CHAPTER IV. GROWTH BY THE STEM. ORIGIN OF THE STEM.-THE GROWING POINT.-PRODUCTION OF WOOD, BARK, PITH, MEDULLARY RAYS.-PROPERTIES OF SAP-WOOD, HEART-WOOD, LIBER, RIND, ETC.-NATURE AND OFFICE OF LEAF-BUDS.—EMBRYOBUDS.—BULBS.-CONVEYANCE OF SAP, AND ITS NATURE. As soon as the root is fully in action, which is shortly after it has begun to lengthen, the vitality of the living point that exists at the bottom of the seed-leaves is excited, and a stem begins to be formed. At first the stem is a mere point of living matter, often invisible to the eye, but sometimes partially developed; in which latter case it is called the plumule. But, as soon as nutritive matter is conveyed into it by the nascent root, all its parts receive an impulse, which forces them into a growth upwards; what matter already exists is distended, enlarged, and solidified; new matter is rapidly generated in all directions from the vital centre, and if it were not for the current setting upwards from the root, it would possibly grow into a spherical figure. Pressed upon, however, by the surrounding earth, impelled upwards by the current of sap ascending from the root, and attracted into the air by the necessity of respiration, the young stem assumes a cylindrical form, its sides having a tendency to solidify, and its point to grow longer. This point, or plumule, or first leaf-bud, soon attracts to itself the food which the root procures from the earth, and a part of the nutritive matter which is stored up in the seed-leaves. It feeds especially upon the latter until they are exhausted, and by the time this happens it is clothed with leaves which are themselves able to feed it after the seed-leaves 34 ORIGIN OF STEM AND WOOD. have perished. In brief, the stem is a branch produced by the first leaf-bud which the embryo plant possesses. When the stem is first called into existence, it is merely a vegetable cell, afterwards increased into a small portion of cellular tissue: an organic substance, possessing neither strength nor tenacity, and altogether unsuited to the purposes for which the stem is destined. If the stem consisted exclusively of such matter it would have neither toughness nor strength, but would be brittle like a mushroom, or like those parts of plants of which cellular tissue is the exclusive component, such for example as the club-shaped spadix of an Arum, or the soft prickles of a young Rose branch. Nature, however, from the first moment that the rudiment of a leaf appears upon the growing point of a stem, occupies herself with the formation of woody matter, consisting of tough tubes of extreme fineness, which take their rise near the leaves, and which, thence passing downwards through the cellular tissue, are incorporated with the latter, to which they give the necessary degree of strength and flexibility. In trees and shrubs, they combine intimately with each other, and so form what is properly called the wood and inner bark; in herbaceous and annual plants, they constitute a lax fibrous matter. No woody matter appears till the first leaf, or the seed-leaves, have begun to act; it always arises from near their bases; it is abundant, or the contrary, in proportion to the strength, number, and development of the leaves; and in their absence is absent also as a general rule. The exceptional cases are those of "leafless" plants; that is to say, of plants in which leaves never advance beyond the condition of scales, and usually drop off soon after their formation. To this class belong green succulent plants like Stapelias, Cacti, and many Euphorbias. Here the bark is excessively developed, and has the colour, texture, and structure of leaves, of which it performs the functions. Such plants form true wood, but of little solidity and in small quantity compared with their bulk. It is also found that in them the wood has a lateral communication with every leaf-bud, as that of ordinary plants has with every leaf. When woody matter is first plunged into the cellular tissue of the nascent stem, it forms a circle a little within the circum ference of the stem, whose interior it thus separates into two parts namely, the bark or the superficial, and the pith or the central, portion; or, in what are called Endogens, into a superficial coating analogous to bark, and a central confused mass of wood and pith intermingled. The effect of this, in Exogens, is, to divide the interior of a perennial stem into three parts, the pith, the wood, and the bark. Since the cellular tissue of the stem is not sensibly lengthened more in one direction than in another, and as it is that kind of organic matter, which, in stems, chiefly increases laterally, it is sometimes convenient to speak of it under the name of the horizontal system; and, for a similar reason, to designate the woody tubes which are plunged among it, and which increase by addition of new tubes having the same direction as themselves, as the perpendicular system. Wood properly so called, and liber or inner bark, consist, in Exogens, of the perpendicular system, for the most part; while the pith and external rind or bark are chiefly formed of the horizontal system. The two latter are connected by cellular tissue, which, when it is pressed into thin plates by the woody tubes that pass through it, acquires the name of medullary rays. It is important, for the due explanation of certain phenomena connected with cultivation, to understand this point correctly, and to remember that, while the perpendicular system is distributed through the wood and bark, the horizontal system consists of pith, outer bark, and the medullary processes which connect these two in Exogens, and of irregular cellular tissue analogous to medullary rays in Endogens. So that the stem of a plant is not inaptly compared to a piece of linen, the horizontal cellular system representing the woof, and the woody system the warp. Whenever the stem is wounded, the injury is repaired by the cellular or horizontal system, which forms granulations that eventually coalesce into masses (Fig. VII. A), within which the perpendicular system or woody matter (B) is subsequently developed. Thus the restoration of the communication between the two sides of an annular excision is effected by granulations of the upper and lower lips, and of the medullary rays, which 36 RENOVATION OF WOOD. finally run together over the wood (Fig. VII. B), and form a coating below which new liber and alburnum may be generated. In cuttings, the "callus," which forms at the end placed in the ground, is the cellular horizontal system, preparing for the reception of the perpendicular system, which is to pass downwards in the form of roots. Many plants will endure extensive lacerations of their surface, and close up such wounds with great facility. The well known fact of large inscriptions cut in trees deeper than the bark (which inscriptions were effected by removing very broad spaces of the bark and wood) being covered over in time by new bark and wood, so as to be no longer visible from the outside, sufficiently prove this. In such cases, however, the reparation of the injury takes place chiefly, if not exclusively, by the annual addition of new matter to the lips only of the wound, the effect of which is to reduce its area annually till at last the wound is closed. RENOVATION OF WOOD. 37 "Certain it is," says Mr. Towers, "that the bark of trees, when wounded or cut in amputation of branches proceeding from the trunk, converges from all points, and not solely, as some assert, from the uppermost cross incision. The Elm-tree may furnish the best examples for investigation, some of which are to be seen in every hedge-row. In the public road leading from Waddon to Mitcham Common, there stand several large Elms in front of a gentleman's house. A wound was made in one of them fully eighteen inches long, and in the middle five or six wide, by which the bark stripped off to that extent, exposed the wood below it. The young liber came rolling forward on every side, A line of posts and is now seen approaching pretty equally, though with projections of a redder colour, which mark the more recent processes. and rails, with a chain at top, extends along the front, close to the row of trees. An abrasion or wound had been made in close contact with a part of the chain, which now is buried, and firmly fixed in the bark, by a knotty boss formed of cortical matter." A striking instance of the power of reparation by mere superficial increase has been recorded by the Rev. M. J. Berkeley: "A vigorous Oak had been mischievously barked all round, and to such an extent as to preclude all probability of ultimate union of the severed edges. The tree, however, for a year or two seemed to suffer very little from the injury, as new tissue was thrown out from the exposed extremities of the medullary rays; and to such an extent, that had not the parties who |