part of a branch be deprived of leaves, the branch will die down to the point where leaves have been left, and below that will flourish. Hence an inference is drawn, that the wood is not formed out of the bark as a mere deposit from it; but that it is produced from matter elaborated in the leaves and sent downwards, either through the vessels of the inner bark, along with the matter for forming the liber, by which it is subsequently parted with; or that it and the liber are transmitted distinct from one another, the one adhering to the alburnum, the other to the bark. I know of no proof of the former supposition; of the latter there is every reason to believe the truth. Knight is of opinion that two distinct sets of vessels are sent down, one belonging to the liber, the other to the alburnum; and if a branch of any young tree, the wood of which is formed quickly, be examined when it is first bursting into leaf, these two sets may be distinctly seen and traced. Take, for instance, a branch of Lilac in the beginning of April, and strip off its bark: the new wood will be distinctly seen to have passed downwards from the base of each leaf, diverging from its perpendicular course, so as to avoid the bundle of vessels passing into the leaf beneath it; and, if the junction of a new branch with that of the previous year be examined, it will be found that the wood already seen proceeding from the base of the leaves, having arrived at this point, has not stopped there, but has passed rapidly downwards, adding to the branch an even layer of young ligneous matter, and turning off at every projection which impedes it, just as the water of a steady but rapid current would be diverted from its course by obstacles in its stream. Again, in Guaiacum wood, the descending tubes of pleurenchyma cross and interlace each other, in a manner that is unintelligible upon the supposition of wood being formed by the mere deposit of secreted matter. If the new wood were a mere deposit of the bark, the latter, as it is applied to every part of the old wood, would deposit the new wood equally over the whole surface of the latter, and the deviation of the fibres from obstacles in their downward course would scarcely occur. This, therefore, in my mind, places the question as to the origin of the wood beyond all further doubt. Or, if

further evidence were required, it would be furnished by a case adduced by Achille Richard, who states that he saw, in the possession of Du Petit Thouars, a branch of Robinia Pseudacacia on which R. hispida had been grafted. The stock had died; but the scion had continued to grow, and had emitted from its base a sort of plaster formed of very distinct fibres, which surrounded the extremity of the stock to some distance, forming a kind of sheath; and thus demonstrating incontestably that wood does descend from the base of the scion to overlay the stock. The singular mode of growth in Pandanus is equally instructive. In that plant, the stem, next the ground, is extremely slender; a little higher up it is thicker, and emits aerial roots, which seek the soil and act as stays upon the centre. As the stem increases in height, it also increases notably in diameter, continuing to throw out aerial roots. If the roots were pruned away, the stem would be an inverted cone; but, if we add to the actual thickness of the base of the stem the capacity of the aerial roots at that part, the two together will be about equal to the capacity of the stem at the apex; which suggests the idea that the woody matter that descends from the leaves may really be their roots, passing through the horizontal cellular system of the An analogous but much more remarkable case is the following mentioned by me in the Penny Cyclopædia, article Endogens, vol. ix. p. 396. In an unpublished species of Barbacenia from Rio Janeiro, allied to B. purpurea, the stems appear externally like those of any other rough-barked plant, only that their surface is unusually fibrous and ragged when old, and closely coated by the remains of sheathing leaves when young. Upon examining a transverse section of it, the stem is found to consist of a small, firm, pale, central circle having the ordinary endogenous organisation, and of a large number of smaller and very irregular oval spaces pressed closely together, but having no organic connection; between these are traces of a chaffy ragged kind of tissue which seems as if principally absorbed and destroyed. fig. 192.)

stem.

