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 the 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 ^'oots 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 medullaiy z'ays of the 



