Chapter VI: The Branch

§ 1. We have hitherto spoken of each shoot as either straight or only warped by its spiral tendency; but no shoot of any length, except those of the sapling, ever can be straight; for, as the family of leaves which it bears are forced unanimously to take some given direction in search of food or light, the stalk necessarily obeys the same impulse, and bends itself so as to sustain them in their adopted position, with the greatest ease to itself and comfort for them.

In doing this, it has two main influences to comply or contend with: the first, the direct action of the leaves in drawing it this way or that, as they themselves seek particular situations; the second, the pressure of their absolute weight after they have taken their places, depressing each bough in a given degree; the leverage increasing as the leaf extends. To these principal forces may frequently be added that of some prevalent wind, which, on a majority of days in the year, bends the bough, leaves and all, for hours together, out of its normal position. Owing to these three forces, the shoot is nearly sure to be curved in at least two directions;[1] that is to say, not merely as the rim of a wine-glass is curved (so that, looking at it horizontally, the circle becomes a straight line), but as the edge of a lip or an eyebrow is curved, partly upward, partly forwards, so that in no possible perspective can it be seen as a straight line. Similarly, no perspective will usually bring a shoot of a free-growing tree to appear a straight line.

§ 2. It is evident that the more leaves the stalk has to sustain, the more strength it requires. It might appear, therefore, not unadvisable, that every leaf should, as it grew, pay a small tax to the stalk for its sustenance; so that there might be no fear of any number of leaves being too oppressive to their bearer. Which, accordingly, is just what the leaves do. Each, from the moment of his complete majority, pays a stated tax to the stalk; that is to say, collects for it a certain quantity of wood, or materials for wood, and sends this wood, or what ultimately will become wood, _down_ the stalk to add to its thickness.

§ 3. "Down the stalk?" yes, and down a great way farther. For, as the leaves, if they did not thus contribute to their own support, would soon be too heavy for the spray, so if the spray, with its family of leaves, contributed nothing to the thickness of the branch, the leaf-families would soon break down their sustaining branches. And, similarly, if the branches gave nothing to the stem, the stem would soon fall under its boughs. Therefore, by a power of which I believe no sufficient account exists,[2] as each leaf adds to the thickness of the shoot, so each shoot to the branch, so each branch to the stem, and that with so perfect an order and regularity of duty, that from every leaf in all the countless crowd at the tree's summit, one slender fibre, or at least fibre's thickness of wood, descends through shoot, through spray, through branch, and through stem; and having thus added, in its due proportion, to form the strength of the tree, labors yet farther and more painfully to provide for its security; and thrusting forward into the root, loses nothing of its mighty energy, until, mining through the darkness, it has taken hold in cleft of rock or depth of earth, as extended as the sweep of its green crest in the free air.

§ 4. Such, at least, is the mechanical aspect of the tree. The work of its construction, considered as a branch tower, partly propped by buttresses, partly lashed by cables, is thus shared in by every leaf. But considering it as a living body to be nourished, it is probably an inaccurate analogy to speak of the leaves being taxed for the enlargement of the trunk. Strictly speaking, the trunk enlarges by sustaining them. For each leaf, however far removed from the ground, stands in need of nourishment derived from the ground, as well as of that which it finds in the air; and it simply sends its root down along the stem of the tree, until it reaches the ground and obtains the necessary mineral elements. The trunk has been therefore called by some botanists a "bundle of roots," but I think inaccurately. It is rather a messenger to the roots.[3] A root, properly so called, is a fibre, spongy or absorbent at the extremity, which secretes certain elements from the earth. The stem is by this definition no more a cluster of roots than a cluster of leaves, but a channel of intercourse between the roots and the leaves. It can gather no nourishment. It only carries nourishment, being, in fact, a group of canals for the conveyance of marketable commodities, with an electric telegraph attached to each, transmitting messages from leaf to root, and root to leaf, up and down the tree. But whatever view we take of the operative causes, the external and visible fact is simply that every leaf does send down from its stalk a slender thread of woody matter along the sides of the shoot it grows upon; and that the increase of thickness in stem, proportioned to the advance of the leaves, corresponds with an increase of thickness in roots, proportioned to the advance of their outer fibres. How far interchange of elements takes place between root and leaf, it is not our work here to examine; the general and broad idea is this, that the whole tree is fed partly by the earth, partly by the air;--strengthened and sustained by the one, agitated and educated by the other;--all of it which is best, in substance, life, and beauty, being drawn more from the dew of heaven than the fatness of the earth. The results of this nourishment of the bough by the leaf in external aspect, are the object of our immediate inquiry.

