248 
PHYSIOLOGY. 
BOOK II. 
finally, in the bark, as in all exogenous plants ; and cellular 
tissue is the principal, or exclusive, constituent of these. 
In the form of vasiform tissue, when it is collected 
together into hollow cylinders, it serves for the rapid trans- 
mission of fluids in the direction of the stem ; and it is well 
worth notice that the size of the tubes of vasiform tissue 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 vasiform 
tissue is unusually large and abundant; in ordinary trees 
much less so; and in herbaceous plants it hardly exists. 
Vasiform tissue eventually ceases to convey fluid, and becomes 
filled with air. 
Woody tiS!^ue 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, form- 
ing 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 combined to 
