

MEDULLARY SHEATH. BARK. 99 



quently found imbedded in solid rock; for they thus afford 

 evidence, that the temperature of this quarter of the globe was 

 both higher and .more equable at the time they grew here, 

 than it is at present ; an opinion which is equally supported, 

 by the nature of the fossil remains of Animals existing at the 

 same period. 



135. Between the pith and the adjacent layer of wood, a 

 delicate membrane may be traced, which is termed the medul- 

 lary sheath. This consists almost entirely of spiral vessels, 

 which are seldom found in any other part of an Exogenous 

 stem, except when they pass off from this, towards the origins of 

 the leaves. Their office is a very important one, as will be 

 hereafter seen (. 324). 



136. The wood is inclosed by the bark, which is, like it, 

 formed in regular layers ; though these are much thinner, and 

 cannot be so plainly distinguished. The layers of bark are 

 formed from the interior, so that the oldest are on the outside. 

 These are gradually lost, either by decay, or by falling off ; so 

 that it is very seldom, that the same number can be traced in 

 the bark as in the wood, although an additional one is formed in 

 each at the same time. As the new layer of wood is formed on 

 the outside of the previous one (at the point, therefore, at which 

 it is in contact with the bark), and as the new layer of bark 

 is added to the inside of the previous one (at the point, there- 

 fore, at which it was in contact with the wood), it is obvious 

 that they are produced at the same spot ; and that the newest 

 layers of both will always be in contact with each other. Their 

 production seems to take place in somewhat of this manner. At 

 the end of the spring, the bark becomes loosened from the wood, 

 with which it was previously in close contact, and a glutinous 

 fluid, termed the cambium, is found bet ween them (see Fig. 47). 

 This may be observed, by stripping the bark from almost any 

 twig, at that season. The cambium is gradually organised into 

 cells, and from these are formed the ducts and cellular portion 

 of the woody layer, and the cellular portion (which is much the 

 greatest) of the layer of bark. Later in the year, the woody 

 tubes grow downwards from the leaves, obtaining nourishment 



H2 



