA INTERIORA.
Besides the lymphatic vessels and the cellular substance, we find the juice vessels, or vasa propria, in the bark. In those trees which are famous for medicinal virtues, they are usually very large; they carry the milky juices of the sumach, and in them is lodged the finest and highest-flavoured turpentine in all the kinds of pine. Dr. Hill thinks that a tree of that genus exhibits them best, and the more, as the turpentine which fills them may be perfectly dissolved in spirit of wine. The pinus orientalis is the species in which these vessels are most distinctly seen.
OF THE BLEA.
This is that part of the tree which is formed into wood, and therefore lies between it and the bark, and may be separated from them by maceration.
A longitudinal piece of the blea, when examined by the microscope, exhibits a number of vessels running parallel to each other, the interstitial spaces being filled with a floccose, white, formless substance, of which Dr. Hill suspects even the vessels themselves to be formed. Innumerable small openings or mouths may be discovered in these vessels, suited to imbibe the moisture which is so essential to the life and health of plants. These mouths cannot be well discerned, except when they are opened by the season of the year, either before the first leaves of spring, or in the midsummer shooting time; though a small quantity of moisture will keep them open at that time, yet no quantity would be sufficient at an improper season.[136]
[136] Hill’s Construction of Timber, p. 47.
The blea is a zone more or less perfect, which lies under the bark, and covers or surrounds the wood, and is principally distinguished from it by being less dense. In some species the difference between the blea and the wood is very remarkable, in others it is less so.
The ancient botanists, struck with the difference they observed between the wood and the blea, compared this substance to the fat in animals. Malpighi, Grew, and Du Hamel considered it as the wood not yet arrived to a state of perfection. It is organized in a manner similar to the wood, and possessing the same vessels disposed nearly in the same manner. The juice vessels of this part may be separated from it by maceration; Dr. Hill says, that in this state they appear perfect cylinders, with thick white coats, the surface perfectly uniform.
OF THE WOOD.
When the bark and the blea are taken away, we come to the wood, which is a solid substance, on which the strength of the tree depends, and which has been considered by naturalists as being to the tree what bones are to the animal. The wood, in a general view may be considered as formed of strata, which are inclosed one within the other; these strata consist of ligneous fibres or lymphatic vessels, the cellular web or tissue, vasa propria, and what have been called the air vessels. It is more difficult to investigate the construction of the wood than that of the other parts, because the texture is in general much harder, and therefore not so easily separated, requiring very long macerations, and many subjects, before one may be found fit for examination.
If a transverse section of almost any kind of wood be examined, we shall perceive these strata very clearly and sensibly distinguished from one another. It has been generally supposed that each of these is the product of one year’s growth; though, if we cut the same wood obliquely, it will be found that each of these strata is compounded of smaller ones, which are therefore not so easy to discover as the larger. By macerating rotten pieces of trees, the wood may be divided into an immense number of leaves or strata, thinner than the finest paper.
If the foregoing strata be examined in their detached state by the microscope, we shall find them to be composed of longitudinal fibres; some pieces of rotten wood, after maceration, will divide of themselves into very fine longitudinal fibres; the existence of these is further proved by the facility with which wood may be split in the direction of these fibres. From hence we may collect, that the ligneous strata are formed of small fibres or vessels, collected together in fascicles, like the bark: in some trees they are parallel to each other, in others they are disposed more obliquely, crossing and forming an irregular kind of network. There is great probability that this reticular disposition exists in all trees, though it may be difficult to discover it in many on account of the fineness of the meshes, the hardness of the wood, and the sameness of colour in the constituent fibres.
We are here only speaking of the lymphatic vessels or ligneous fibres of the wood, which exist in it as well as in the bark, though in different states; for the ligneous fibres are always harder and less flexible than the cortical ones. Malpighi thinks they differ in another particular, namely, that a juice or fluid issues from the cortical fibres, while none is found in those of the wood. In this it would appear from the observations of Du Hamel, that he was mistaken.
A transverse section of wood generally appears formed of a number of rays proceeding from the corona to the bark, which are intersected at different distances by concentric circles, interspersed with vessels of varying magnitude: the variations in this structure afford much pleasure to the curious observer, and throw considerable light upon the nature and properties of timber; for it is by means of a variety of strainers that different juices are prepared from the same mass. Matter, considered as matter, has no share in the qualities of bodies. It is from the arrangement of it, or the recipient forms given to it, that we have so many different substances. According to the modifications that these receive, we shall find the same light, air, water, and earth, manifesting themselves in one by a deadly poison, and in another by the most salubrious food. A lemon ingrafted upon an orange stock, is capable of changing the sap of the orange into its own nature, by a different arrangement of the nutritive juices. One mass of earth will give life and vigour to the bitter aloe, to the sweet cane, the cool house-leek, and the fiery mustard, the nourishing grain, and the deadly night-shade.
