318 
PHYSIOLOGY. 
BOOK II. 
In endogenous trees there is no cambium, and yet wood is 
formed in abundance ; and in the centre, not in the circum- 
ference ; 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 I 
of this learned botanist is true, no doubt, but does not affect ! 
the question in dispute. 1 
Aware of the difficulties in the way of the common explan- | 
ations of the formation of wood, Du Petit Thouars, an in- j 
genious 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 i 
French philosopher to his aid, to be considered legitimately j 
as his own. The attention of Du Petit Thouars appears to j 
have been first especially called to the real origin of wood by j 
having remarked, in the Isle of France, that the branches 1 
emitted by truncheons of Dracaena (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 gra- 
dually 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 or- 
ganisation 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. 
