CHAP. III. 
ORIGIN OF WOOD. 
265 
urged, that, in grafted plants, the scion often overgrows the 
stock, increasing much the more rapidly in diameter, or that 
the reverse takes place, as when the Pavia lutea is grafted 
upon the common horse-chestnut, — and that these circum- 
stances are inconsistent with the supposition that the wood is 
organic matter engendered by leaves. To these statements 
there is nothing to object as mere facts, for they are true ; 
but they certainly do not warrant the conclusions that have 
been drawn from them. One most important point is over- 
looked by those who employ such arguments, namely, that 
in all plants there are two distinct simultaneous systems of 
growth, the cellular and the fibro- vascular, of which the former 
is horizontal, and tlie latter vertical. The cellular gives 
origin to the pith, the medullary rays, and the principal part 
of the cortical integument ; the fibro- vascular, to the wood and 
a portion of the bark ; so that the axis of a plant may be not 
inaptly compared to a piece of linen, the cellular system being 
the woof, the fibro-vascular the warp. It has also been 
proved by Knight and De Candolle that buds are exclusively 
generated by the cellular system, while roots are evolved 
from the fibro-vascular system. Now, if these facts are rightly 
considered, they will be found to offer an obvious explanation 
of the phenomena appealed to by those botanists who think 
that wood cannot be matter generated in an organic state by 
the leaves. The character of wood is chiefly owing to the 
colour, quantity, size, and distortions of the medullary rays, 
which belong to the horizontal system : it is for this reason 
that there is so distinct a line drawn between the wood of the 
graft and stock ; for the horizontal systems of each are con- 
stantly pressing together with nearly equal force, and uniting 
as the trunk increases in diameter. As buds from which 
new branches elongate are generated by cellular tissue, they 
also belong to the horizontal system : and hence it is that the 
stock will always produce branches like itself, notwithstanding 
the long superposition of new wood which has been taking 
place in it from the scion. 
The case of a ring of red bark always forming red wood 
beneath it, is precisely of the same nature. After the new 
bark has adhered to the mouths of the medullary rays of the 