The wood may be considered as composed of two parts, ligneous and parenchymatous. The former has already been treated of; the latter is that which is disposed into rays, running as it were between the ligneous fibres, and interweaving with them; it originates either with the pith or corona. There is a very great diversity in these radial insertions; in some trees there are very few, while they abound in others; in some they are very fine, in others very thick. In texture, they seem similar to the blebs of the bark, only that here they are so crowded and stretched out as to appear like parallel threads, somewhat similar to a net when drawn tight.
OF THE CORONA.
Dr. Hill gives this name to that circle which surrounds the pith, and separates it from the wood; although in his opinion it differs greatly from both, and in its composition has no resemblance to either. It is, according to him, the most important part in the whole vegetable fabric, by which the propagation and increase of the branches, buds, and shoots, are carried on.[137]
[137] Hill on the Construction of Timber, p. 55.
It has been usual to suppose the pith of vegetables to be the part in which these wonderful sources of increase reside, but this is not the case; and he asserts, that so far from being prior to the other parts, it is in reality posterior to some of them in the time of its formation.
The corona is not so uniform as the other parts, nor is it constituted exactly similar in all trees. It is placed between the pith and wood in all vegetables, forming a ring, whose outline is more or less regulated. The general circle is cellular, composed of blebs and vessels, like the bark and the rind, and is perfectly similar to them, only that at different distances oblong clusters of different vessels are placed amongst it. These clusters are usually eight or ten in number, and give origin to the angles of the corona. They are not uniform, or of one kind of vessels, as in the bark, but each has two distinct sorts, the exterior one answering to the blea, and the interior, to the wood of trees; and within each of these are disposed vessels not unlike those in the blea and wood, though often larger than they are found in those parts.
Thus each cluster is composed of all the essential parts of the succeeding branch, and the intermediate parts of the circle are absolutely bark and rind; they are ready to follow and clothe the cluster when it goes off in the form of a shoot, because it will then need their covering and defence, though in its present inclosed state it does not. It is from this construction, that a tree is ready at all times and in all parts to shoot out branches, and every branch in the same manner to send out others; for the whole trunk, and the branch in all its length, have this course of eight or ten clusters of essential vessels ready to be protruded out, and the proper and natural integuments as ready to cover them. In some trees, these parts are more evident, in others more obscurely arranged. Dr. Hill says, the bocconia, or parrot-wood of the West-Indies, and the greater celandine, are proper subjects for opening this great mystery of nature. On the corona and its clusters depend that property of vegetables, that they can be produced entire from every piece. These clusters follow the course of the other portions of the tree; they are therefore everywhere; they are always capable of growing, and their growth, even in a cutting of the smallest twig, cannot produce a leaf, or any other part of a vegetable alone, but must afford the whole; for they are complete bodies, and the whole is there waiting only for the opportunity of extension, by obtaining sufficient nourishment. For the knowledge we have of this part we are altogether indebted to Dr. Hill. It remains for future observers to confirm, or disprove his observations.
OF THE PITH.
The pith is found in the center of every young shoot of a tree; it is large in some, less in others, but present in all. It is placed close within the corona.
It seems to be nothing more than a congeries of the cellular tissue; it is generally found near the center of the tree, inclosed as it were within a tube; in general, the cells of the pith are larger than those of the cellular tissue, with which, according to Du Hamel, it communicates. For the rays which extend from the pith to the bark are, in his opinion, produced from it. Thus, though it may differ in name from the parenchymatous parts of the bark, and the radial insertions in the wood, yet it is of the same nature and texture, and is continuous with them; so that, according to this idea, the skin, the parenchyma, the insertions, and the pith, are all one piece of work, filled up in divers manners with the vessels.
The bark and the wood grow thicker every year, while the pith, on the contrary, grows more slender, so that in a branch of one year it is of a larger size than it is in the same branch when two years old, and so on. In very young branches, while in an herbaceous state, the pith forms the greatest part of its substance; but when the fibres are stronger, the pith becomes less succulent, and surrounded with a tube of wood; when the branch has arrived to a certain age, it is so compressed as to be almost annihilated. In examining different branches that proceed from others in their first state, a small communication between the pith of the one and the other will be found; but this communication is generally entirely closed up in the second or third year.[138] The cells of which the pith is formed are at first entirely one connected body; but as the plant grows up, it is often so broken and ruptured, as to remain no longer a continuous substance.
