GENEKAL CONSIDERATIONS. [BOOK H. 



cellular system of the stem takes place, and each bud sends 

 down organisable matter within the bark and above the wood 

 of the shoot from which it sprang : thus forming on the one 

 hand a new layer of wood, and on the other a fresh deposit of 

 liber. 



In order to facilitate this last operation, the old bark and 

 wood are separated in the spring by the exudation from both 

 of them of the glutinous slimy substance called cambium; 

 which appears to be expressly intended, in the first instance, 

 to facilitate the development of the subcortical tubular tissue ; 

 and, in the second place, to assist in generating the cellular 

 tissue by which the horizontal dilatation of the axis is caused, 

 and which maintains a communication between the bark and 

 the centre of the stem. This communication has, by the 

 second year, become sufficiently developed to be readily dis- 

 covered, and is effected by the medullary rays spoken of in 

 the last book. It will be remembered that there was a time 

 when that which is now bark constituted a homogeneous 

 body with the pith ; and that it was after the leaves began to 

 come into action that the separation which now exists between 

 the bark and pith took place. At the time when the latter 

 were indissolubly united they both consisted of cellular tissue, 

 with a few spiral vessels upon the line indicative of future 

 separation. When a deposit of wood was formed from above 

 between them they were not wholly divided the one from the 

 other, but the deposit was effected in such a way as to leave 

 a communication by means of cellular tissue between the 

 bark and the pith ; and, as this formation, or medullary ray, 

 is at all times coetaneous with that of the wood, the commu- 

 nication so effected between the pith and bark is quite as 

 perfect at the end of any number of years as it was at the 

 beginning of the first ; and so it continues to the end of the 

 growth of the plant. 



The sap which is drawn from the earth into circulation by 

 the unfolding leaves is exposed, as in the previous year, to 

 the effect of air and light; is then returned through the 

 petiole to the stem, and sent downwards through the bark, 

 to be from it either conveyed to the root, or distributed 

 horizontally by the medullary rays to the centre of the stem. 



