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 it feels 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 the store is exhausted, and by the time this happens it is clothed with leaves which are themselves able to feed it after the seed leaves have perished. In brief, the stem is a branch produced by the first leaf-bud which the embryo plant

possesses.

43. When the stem is first called into existence, it is merely 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 such matter formed exclusively its solid contents, the stem 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 clubshaped 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 in 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 their bases; it is abundant, or the contrary, in proportion to the strength, number, and developement of the leaves; and in their absence is absent also.

44. When woody matter is first plunged into

the cellular tissue of the nascent stem, it forms a circle a little within the circumference 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.

45. As the cellular tissue of the stem is not sensibly lengthened more in one direction than in another, and as it is the only kind of organic matter that, in stems, 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 only increase by addition of new tubes having the same direction as themselves, as the perpendicular system.

46. 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.

47. Whenever the stem is wounded, the injury is repaired by the cellular or horizontal system, which forms granulations that eventually coalesce into masses (fig. 2. A), within which the perpendicular system or woody matter is subsequently developed. Thus the restoration of the communication between the two sides of an annular excision

is effected by granu

2

medullary rays, which finally run together over the wood (fig. 2. 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 below 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 the circle annually to a less diameter, till at last the centre is closed up.

48. In the bark of trees and shrubs, two distinct parts are found: the one external and cellular; and the other internal, resting upon the wood, and consisting of woody matter mixed with cellular. The external is the rind or cortical integument, the internal is the liber. These two parts grow independently of each other, by their inner faces; the rind belonging exclusively to the