STEM IN DICOTYLEDONS.] BOTANY 101 altered. In old trees there is a marked division between the central heart-ivood or duramen, and the external sap- wood or alburnum, the former being hard and dense, and often coloured, with its tubes dry and thickened, while the latter is less dense, is of a pale colour, and has its tubes permeable by fluids. The difference of colour between these two kinds of woods is often very marked. In the Ebony tree the duramen or perfect-wood is black, and is the part used for furniture, while the alburnum is pale ; in the Beech, the heart-wood is light-brown ; in the Oak, deep-brown; in Judas tree, yellow ; in Guaiacum, greenish. The alteration in colour is frequent in tropical trees. In trees of temperate climates, called white-ivood, as the Willow and Poplar, no change in colour takes place ; this is also the case in the Chestnut and Eombax. The relative proportion of alburnum and duramen varies in different trees. The heart-wood is more useful than the sap-wood, and is less liable to decay. From the mode in which the woody layers are formed, it is obvious that each vascular zone is moulded upon that which precedes it ; and as, in ordinary cases, each woody circle is completed in the course of one year, it follows that, by counting the concentric circles, the age of a tree may be ascertained. Thus fig. 75 represents an oak eight years old, having eight woody layers b. This computation can only be made in trees having marked separations between the circles. There are, however, many sources of mistake. In some instances, by interruption to growth, several circles may be formed in one year, and thus lead to an erroneous estimate. Care must be taken to have a complete section from the bark to the pith, for the circles TT Horizontal section of the stem of an Sometimes vary 111 diameter at oak eight years old. 6, wood, show ing concentric circles or zones, separ ated by points which correspond to the opening of the large pitted vessels; e, bark, showing also eight concentric circles, thinner and less distinct. The wood and bark are traversed by me dullary rays, some of which are the different parts of their course, and a great error might occur from taking only a few rings, or circles, and then estimat ing for the whole diameter of the tree. When by the ac tion of severe frost, or other primary rays extending from the bark to the pith, while others (the secondary rays) reach only a certain way inwards. causes, injury has been done to the tender cells from which the young wood is developed, while, at the same time, the tree continues to live, so as to form perfect woody layers in subsequent years, the date of the injury may be ascertained by counting the number of layers which intervene between the imperfectly formed circle and the bark. Inscriptions made in the wood become covered, and may be detected in after years when a tree is cut down ; so also wires or nails driven into the wood. As the same development of woody layers takes place in the branches as in the stem of an exogenous tree, the time when a branch was first given off may be computed by counting the circles on the stem and branch respectively. If there are fifty circles, for instance, in the trunk, thirty in one branch and ten in another, then the tree must have been twenty years old when it produced the first, and forty when it formed the other. In exogenous stems the pith is not always in the centre. The layers of wood on one side of a tree may be larger than those on the other, in consequence of their fuller exposure to light and air, or the nature of the nourishment conveyed, and thus the pith may become excentric. Zones vary in .size in different kinds of trees, and at different periods of a plant s life. Soft wooded trees have usually broad zones, and old trees form smaller zones than young ones. There are certain periods of a plant s life when it seems to grow most vigorously, and to form the largest zones. This is said to occur in the Oak between twenty and thirty years of age. Cambium. External to the layers of wood, and between Cambium, them and the bark, there is a layer of thin-walled cells in which the protoplasm and cell-sap remain, and consequently they are capable of division and growth. To this layer the name of cambium has been given. This cambium layer marks the separation between the wood and the bark, and may be regarded as constituting the active formative tissue of Dicotyledonous stems. It constitutes the thicken ing zone, by means of which the stem is enlarged, the cambium cells situated most internally being subservient to the purposes of the wood formation, while the external ones give origin to the new bark. When these cells are carry ing on the process of growth with activity, during the flow of the sap in spring, the bark can be easily separated from, the wood. The Bark or Cortical System lies external to the wood, The bark and, like it, consists of several layers. In the early state it is entirely cellular, and is in every respect similar to the pith; but as the fibro-vascular bundles are developed, the bark and pith are separated, and the former gradually becomes altered by the formation of secondary deposits. We find in the cortex, as in the wood portion of the stem, fibro-vascular along with cellular tissue. But the position and relative proportion of these two systems is reversed. In the bark the cellular system is external, and is much developed, while the fibro-vascular is internal, and occupies comparatively a small space. The cellular portion of the bark consists of an external layer, or epiphlceum, and the cellular envelope, or mesophlceum, while the vascular system forms the internal portion called liber, or endophlceum. The endophlceum, liber, or inner bark, is formed from the secondary cortex of the young stem. It consists mainly of thick or thin walled woody fibres, commonly known as bast-fibres, mixed with elongated cellular tissue and frequently with laticiferous vessels. It is separated from the wood by the cambium layer. The pleurenchymatous tubes are thickened so as to be flexible, but are not ligni- fied, and are thus very tenacious. The endophlceum of the Lime-tree and of Antiaris saccidora (the Sack tree of Coorg) is used to form mats, cordage, and bags ; and the tough ness of the fibres of the inner bark of flax, hemp, and of many of the Nettle and Mallow tribes, render them fit for various manufacturing purposes. The endophlceum is sometimes, from its uses, called the bast-layer. Occasionally it is continuous and uninterrupted, as in the Vine and Horse-chestnut; at other times, as in the Oak, Ash, and Lime, the fibres are separated during the progress of growth, and form a sort of network, in the interstices of which the medullary rays are seen. The fibres of the Lace-bark tree (Lagetta lintearia) are similar. In fig. 76 is represented the bark of Daphne Laureola, -/ indicating the woody fibres of liber, and r the medullary rays. The endophlceum increases by layers on its inside, which are thin, and may be separated like the leaves of a book. The outer layer of bast- fibres betwixt the endophlceum and the outer bark has been termed the cortical sheath, corresponding to the medullary sheath on the inside of the stem. The outer bark is formed from the primary cortex; it is always cellular, and is divided into two layers, the epi- Network fonned by liber of Daphne Laurtola. /, /, fibrous bundles ; >, r,
inedullarv ravs.