CHAP. II. 
EXOGENOUS STEMS. 
91 
it, it is often manufactured into cordage, especially in trees 
and shrubs of the natural order Malvaceae. The Russia 
mats of commerce are manufactured from the thin laminae 
into which the endophloeum of Tilia europaea readily sepa- 
rates. The Lace bark of Jamaica, remarkable for its beau- 
tiful lace-like appearance when gently pulled laterally, and 
for its great toughness, whence it is often twisted into whip- 
lashes, is the laminated liber of Lagetta lintearia. 
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 V asa propria are exceedingly common in the 
bark, but there is no authenticated instance of any spiral or 
other vessels having been found in it; except in Nepenthes, 
in which they occur in almost every part, and exist in no in- 
considerable numbers in the bark. 
Beneath the bark, and above the wood, is interposed in the 
spring a mucous viscid layer, which, when highly magnified, 
is found to contain numerous minute transparent granules, 
and to exhibit faint traces of a delicate cellular organisation. 
This secretion is named the Cambium, and appears to be 
exuded both by the bark and wood, certainly by the latter ; 
but Dutrochet says only by the former, founding his opinion 
upon the presence of cambium in bark nodules, which, he 
says, have no communication with the wood of the parent 
tree ; see page 80. 
The cellular system of the pith and that of the bark are, in 
the embryo and youngest shoots, in contact ; but the woody 
system, as it forms, gradually interposes between them, till 
after a few weeks they are distinctly separated, and in very 
aged trunks are sometimes divided by a space of several feet ; 
that is to say, by half the diameter of the wood. But whatever 
may be the distance between them, a horizontal communi- 
cation of the most perfect kind continues to be maintained. 
When the woody system is first insinuated into the cellular 
system, dividing the pith and cortical integument, it does not 
