232 



LETTERS ON TREES. 



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 over- 

 grows 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 stich argimients, 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 cor- 

 tical 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 shewn by 

 Knight and De Candolle that buds are exclusively generated 

 by the cellular system, while roots are evolved by the fibro- 

 vascular. Now, if these facts are rightly considered, they will 

 be found to ofi'er 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 gi'aft and stock ; for the 

 horizontal systems of each are constantly pressing together 

 with nearly equal force, and uniting as the tmnk increases in 

 diameter. As buds from which new branches elongate are 