(See

A vertical section of the thickest part of this stem exhibits, in addition to a pale, central, endogenous column, woody

branches crossing each other or lying parallel, after the manner of the ordinary ligneous tissue of a Palm stem

(fig. 193.), only the bundles do not adhere to each other, and are not embodied, as usual, in a cellular substance. These bundles may be readily traced to the central column, particularly in the younger branches (fig. 191.), and are plainly the roots of the stem, of exactly the same nature as those aerial roots which serve to stay the stem of a Screw Pine (Pandanus). When they reach the earth, the woody bundles become more apparently roots, divided at their points into fine segments, and entirely resembling on a small scale the roots of a Palm-tree. The central column is much smaller

at the base of the stem, than near the upper extremity. Nothing can well show more distinctly than this, that the woody bundles of the endogenous stem are a sort of roots emitted by the leaves, plunging down through their whole length into the cellular substance of the stem in ordinary cases; but, in Barbacenia, soon quitting the stem, and continuing their course downwards on the outside. The observation of Du Petit Thouars, that, when Dracænas push forth branches, each of the latter produces from its base a quantity of fibres, which are interposed between the cortical integument and the body of the wood, forming a sort of plaster analogous to what is found in the graft of an Exogen; and that, of the fibres just mentioned, the lowermost have a tendency to descend, while those originating on the upper side of the branch turn downwards, and finally descend also; had already rendered the above-mentioned conclusion probable. The case of Barbacenia can scarcely leave a doubt upon the subject; and leads to the important conclusion, that the theory of the wood of Exogens being also a state of roots belonging to the leaves of the stem is well founded also.

Mirbel, who formerly advocated the doctrine of wood being deposited by bark, has candidly admitted the opinion to be no longer tenable; and he has suggested, in its room, that wood and bark are independent formations, which is no doubt true; but he adds, created out of cambium, in which it is impossible to concur, if by cambium M. Mirbel means the viscid secretion found in the spring between the bark and wood of Exogens; for the following reasons: - All the writers hitherto mentioned have considered the formation of wood exclusively with reference to exogenous trees, and to such only of them as are the common forest plants of Europe. Had they taken into account exotic trees or any endogenous plants, they would have seen that none of their theories could apply to the formation of wood in the latter tribe. In many Exogens of tropical countries, wood is not deposited in regular circles all round the axis, but only on one side of the stem, or along certain lines upon it: were it a deposit from the bark, or a metamorphosis of cambium, it would necessarily be deposited with some kind of uniformity.

In endogenous trees there is no cambium, and yet wood is formed in abundance; and in the centre, not in the circumference so that bark can have, in such cases, nothing to do with the creation of wood.

But, if the word cambium is employed by M. Mirbel as an equivalent for organic mucus (see p. 1.), then the statement of this learned botanist is true, no doubt, but does not affect the question in dispute.

Aware of the difficulties in the way of the common explanations of the formation of wood, Du Petit Thouars, an ingenious French physiologist, who had possessed opportunities of examining the growth of vegetation in tropical countries, proposed a theory, which, although in many points similar to one previously invented by his countryman, De la Hire, is nevertheless, from the facts and illustrations brought by the French philosopher to his aid, to be considered legitimately as his own. The attention of Du Petit Thouars appears to have been first especially called to the real origin of wood by having remarked, in the Isle of France, that the branches emitted by truncheons of Dracana (with which hedges are formed in that colony) root between the rind and old wood, forming rays, of which the axis of the new shoot is the centre. These rays surround the old stem; the lower ones at once elongate greatly towards the earth, and the upper ones gradually acquire the same direction; so that at last, as they become disentangled from each other, the whole of them pass downwards to the soil. Reflecting upon this curious fact, and upon others which I have not space to detail, he arrived at this conclusion; that it is not merely in the property of increasing the species that buds agree with seeds, but that they emit roots in like manner; and that the wood and liber are both formed by the downward descent of bud-roots, at first nourished by the moisture of the cambium, and finally embedded in the cellular tissue which is the result of the organisation of that secretion. That first tendency of the embryo, when it has disengaged itself from the seed, to send roots downwards and a stem and leaves upwards, and to form buds in the axils of the latter, is in like manner possessed by the buds themselves; so that plants increase in size by an endless repetition of the same phenomenon.