§ 5. Hitherto we have considered the shoot as an ascending body, throwing off buds at intervals. This it is indeed; but the part of it which ascends is not seen externally. Look back to Plate 51. You will observe that each shoot is furrowed, and that the ridges between the furrows rise in slightly spiral lines, terminating in the armlets under the buds which bore last year's leaves. These ridges, which rib the shoot so distinctly, are not on the ascending part of it. They are the contributions of each successive leaf thrown out as it ascended. Every leaf sent down a slender cord, covering and clinging to the shoot beneath, and increasing its thickness. Each, according to his size and strength, wove his little strand of cable, as a spider his thread; and cast it down the side of the springing tower by a marvellous magic--irresistible! The fall of a granite pyramid from an Alp may perhaps be stayed; the descending force of that silver thread shall not be stayed. It will split the rocks themselves at its roots, if need be, rather than fail in its work.

So many leaves, so many silver cords. Count--for by just the thickness of one cord, beneath each leaf, let fall in fivefold order round and round, the shoot increases in thickness to its root:--a spire built downwards from the heaven.

And now we see why the leaves dislike being above each other. Each seeks a vacant place, where he may freely let fall the cord. The turning aside of the cable to avoid the buds beneath, is one of the main causes of spiral curvature, as the shoot increases. It required all the care I could give to the drawing, and all Mr. Armytage's skill in engraving Plate 51, to express, though drawing them nearly of their full size, the principal courses of curvature in even this least graceful of trees.

§ 6. According to the structure thus ascertained, the body of the shoot may at any point be considered as formed by a central rod, represented by the shaded inner circle, _a_, Fig. 36, surrounded by as many rods of descending external wood as there are leaves above the point where the section is made. The first five leaves above send down the first dark rods; and the next above send down those between, which, being from younger leaves, are less liable to interstices; then the third group sending down the side, it will be seen at a glance how a spiral action is produced. It would lead us into too subtile detail, if I traced the forces of this spiral superimposition. I must be content to let the reader peruse this part of the subject for himself, if it amuses him, and lead to larger questions.

§ 7. Broadly and practically, we may consider the whole cluster of woody material in Fig. 36 as one circle of fibrous substance formed round a small central rod. The real appearance in most trees is approximately as in _b_, Fig. 36, the radiating structure becoming more distinct in proportion to the largeness and compactness of the wood.[4]

Now the next question is, how this descending external coating of wood will behave itself when it comes to the forking of the shoots. To simplify the examination of this, let us suppose the original or growing shoot (whose section is the shaded inner circle in Fig. 36) to have been in the form of a letter Y, and no thicker than a stout iron wire, as in Fig. 37. Down the arms of this letter Y, we have two fibrous streams running in the direction of the arrows. If the depth or thickness of these streams be such as at _b_ and _c_, what will their thickness be when they unite at _e_? Evidently, the quantity of wood surrounding the vertical wire at _e_ must be twice as great as that surrounding the wires _b_ and _c_.

§ 8. The reader will, perhaps, be good enough to take it on my word (if he does not know enough of geometry to ascertain), that the large circle, in Fig. 38, contains twice as much area as either of the two smaller circles. Putting these circles in position, so as to guide us, and supposing the trunk to be bounded by straight lines, we have for the outline of the fork that in Fig. 38. How, then, do the two minor circles change into one large one? The section of the stem at _a_ is a circle; and at _b_, is a circle; and at _c_, a circle. But what is it at _e_? Evidently, if the two circles merely united gradually, without change of form through a series of figures, such as those at the top of Fig. 39, the quantity of wood, instead of remaining the same, would diminish from the contents of two circles to the contents of one. So for every loss which the circles sustain at this junction, an equal quantity of wood must be thrust out somehow to the side. Thus, to enable the circles to run into each other, as far as shown at _b_, in Fig. 39, there must be a loss between them of as much wood as the shaded space. Therefore, half of that space must be added, or rather pushed out on each side, and the section of the uniting branch becomes approximately as in _c_, Fig. 39; the wood squeezed out encompassing the stem more as the circles close, until the whole is reconciled into one larger single circle.

