FUNCTION.] OF BOTHEENCHYM AND PLEURENCHYM. 173 



Arrow-root; in rhizomes, as in the Ginger; in soft stems, 

 such as those of the Sago-Palm and Sugar-cane; in albumen, 

 as that of Corn ; in pith, as in the Cassava ; in the disk of the 

 flower, as in Amygdalus ; and, finally, in the bark, as in all 

 exogenous plants; and cellular tissue is the principal, or 

 exclusive, constituent of these parts. 



In the form of articulated BOTHRENCHYM, when 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 articulated both- 

 renchym, and their abundance, are usually in proportion to 

 the length to which 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 ordinary 

 trees much less so ; and in herbaceous plants it hardly exists. 

 Bothrenchym eventually ceases to convey fluid, and becomes 

 filled with air. The use of other kinds of bothrenchym is 

 not known. 



PLEURENCHYM 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. 



