38 THIRD REPORT — 1833. 



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 tliat have been drawn from them. One most 

 important point is overlooked by those who employ these argu- 

 ments, namely, that in all plants there are two distinct simul- 

 taneous systems of growth, the ceUular 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 Mr. Knight* and M. De Candollef 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 expla- 

 nation of the pha?nomena produced 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 be- 

 long 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 in- 

 creases in diameter. As buds from which new branches elon- 

 gate are generated by cellular tissue, they also belong to the 

 horizontal system ; and hence it is that the stock will ahvays 

 produce branches like itself, notwithstanding the long super- 

 position of new wood which has been taking place in it froni 

 the scion. 



The case of a ring of red bark always forming red wood be- 

 neath 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 hoi-izontal system, through which the wood de- 

 scends, which thus acquires a colour that does not properly 

 belong to it. With regard to the instances of grafts over- 

 growing their stock, or vice versa, it is obvious that these are 

 susceptible of explanation upon the same principle. If the hori- 

 zontal system of both stock and scion has an equal power of 



• Philosophical Transaciiuns, 1S05, p. 257. 

 •{■ Physiohgic Vcnitcde, p. 158. 



