THE STEM. 51 
+ 
horizontal diffusion of the fluids, connecting the bark and 
‘central parts. 
163. The wood gives passage to the sap, from the root to 
the buds and leaves. It has been supposed that part of the 
sap rises through that part of the wood next to the pith (the 
medullary sheath), and is conveyed thence to the buds, being 
somewhat altered in its properties during its course, and 
rendered fit for promoting the growth and evolution of the 
leaves: The rest of the sap rises through the alburnum 
__ (the vessels of which, being young, soft, and not compressed, 
are well adapted for the passage of fluids), and is conveyed 
to the leaves when formed, there to undergo changes which 
render it fit to promote the growth of the plant. Little is 
known with respect to the particular functions of each part 
of the wood, excepting the alburnum, the latest formed lay- 
ers of which give passage upwards to the sap. Mr. Knight 
made a circular incision several inches in depth in a tree, 
so as to divide completely both the bark and alburnum, 
_ which so completely intercepted the ascent of the sap, that 
no vegetation took place in the tree in the following spring. 
If the alburnum be removed and the bark left, the plant 
will also die. Thus it would seem that the new layer of al- 
burnum is formed for the nourishment of the plant, by con- 
veying the sap to the leaves; and this explains why a serious 
injury of this part is so fatal. The wood also contains many 
of the secretions of the plant, particularly the older layers. 
5. The Pith or Medulla, and Medullary Canal. - 
164. The pith is contained in the medullary sheath or 
- longitudinal direction. In all plants the pith consists of “= 
The cells are, in ge : 
form in section: in the young shoots of trees, and in herba- — 
ear as the plant grows older, and then they contain 
canal, which is in the centre of the tree, and the sides of 
_ which are composed of vessels, chiefly spiral, disposed ina 
cellular tissue alone, and is of a light and spongy character. _ Ss 
neral, very regular, and of an hexagonal 
