bark, occupied by a viscid fluid or cambium , as imagined by the older writers; but, as the simplest in¬ 
spection of a carefully made section of a dicotyledonous shoot will show, the so-called cambium is 
nothing but the extremely delicate cellular tissue in which resides the unlimited power of developing 
into wood internally and bark externally, which results in the production of annual rings. But even 
if we take a modified view, that the wood of each successive layer is the organic prolongation down¬ 
wards of the wood of the shoots formed in the same year, and is directly continuous with that, the fact 
of the fibres being continuous, as may often be observed, proves nothing as to the mode or direction of 
growth; for the fibres of wood run down in continuous lines, not because they have grown down in 
these lines, but because this is the appointed plan of them development. We may say roughly, in ani- 
y, t at cei tain, vessels, or nerves, run into such and such a limb; but we know that this is 
only a convenient mode of expression, and that it is not intended to indicate that those structures were 
formed by a gradual budding out and branching, like that of the growth of the branches of a tree. 
Just so with the fibres we see running down from a branch towards the roots; they are intended to 
form the medium of connection between the roots and the branches, and therefore are developed in the 
way best suited for that purpose; that is to say, in the form of continuous bundles of fibres extending 
down in this direction, and thickest where the activity of the growth of the tree is greatest. There is as 
much reason to imagine that the projections and buttress-like prominences on the trunks of old trees, under 
large branches, arise/rom the large quantities of fluid drawn up through these tracts by the great evapo¬ 
ration from the leaves above, as that the cause lies in an elaborated sap of the leaves having descended. 
Of the downward course of the sap, and its physiological characters, we know almost nothing be¬ 
yond the mere fact of its existence; but the theory of the downward growth of the wood, actual 
mechanical elongation of individual fibres downwards, is negatived by all experiment, for the only 
experimental facts brought in aid of it are those of wounds on trees, where conditions are setup which 
do not exist in the regular course of nature ; for the cut ends of the fibres of the upper lip of the wound 
are set free to elongate, which never can be the case in an uninjured tree; and, moreover, in these 
very cases it is never the ends of the fully-formedfibres which grow, but the cambium-cells of the ex¬ 
treme outer limit of the wood, which develop outwards, and gradually fold over downwards, so as to 
heal the wound by covering it in. 
If any further proof of the origin of the wood had been necessary to unprejudiced observers, a paper 
lately published by M. Trecul, in the Comptes Rendus (Feb. 16, 1852), would have removed all doubt. 
He laid before the French Academy a statement, with an illustrative specimen, which showed, beyond 
question, that both the wood and the bark are formed from the surface on which they lie , quite inde¬ 
pendently of the tissues higher up. Iii the specimen (of Xyssa angulisans, Michx.), it was shown that 
new layers of bark and wood had formed on the surface of the previously decorticated alburnum, and 
below the decorticated part as well as above. We may give the chief particulars of his description :— 
The stem, which the author had brought from Louisiana, was deprived of its bark to the extent 
of about eighteen inches, but nevertheless continued to vegetate. When cut in the month of Sep¬ 
tember, it bore leaves and fruit. New layers of wood had been formed above and below the barked 
surface. In addition to these, others presented themselves on the surface whence the bark had been 
stripped. On this surface were seen oblong or hemispherical prominences, covered with greyish bark, 
varying in size from about one-sixth of an inch long and one-twelfth broad, to fourteen inches long and 
one to two inches broad. The cortical portion coating them, now dried up, might be readily detached, 
and displayed, after its removal, a more dense tissue adhering strongly to the wood. 
The tissue of one of the smallest of these prominences, examined by the microscope, proved to be 
what might be expected ; the outer easily removeable part was bark parenchyme ; the inner adherent 
parts of the new formations were true wood, even containing vessels, this wood being directly con¬ 
tinuous with the fibro-vascular bundles on which it lay, while the medullary rays of the stem, in some 
cases, extended out into these new structures. The new structures did not adhere by their whole 
surface to the wood, but by the central part, whence the new growth seemed to have radiated, as from 
a centre, upwards, downwards, and to each side. In one of the largest of the protuberances, on which the 
