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 tissue is apparently destined for the conveyance 

 of fluid upwards or downwards, from one end of a body 

 to anothei', and for giving firmness and elasticity to every 

 part. 



That it is intended for the conveyance of jiidd 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 fii"st 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 dowiiNvards 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 elasticitg 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 firnuiess or tenacity itself. To this evidence might be 

 ivdded, the admirable manner in wliich it is combined to 