§ 9. I fear the reader would have no patience with me, if I asked him to examine, in longitudinal section, the lines of the descending currents of wood as they eddy into the increased single river. Of course, it is just what would take place if two strong streams, filling each a cylindrical pipe, ran together into one larger cylinder, with a central rod passing up every tube. But, as this central rod increases, and, at the same time, the supply of the stream from above, every added leaf contributing its little current, the eddies of wood about the fork become intensely curious and interesting; of which thus much the reader may observe in a moment by gathering a branch of any tree (laburnum shows it better, I think, than most), that the two meeting currents, first wrinkling a little, then rise in a low wave in the hollow of the fork, and flow over at the side, making their way to diffuse themselves round the stem, as in Fig. 40. Seen laterally, the bough bulges out below the fork, rather curiously and awkwardly, especially if more than two boughs meet at the same place, growing in one plane, so as to show the sudden increase on the profile. If the reader is interested in the subject, he will find strangely complicated and wonderful arrangements of stream when smaller boughs meet larger (one example is given in Plate 3, Vol. III., where the current of a smaller bough, entering upwards, pushes its way into the stronger rivers of the stem). But I cannot, of course, enter into such detail here.

§ 10. The little ringed accumulation, repelled from the wood of the larger trunk at the base of small boughs, may be seen at a glance in any tree, and needs no illustration; but I give one from Salvator, Fig. 41 (from his own etching, _Democritus omnium Derisor_), which is interesting, because it shows the swelling at the bases of insertion, which yet, Salvator's eye not being quick enough to detect the law of descent in the fibres, he, with his usual love of ugliness, fastens on this swollen character, and exaggerates it into an appearance of disease. The same bloated aspect may be seen in the example already given from another etching, Vol. III., Plate 4, Fig. 8.

§ 11. I do not give any more examples from Claude. We have had enough already in Plate 4, Vol. III., which the reader should examine carefully. If he will then look forward to Fig. 61 here, he will see how Turner inserts branches, and with what certain and strange instinct of fidelity he marks the wrinkled enlargement and sinuous eddies of the wood rivers where they meet.

And remember always that Turner's greatness and rightness in all these points successively depend on no scientific knowledge. He was entirely ignorant of all the laws we have been developing. He had merely accustomed himself to see impartially, intensely, and fearlessly.

§ 12. It may, perhaps, be interesting to compare, with the rude fallacies of Claude and Salvator, a little piece of earliest art, wrought by men who could see and feel. The scroll, Fig. 42, is a portion of that which surrounds the arch in San Zeno of Verona, above the pillar engraved in the _Stones of Venice_, Plate 17, Vol. I. It is, therefore, twelfth, or earliest thirteenth century work. Yet the foliage is already full of spring and life; and in the part of the stem, which I have given of its real size in Fig. 43, the reader will perhaps be surprised to see at the junctions the laws of vegetation, which escaped the sight of all the degenerate landscape-painters of Italy, expressed by one of her simple architectural workmen six hundred years ago.

We now know enough, I think, of the internal conditions which regulate tree-structure to enable us to investigate finally, the great laws of branch and stem aspect. But they are very beautiful; and we will give them a separate chapter.

FOOTNOTES:

[1] See the note on Fig. 11, at page 17, which shows these two
directions in a shoot of lime.

[2] I find that the office and nature of cambium, the causes of the
action of the sap, and the real mode of the formation of buds, are
all still under the investigation of botanists. I do not lose time in
stating the doubts or probabilities which exist on these subjects.
For us, the mechanical fact of the increase of thickness by every
leaf's action is all that needs attention. The reader who wishes for
information as accurate as the present state of science admits, may
consult Lindley's _Introduction to Botany_, and an interesting little
book by Dr. Alexander Harvey on _Trees and their Nature_ (Nisbet &
Co., 1856), to which I owe much help.

[3] In the true sense a "mediator," ([Greek: mesitês]).

[4] The gradual development of this radiating structure, which is
organic and essential, composed of what are called by botanists
medullary rays, is still a great mystery and wonder to me.