Internal Modifications. 269 



tion is named the Cambium, and appears to be exuded both by 

 the bark and wood, certainly by the latter. The cellular system 

 of the pith and that of the bark are, in the embryo and youngest 

 shoots, in contact ; but the vascular system, as it forms, gradu- 

 ally interposes between them, till after a few weeks they are dis- 

 tinctly separated, and in very aged trunks sometimes they are 

 divided by a space of several feet ; that is to say by half the 

 diameter of the wood. But whatever may be the distance be- 

 tween them, a horizontal communication of the most perfect 

 kind continues to be kept up. When the vascular system is 

 first insinuated into the cellular system, dividing the pith and 

 the bark, it does not completely separate them, but pushes aside 

 a quantity of cellular tissue, pressing it tightly into thin, vertical, 

 radiating plates ; as the vascular system extends, these plates in- 

 crease outwardly, continuing to maintain the connection between 

 the centre and the circumference. Botanists call them medul- 

 lary rays ; and carpenters the silver grain. In horizontal 

 sections of an exogenous stem, they are seen as fine lines radi- 

 ating from the centre to the circumference ; in longitudinal sec- 

 tions they give that glancing satiny lustre which is in all dis- 

 coverable, and which gives to some, such as the. Plane and Syca- 

 more, a character of remarkable beauty. 



The vascular system is confined to the space between the pith 

 and bark, where it chiefly consists of ducts and woody fibre col- 

 lected into compact, wedge-shaped, vertical plates, the edges of 

 which rest on the pith and bark, and the sides of which are in 

 contact with the medullary rays (fig. 5, a). The vascular sys- 

 tem of a stem one year old consists of a zone of wood lying 

 between the pith and the bark, lined on the inside by the medul- 

 lary sheath, and separated into wedge-shaped vertical plates by 

 the medullary rays that pass through it. All that part of the first 

 zone which is on the outside of the medullary sheath is com- 

 posed of woody fibre and ducts intermixed in no apparent order; 

 but the ducts are generally in greater abundance next the medul- 

 lary sheath, or confined to that side of the zone, and the woody 

 fibre alone forms a compact mass on the outside. The second 

 year another zone is formed on the outside of the first, with 

 which it agrees exactly in structure, except that there is no 

 medullary sheath ; the third year a third zone is formed on the 

 outside of the second, in all respects like it ; and so on one zone 



