70 
ORGANOGRAPHY. 
BOOK I. 
the force of the inward growth, they are separated into 
lozenge-shaped meshes, arranged in such beautiful order, 
as to have acquired for the plant itself the name of the Lace 
Bark Tree. There exists, however, in all cases, a limit to 
the extensibility of the old layers of bark ; and when this is 
reached, the outer bark either splits into deep fissures, as in 
the Oak, the Elm, the Cork, and most of our European trees, 
or it falls away in broad plates, as in the Plane, or it peels off 
in long thin ribands, as in the Birch. 
As there is a double layer of cellular integument and woody 
fibre formed every year, it follows that the age of a tree ought 
to be indicated by the number of such deposits contained in 
its bark. But the arrangement of the zones is so very soon 
disturbed, and the distinction between them becomes so im- 
perfect, that even when the outermost coating is still entire, it 
is scarcely practicable to count the zones; and as soon as the 
outside begins to split or peel off, all traces of their full num- 
ber necessarily disappear. 
That the bark really increases by constant deposits of new 
matter between it and the wood, is demonstrated by introduc- 
ing a piece of metal into the liber of a tree, and watching it 
subsequently : in process of time it will be protruded to the 
outside, and will finally fall away. 
Notwithstanding the fibrous character of a certain portion 
of the bark, it is generally so brittle as to be capable of 
breaking in all directions with a clean fracture, as soon as it 
becomes dry and ceases to live; but in many plants, when 
young, it is so tough as to be applied to different economical 
purposes. The Russia mats of commerce are prepared from 
the liber of two or three species of Tilia, that of numerous 
Malvaceae is manufactured into cordage, and similar pro- 
perties are found in that of many other plants. 
When stems are old, the bark usually bears but a small 
proportion in thickness to the wood; yet in some plants its 
dimensions are of a magnitude that is very remarkable. For 
instance, specimens of Abies Douglasii have been brought to 
Europe twelve inches thick, and these are said not to be of 
the largest size. 
Air cells and Vasa propria are exceedingly common in the 
