262 



VEGETABLE 



which it passes are situated near the inner surface of the 

 bark. They differ widely in their construction, from those 

 by which the sap is elevated, being in the form of straight 

 parallel bundles, running close to the wood. It would appear, 

 therefore, that the spiral form of the interior vessels, consti- 

 tutes a portion of the agency by which the sap is elevated ; 

 this complicated tissue being unnecessary in those by which 

 the fluid descends. 



A new layer of these vessels is formed every year between 

 the bark and the wood, the former layers being pushed out- 

 ward, by the additional woody matter, or by the growth of 



Fig. 233 



the tree. In consequence of 

 this mechanical force, the paral- 

 lel vessels separate from each 

 other, at a little distance from 

 the wood, and form a mass of 

 net-work, the meshes of which 

 are afterwards filled with cel- 

 lular matter. This mechanism 

 is readily seen by tearing a 

 piece of bark longitudinally. 



Fig. 233 represents the dif- 

 ferent states of the returning ves- 

 sels, a, being a simple tube, b, a bundle of the same, and c, 

 these vessels divided by the expansion of the bark, the spaces 

 between the fibres of the bark being filled up with cellulai 

 matter 



BARK, WOOD AND PITH. 



The Bark, (cortex,) comprehends four distinct layers, or 

 parts, namely, the Epidermis, or external covering ; the Cu~ 

 tis, or inner covering ; the Cellular Integument, or true bark, 

 and the Liber, or inner bark. 



Epidermis. In young and annual plants, this is a thin 

 membrane or pellicle, which extends ovtff the whole external 

 surface, from the fibrils of the roots to the leaves, and deli- 

 cate petals of the flower, and even to the fruit. Its use is 

 to guard the living parts of the plant from external inju- 

 ries. 



In some parts, and especially where it covers the leaves, 

 this membrane is furnished with minute vessels, which are 

 spread over it like a net-work, the form of the mealies dif 



