CHAP. II. 
EXOGENOUS STEMS. 
71 
bark, but there is no well-authenticated instance of any spiral 
vessels having been found in it; except in Nepenthes, in 
which they occur in almost every part, and exist in no incon- 
siderable numbers in the bark. Don states that spiral vessels 
abound in the bark of Urtica nivea, but I have not succeeded 
in discovering them there. 
Beneath the bark and above the wood is interposed in the 
spring a mucous viscid layer, which, when highly magnified, 
is found to consist of 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. 
The cellular system of the pith and that of the bark are, in 
the embryo, and youngest shoots, in contact; but the vascular 
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 whaitever 
may be the distance between them, a horizontal communi- 
cation of the most perfect kind continues to be maintained. 
When the vascular system is first insinuated into the cellular 
system, dividing the pith and bark, it does not completely 
separate them, but pushes aside a quantity of cellular tissue, 
pressing it tightly into thin vertical radiating plates : as the 
vascular system extends, these plates increase outwardly, con- 
tinuing to maintain the connection between the centre and 
the circumference. Botanists call them medullary rays (or 
plates) ; and carpenters, the silver grain. They are composed 
of muriform cellular tissue (Plate I. fig. 7.), often not con- 
sisting of more than a single layer of cellules ; but sometimes, 
as in Aristolochias, the number of layers is very considerable 
(Plate II. fig. 12. a). In horizontal sections of an Exoge- 
nous stem, they are seen as fine lines radiating from the centre 
to the cu'cumference ; in longitudinal sections they produce 
that glancing satiny lustre which is in all discoverable, and 
which gives to some, such as the Plane and the Sycamore, a 
character of remarkable beauty. 
No vascular tissue is ever found in the medullary rays, un- 
less those curious plates described by Griffith in the wood 
F 4 
