LECT. VII.] ANATOMY OF STEMS. 335 



" of its parenchyma ; but still filling up the in- 

 " terstices of the concentric layers, and binding 

 u them together like a cement.** But there is 

 little difficulty in demonstrating the error of both 

 these opinions ; for, examining the alburnum in a 

 very early stage of its formation at the moment it 

 is passing from the gelatinous state in which it is 

 first deposited, we find the rudiments of both the 

 concentric and divergent layers already assuming 

 the form which they afterwards maintain. Were 

 any further reason required to prove that the diver- 

 gent layers do not originate in the pith, it would 

 be found in the fact, that many of them cannot be 

 traced to the pith ; although the more conspicuous 

 of them traverse the whole of the wood, from the 

 pith to the bark. 



Such is the structure of wood in the stems of 

 one year's growth of ligneous dicotyledons ; and 

 it is found nearly of the same structure in the root : 

 a fact, which is rendered evident, not only by the 

 microscope, but also to the unassisted eye, by the 

 decomposition of the divergent layers in the ligne- 

 ous part of roots which have been dug up and long 

 exposed to the action of the atmosphere. The 

 concentric layers, longer resisting the action of 

 the weather, remain after the divergent have dis- 

 appeared, and display a beautiful network, more 

 or less open in its meshes, according to the den- 

 sity or sponginess of the wood. 



