CHAP. V. ORIGIN OF WOOD. 321 
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 shown 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 or- 
ganic 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 constantly 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 
stock, and so identified itself with the horizontal system, it is 
gradually pushed outwards by the descent of woody matter 
from above through it ; but, in giving way, it is constantly 
generating red matter from its horizontal system, through 
which the wood descends, and thus acquires a colour not 
properly belonging to it. With regard to the instances of 
grafts overgrowing their stocks, or vice versa, it seems that 
these are susceptible of explanation on the same principle. 
If the horizontal system of both stock and scion has an equal 
power of lateral extension, the diameter of each will remain 
the same ; but, if one grows more rapidly than the other, 
the diameters will necessarily be different; where the scion 
has a horizontal system that developes more rapidly than that 
of the stock, the latter will be the smaller, and vice versa. 
It is, however, to be observed, that in these cases plants are 
in a morbid state, and will not live for any considerable time. 
Y 
