56 



herbaceous envelope is burst and de- 

 stroyed the next ring of the pith assumes 

 its functions. Indeed in the plane tree, 

 whose epidermis or outer bark sloughs off 

 as it dies, a green outer layer is found 

 to the very foot of the largest trees, 

 doubtless owing to the free admission of 

 light and air. Possibly^ also^ for the deposit 

 of the new external layer of wood and of 

 the new internal layer of bark, it may be 

 necessary to bring together, through the 

 medium of the medullary rays, on the 

 common ground on which the two layers 

 are deposited, and on which these rays of 

 the wood and of the bark meet, if not 

 join, juices the product of chemical de- 

 composition, assimilation, and elaboration 

 in the stem from the upward sap, which 

 have been subjected to respiration and 

 transpiration in the leaf, and to all these 

 processes in the descent of the sap through 

 the bark. 



Let the practical man guard these 



