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. [Illustration: Fig. 478. Piece of a stem of Soft Maple, of a year old, cut crosswise and lengthwise.] [Illustration: Fig. 479. A portion of the same, magnified.] [Illustration: 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,-- 1. The LIBER or FIBROUS BARK, the _Inner Bark_. This contains some wood-cells, or their equivalent, commonly in the form of bast or bast-cells (411, Fig. 444), such as those of Basswood or Linden, and among herbs those of flax and hemp, which are spun and woven or made into cordage. It also contains cells which are named _sieve_-cells, on account of numerous slits and pores in their walls, by which the protoplasm of contiguous cells communicates. In woody stems, whenever a new layer of wood is formed, some new liber or inner bark is also formed outside of it. 2. The GREEN BARK or _Middle Bark_. This consists of cellular tissue only, and contains the same green matter (_chlorophyll_, 417) as the leaves. In woody stems, before the season's growth is completed, it becomes covered by 3. The CORKY LAYER or _Outer Bark_, the cells of which contain no chlorophyll, and are of the nature of _cork_. Common cork is the thick corky layer of the bark of the Cork-Oak of Spain. It is this which gives to the stems or twigs of shrubs and trees the aspect and the color peculiar to each,--light gray in the Ash, purple in the Red Maple, red in several Dogwoods, etc. 4. The EPIDERMIS, or skin of the plant, consisting of a layer of thick-sided empty cells, which may be considered to be the outermost layer, or in most herbaceous stems the only layer, of cork-cells.