Vegetable Organography and Physiology. 55 



lar tissue outside. The bark is also subdivided into three parts ; 

 the inner portion is called the liber ; surrounding this is the cel- 

 lular envelop, and external to all, the epidermis. It is through 

 the liber, the inner portion of the bark, that the principal part of 

 the descending sap is carried, after it has undergone those chemi- 

 cal changes, in its circulation through the leaves, that qualify it 

 for giving support and nourishment to the different parts of the 

 plant. 



On making a horizontal section of the trunk, or branch of an 

 exogenous tree, radii of cellular tissue are seen extending from 

 the centre to the circumference of the wood. These vessels are 

 denominated the medullary rays. Having their origin in the 

 medullary sheath, and consisting of the same elementary tissue 

 with the pith, projections of which pass through the sheath into 

 the medullary rays, they serve to connect this central system of 

 the plant with its circumference. 



The medullary rays perform a very important function in the 

 economy of vegetable growth. That peculiar secretion which is 

 effused in the spring of the year, between the wood and the bark, 

 and which separates the alburnum from the liber, is poured out by 

 these medullary radii. This viscid secretion, when organized, is 

 supposed to constitute the cellular portion of each concentric 

 layer. Whilst this deposition is going on, a fibrous secretion is 

 descending from the expanding buds, and by its adhesion to the 

 first deposit from the medullary rays, constitutes the outer stra- 

 tum of woody fibre and ducts of the new layer. Thus it will 

 be seen, that a triple and diverse circulation is carried on, in ex- 

 ogenous plants, at the same time. During the vernal season, 

 whilst the lymphatic sap is ascending in the greatest quantity, 

 and the elaborated fluid, having undergone the necessary chan- 

 ges, in its circulation through the leaves, is descending and de- 

 positing its nutritious particles in the different parts of the plant, 

 this transverse current is percolating through the horizontal radii, 

 to deposit, on the exterior surface of the alburnum, a new layer 

 of vegetable growth. 



It would be interesting to trace minutely the course of the 

 vegetable circulation, and to investigate the agency or powers by 

 which this circulation is sustained. But the limits orginally de- 

 signed for this paper will not allow of this extended inquiry. We 



