Fig. 475. Diagram of a cross section of a very young exogenous stem, showing six woody bundles or wedges. 476. Same later, with wedges increased to twelve. 477. Still later, the wedges filling the space, separated only by the thin lines, or medullary rays, running from pith to bark.
429. When such a stem grows on from year to year, it adds annually a layer of wood outside the preceding one, between that and the bark. This is exogenous growth, or outside-growing, as the name denotes.
Fig. 478. Piece of a stem of Soft Maple, of a year old, cut crosswise and lengthwise.
Fig. 479. A portion of the same, magnified.
Fig. 480. A small piece of the same, taken from one side, reaching from the bark to the pith, and highly magnified: a, a small bit of the pith; b, spiral ducts of what is called the medullary sheath; c, the wood; d, d, dotted ducts in the wood; e, e, annular ducts; f, the liber or inner bark; g, the green bark; h, the corky layer; i, the skin, or epidermis; j, one of the medullary rays, or plates of silver grain, seen on the cross-section.
430. Some new bark is formed every year, as well as new wood, the former inside, as the latter is outside of that of the year preceding. The ring or zone of tender forming tissue between the bark and the wood has been called the Cambium Layer. Cambium is an old name of the physiologists for nutritive juice. And this thin layer is so gorged with rich nutritive sap when spring growth is renewed, that the bark then seems to be loose from the wood and a layer of viscid sap (or cambium) to be poured out between the two. But there is all the while a connection of the bark and the wood by delicate cells, rapidly multiplying and growing.
[431.] The Bark of a year-old stem consists of three parts, more or less distinct, namely,—beginning next the wood,—