THE ROOTS OF VASCULAR PLANTS. 211 



development of the cellular envelope of the bark. 

 This development of the external cellular tissue appears 

 to arise from this — viz. 1st, the growth of the woody 

 body being less, the cortical body is not distended so 

 much as in the stem ; 2d, the subterranean position of 

 roots, which tends to prevent the drying up and alter- 

 ation of the external tissue. It is also to this position 

 imder ground that the dull and dark appearance of the 

 epidermis of most of them ought to be ascribed. 



We have seen, in speaking of stems, that their shoots 

 grow throughout their whole length, until they abso- 

 lutely cease to elongate. It is not the same with roots ; 

 they only lengthen by their extregiitie^ ^;he position 

 and respective distance of the JHeral radicules be re- 

 marked, we can easily assure ours^es of this important 

 fact. If dots of coloured varnish be marked upon the 

 roots of the Hyacinth, the Bean, &c., or if small pieces 

 of thread be fastened to them at equal distances, we see 

 that all these marks remain at exactly the same distance 

 at which they were placed, and the root is prolonged 

 beyond them ; whence we know that the roots only grow 

 by their extremities. Duharael, who first made this 

 important experiment, has also remarked, that roots 

 when cut never elongate, — a necessary consequence of 

 their never lengthening but by the extremity. It is 

 from this growth of roots solely by their extremities, 

 and of young shoots by their whole length, that Mr. 

 Knight has deduced the most ingenious and plausible 

 explanation of the descending direction of roots, and 

 the ascending one of stems. 



The growth of roots in diameter takes place in each 

 class of plants as in the stems themselves ; thus the 

 roots of Endogens are cylindrical processes, more or 

 less thick ; whilst those of Exogens are simple, or 

 branched reversed cones. 



p2 



