LETTER XII. 



131 



Fig. 17. 



tissue of the far greater part of the Cambium-layers 

 (and which give strength and 

 stabiHty to the stems and 

 roots, and preserve them 

 from decay), would be en- 

 tirely accessory, and would 

 require some proper name 

 common to the portions both 

 above and below ground. 

 At all events, as regards 

 this whole mass of ligneous 

 tissue, whatever name we 

 may give it, it is one and 

 the self - same structure 

 throughout, and identical in 

 its character with true and y_ 

 genuine roots. 



14. With regard to the 

 transformation of the Cam- 

 bium-layer, it is effected 

 after this manner : Such of 

 the cells composing it as 

 are destined to become cellu- 

 lar tissue merely, undergo no particular change beyond 

 a full development, and that in the transverse direc- 

 tion of the axis of the tree. Those of them, on the 

 other hand, that are to form woody tissue, undergo 



