298 
PHYSIOLOGY. 
BOOK II. 
In the form of articulated bothrenchyma, when it is col- 
lected together into hollow cylinders, it serves for the rapid 
transmission of fluids in the direction of the stem ; and it is 
well worth notice, that the size of the tubes of articulated 
bothrenchyma, and their abundance, are usually in proportion 
to the length to which the fluid has to be conveyed. Thus 
in the Vine, Phytocrene, the common Cane, and such plants, 
the pitted tissue is unusually large and abundant ; in or- 
dinary trees much less so ; and in herbaceous plants it hardly 
exists. Bothrenchyma eventually ceases to convey fluid, and 
becomes filled with air. The use of other kinds of bothren- 
chyma is not known. 
Pleurenchyma is apparently destined for the conveyance 
of fluid upwards or downwards, from one end of a body to 
another, and for giving firmness and elasticity to every part. 
That it is intended for the conveyance of fluid in particular 
channels seems to be proved: 1. from its constituting the 
principal part of all wood, particularly of that which is formed 
in stems the last in each year, and in which fluid first ascends 
in the ensuing season ; 2. from its presence in the veins 
of leaves where a rapid circulation is known to take place, 
forming in those plants both the adducent and reducent 
channels of the sap ; and, 3., from its passing downwards from 
the leaves into the bark, thus forming a passage through 
which the peculiar secretions may, when elaborated, arrive at 
the stations where they are finally to be deposited. Knight 
is clearly of opinion that it conveys fluid either upwards or 
downwards ; in which I fully concur with him : the power of 
cuttings to grow when inverted seems, indeed, a conclusive 
proof of this. Dutrochet, however, endeavours to prove that 
it merely serves for a downward conveyance. 
With regard to its giving firmness and elasticity to every part ^ 
we need only consider its surprising tenacity, as evinced in 
hemp, flax, and the like; and its constantly surrounding and 
protecting the ramifications of the vascular system, which has 
no firmness or tenacity itself. To this evidence might be 
added, the admirable manner in which it is contrived to an- 
swer such an end. It consists, as has been seen, of lignified 
slender tubes, each of which is indeed possessed of but a 
slight degree of strength; but being of different lengths. 
