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Bee. Vegetable Organography and Physiology. 55 
ae ne. ‘= 
Jar 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 
eflused in the spring of the year, between the wood and the bark, - 
and which separates the alburnum from the liber, is pot 
these medullary radii. This viscid secretion, whe Saasaiiand red, i 
supposed to constitute the cellular portion of each concentric 
layer. Whilst this deposition is going on, a fibrous’ ae 
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 
