198 DIFFICULTIES MET. [BOOK n. 



may be' made, the trunk will increase in diameter, but all the 

 wood beneath the ring of red bark will be red, although it 

 must have originated in the leaves of the tree which produces 

 white wood. It is further 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 

 Pavia lutea is grafted upon the common horsechestnut ; and 

 that these circumstances are inconsistent with the supposition 

 that 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 

 which have been drawn from them. One most important 

 point is overlooked 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 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 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 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. 



