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. 

 Tliis 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 sa)', 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 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 circumference ; 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 



