10 
PLANTS. 
they likewise become more compact, and in no par- 
ticular differ from the real wood. The tree, by its 
perpetual increase in strength and circumference, 
forces the fibres of the bark to stretch and extend 
themselves, so that the outward surface sometimes 
bursts with a surprising noise : this occasions the 
crevices, which are always enlarging the external 
bark in proportion to the growth of the tree. 
Immediately under the bark lies the wood, which 
forms by far the greatest part of the trunk and 
large branches of trees. The layers of which it 
consists, and which, as we have just remarked, de- 
termine the age of the tree, are chiefly composed of 
longitudinal fibres that once afforded a passage for 
the sap, but whose orifices are obliterated by com- 
pression, and become solid and impervious. The 
pith, which in the large trunks of many trees dis- 
appears altogether, occupies the centre of the wood, 
and consists of a system of little cells separated by 
interstices or partitions of a very thin texture. Ana- 
% 
tomists have compared the cells to bladders. 
We have observed that the bark, as well as the 
sappy parts, is composed of long rows of tubes or 
hollow fibres, that ascend and join together, or have 
a communication with one another by the agency 
of transverse fibres ; which consequently leaves se- 
veral spaces between these fibres. All these kinds 
of open meshes are filled with little vessels, or bags, 
of an oval form, pierced at the two extremities, and 
joined to one another at each end like a string of 
beads; ranged at the same time in heaps one above 