[138] Du Hamel Physique des Arbres, tom. 1, p. 38.
This, as well as many other particulars in the history of the pith, corroborates the opinion of Dr. Hill,[139] who thinks it is formed for the purpose of moistening the clusters of the corona, and regulating its extension; it has been supposed coeval with, or primordial to all the other parts, but he thinks it is postnate, and comes after them in the order of time, as well as in its uses; that exhaled air gives origin to its blebs, and the thickness of the juices cloathing the bubble, gives it form and substance. The first season is the time of its greatest use, and it immediately after begins to decay.
[139] Hill’s Construction of Timber, p. 66.
The pith has in general been represented as much more complex than it really is. It consists of a range of bladders lying one over the other. The membrane is simple, the outline single; but as it is very difficult to procure it in this simple state, it is often seen and represented under a variety of irregular, though pleasing forms, which are occasioned by the intersections of the outlines of the blebs, as seen one over another.
A cluster in any part of the corona, protruding itself onward and outward in the growing season,[140] carries a part of the circle out with it. The cluster itself is a perfect piece of the wood and blea, and the bark which follows it out in its progress perfectly clothes it; thus is the first protrusion of the shoot made, but all this while there is no pith. The continuation of growth is made by the extension of all the parts obliquely upwards; in the course of this extension they hollow themselves into a kind of cylinder, of the form of the future branch, and by this disposition a small vacancy is made in their center. This enlarges as they increase, and as it enlarges it becomes filled with the exudation of those little bladders which remain and constitute the pith, fed from the inner coat of the pith, which already begins to form itself into a new corona. Grew seemed to think, that in some instances the pith was of posterior growth to the other parts, and derived its origin from the bark; and that the insertions of the bark running in between the rays of the wood meet in the center, and constitute the pith.
[140] Hill’s Construction of Timber, p. 99.
OF THE SAP VESSELS.
The most numerous and the largest apertures are generally to be found in the wood, which are perceived very distinctly in a transverse section, in which the ends of the vessels are seen as cut through by the knife. The scarlet oak of America is recommended as a proper object for exhibiting them. If a short cylinder of a three years branch of this oak, a little macerated, be hollowed away with a chissel, we shall see what a large portion of the wood is occupied by these vessels; they are thick and strong, and it is easy, with some care and attention, to loosen several of them.
If a number of these thus separated be put into a vial of rain water, and frequently shook for several days, some will at length be found perfectly clean; these are then to be put into spirit of wine, and when that has been two or three times changed, they will be in a condition to be viewed for understanding their structure; another method of preparation has already been shewn in page 162.
These are the vessels which have been called by some writers air, by others, tracheal vessels. It is, however, to be remarked, that most of those who have considered them as air vessels, refer us to the tree while in a more herbaceous state; in this case they say, that we shall find these parts filled with a fine spiral filament. As these vessels are often to be found empty, they have been supposed to answer the purposes of lungs to the plant. Malpighi asserts, that if they be examined in winter, they often exhibit a vermicular motion, which astonishes the spectator.
Those who suppose the corona to contain the whole structure of the tree in miniature, and that it is the embryo of future shoots, suppose it to contain the vessels proper for each part, a subject that must be left to the decision of future observers.
OF THE VASA PROPRIA INTIMA.
These are the only vessels which remain to be spoken of. They are large, conspicuous, and important; their natural place is in the blea, though they are sometimes repeated in the wood and the corona. Their coats are thicker than those of any other vessels.[141] It is not difficult, after a successful maceration, to separate some of these vessels from the blea; in this state they appear perfect cylinders, with thick white coats, of a firm, solid, and uniform texture.
[141] Hill’s Construction of Timber, p. 83 and 85.
It has generally been supposed, that each of those concentric circles, which are to be observed in the transverse section of almost every tree, was the product of one year, or the quantity of wood added to the tree in that space; here, however, Dr. Hill differs again from the general opinion.
From what has been said, we may deduce the following general ideas relative to the organization of trees. The most obvious and remarkable parts of a plant, or tree, are the root, the stem, the branches, the leaves, the flower, and the fruit. The component parts of these divisions are not complicated; they are simple when compared with those of an animal, and this because the offices of the vegetable are fewer than those of the animal.
The interior part may be considered as consisting of ligneous fibres, interspersed with a vast number of bladders, which are here named the cellular tissue, the vasa propria, and the sap vessels; though these are considered by some writers as mere air vessels.