or ducts, and of fibre, annually deposited outside each other. It consists of two parts, namely:

1. The central layers which are harder, more colored, and evidently older than those near the circumference: these form what workmen call the heart of the wood, and naturalists true wood, or lignum.

2. The external layers, which being incompletely formed, are softer, whiter, and younger than the former, and constitute what is called the Alburnum.

In some trees, especially in those which are not very hard, the line of demarkation between the true wood and the alburnum is not very perceptible; in the hard woods it is well marked both by texture and color, as in ebony, in which the wood is jet black and the alburnum white.

Every layer both of the wood and alburnum, if we except the medullary portion, is composed of vessels and fibres intermixed with elongated cellular tissue. The sole organic difference between the wood and the alburnum, is, that in the former, the interior of the cells and perhaps of the vessels, is encrusted, while in the latter it is empty or only filled with juices scarcely solidified. M. Dutrochet has proved that the different degrees of hardness between divers woods, and between the wood and the alburnum, is owing to the nature of the juice contained in their tissue, and not to the tissue itself, which is identical in both. The tissue of the box and the poplar, though these woods differ so much in density, become perfectly similar when the matter they contain has been dissolved out by nitric acid. The spaces which after maceration appear to exist between the woody layers, are not really such; but were filled with cellular tissue, analogous, for each annual layer, to the

central pith of the first year's growth. Each woody layer, being, in the Exogenous trees of cold or temperate climates, the produce of one year, the number of concentric zones in a transverse cutting of a stem will show the number of years during which that part of the tree has existed. To know the entire age of the tree itself, it must be cut exactly at the crown, since of course the higher portions of the stem were not in being when the deposits on the lower were formed. An inscription graven on the trunk of a tree, and penetrating to the alburnum becomes covered by new woody layers, and may be discovered unaltered: thus Reisel found in 1675, some capital letters in the centre of a beech tree.* The nourishment of the tree being entirely performed by the young or sap wood (the alburnum) is carried on when age and decay have deprived it of its heart wood. Thus we see the hollow trunk of an oak or willow capable of sustaining large branches, and putting forth foliage almost as luxuriant as when in its prime.

29. The cortical system (or Bark) of Exogenes is organized in a manner analogous to that of the central, or ligneous system-every stem acquiring a cortical, as well as a ligneous zone annually; but while each fresh woody layer is deposited on, and externally to, that of the year before, each layer of the bark is produced on the inner side of that pre

*There is a singular illustration of the manner in which the older portions of a stem are completely enveloped in the later deposits of woody matter, to be seen in a part of the stem of the Wellington Tree, presented to the British Museum by Mr. Children. A chain had been passed round the trunk when it was a sapling, and was so entirely buried in the layers of succeeding years, that it was only by the violent resistance the chain made to the tools of the workmen who were sawing the tree, that its existence was discovered.

viously formed. The younger and more flexible portion is called the Liber, and is deposited on the alburnum of the wood; the older layers are pushed outwards, and are the cortical layers, or true bark : they represent in the bark, what the heart wood is in the central portion, but with this great difference, that the woody layers being deposited beyond each other in the order of their formation, remain perfectly entire; while the layers of bark, acquiring fresh zones from within, undergo considerable distension-thus, although the number of cortical layers equals those of the woods, their fate is very different; those of the bark distended by the growth of the tree after the first year, always present more or less flexuous fibres, and this tendency augments with age, while on the contrary the fibres of the wood continue straight and rigid. The woody layers remain in the state of alburnum till they have acquired their proper hardness,-the layers of bark on the contrary, soon lose their freshness, and never attain the same degree of solidity. The first, placed beyond the reach of atmospheric influence, preserve -the appearance of life; the latter, exposed to the action of the air and light, soon dry up and split. This* difference in the mode of growth accounts for the different results of such experiments in this part of the tree, as were before mentioned as having been tried in the wood-if an inscription be made on the bark only, the letters without lengthening, gradually become thicker, larger, further apart, and are at last effaced. The secretions of a plant are often deposited in the bark.

30. The Medullary Rays, formed of compressed

*It will be therefore observed that, strictly speaking, it is the woody portion only of Exogenes to which the term applies, as the bark follows the laws of the Endogenous tribes.

parallelograms of cellular tissue, connect the centre and circumference of the trunk: they strengthen the tissue, and convey secreted nutritive matter in a horizontal direction. They are distinctly perceptible in a section of a woody stem. Sometimes they can be traced from the central pith to the extreme circumference, but ordinarily the line is interrupted.

All

31. Stems vary extremely in appearance in different plants-sometimes they run under the ground, and are improperly called creeping roots; occasionally they lie prostrate, and send roots into the earth underneath them ;—the term rhizoma is then applied to them; and sometimes they are much swollen, and called a tuber;—or if they (or rather their leaf buds) (35) thicken below the ground, a corm. these forms of stem have been called roots; but there are two marked distinctions between these and true roots. They have what are termed nodes, which are the points at which the leaf buds are formed, as well as leaf buds, which are never found on roots properly so called. Scales being the rudiments of leaves, no proper root can be scaly.

32. The stems of Endogenous plants, considered generally, have these common characters.

1. They are composed of one single homoge

neous mass.

2. They have no true medullary channel nor distinct medullary rays.

3. Their older fibres are on the circumference, and the newer deposits in the centre, from which latter circumstance they take their name.

They are less marked in character, and present less regularity of structure than the Exogenes. Thus one species, the Palm, will afford a sufficient idea of the whole class. This stem is generally upright, strong, simple, regularly cylindrical, and crowned at

its summit with a bunch of leaves; transversely divided, it appears to be composed of scattered fibres, mixed with cellular tissue, which unites them together. At a glance it is obvious that the fibres of the circumference are more close, of a firmer consistence, and older than the inner ones, which are distant, soft, and surrounded by a loose cellular tissue. Each fibre consists of a bundle of trachea, and rayed and dotted vessels. The difference in consistence between the circumference and the centre of the trunk is always perceptible, sometimes very remarkable; for instance, there are some palms whose exterior is so hard that a hatchet can make no impression on it, while the inside is a loose spongy tissue, quickly decaying in a humid air. The circumference of the palms corresponds to the wood of our trees, while the centre is a species of alburnum. It is from this central alburnum that the leaves and flowers spring, or in a word, it is from the centre that the development of all the parts takes place. Immediately on the appearance of the plant a first row of leaves is put forth, attached to the crown by a layer of fibres--the next year a second row is produced within the former, and distends them-it is the same with the succeeding seasons, till the period when the outer layer, having acquired by age the hardness of perfect wood, and no longer admitting of further distension, is incapable of any increase of diameter.

33. A Leaf has two distinct parts-the Petiole, or stalk, and the Lamina, called also the blade or limb; the former consists of fibres proceeding from the stem, and enclosed in a cellular integument; the latter is formed by the ramifications of the fibres of the petiole, and the expansion of its cellular tissue. In exogenous plants the veins branch in various directions, so as to form a kind of network; in the