90 
OHGANOGRAPHY. 
BOOK I. 
cells, less compactly arranged, and easily separating into a 
fine powder when disturbed. 
As strata of cellular tissue, in a peculiar state, may form 
between the lamellae of the Birch and other such trees, so 
may it in other parts of the bark. This causes the sloughing 
of hard thin plates from the bark of the Plane tree ; which 
Mohl explains thus: — Up to its eighth or tenth year, the bark 
of the Plane tree is like that of the Beech ; at that period 
there forms in different parts of the liber a stratum of 
tabular cells, in all respects Analogous to those of the epi- 
phloeum. This new epiphloeum is not exactly parallel with 
that of older date, which exists at the surface of the bark, and 
cuts off an exterior portion, which then dies and drops off in 
the manner with which we are all familiar. The scales pro- 
duced by this formation of epiphloeum inside the liber or 
mesophloeum Mohl calls Rliytidoma^ from pur<j, a wrinkle. 
(Ann. des Sciences, N. S. IX. 290.) 
In some plants the epiphloeum forms regular strata, parallel 
with the axis of the stem, and afterwards separates into strips 
analogous to those of the liber, as in the Juniper, Callis- 
temon lophanthus, &c. In others, a portion of the liber is 
really thrown off annually, as in the Vine, the Honey- 
suckle, &c. 
Hence in exogenous trees, the thickness of the bark is 
annually diminished by one of two causes ; either by an ex- 
foliation of the external and dead portions of the epiphloeum 
only, or by a formation of a second epiphloeum, or false cork, 
among the liber, the result of which is the throwing off the 
parts of the bark lying over it as soon as they die. 
So long as the parts of the bark remain alive, they 
give way to the expansion of the wood wdthin it, by adding 
new tissue to themselves, as has been already stated : but 
when they die, they are necessarily torn into clefts, rents, or 
ribands," as we find in the trunks of trees. 
It will have been seen that the only part of the bark in 
which woody tissue occurs is the endophloeum. Here it is 
often very abundant, and exceedingly tough and thick-sided ; 
in consequence of which it is of great value for many useful 
purposes. When freed from the cellular tissue adhering to 
