LETTER XII. 



" Audi alteram partem." 



February 17, 1855. 



My Dear Sons, 



1. You may remember that at the close of my 

 eighth letter, I made allusion to certain exceptions 

 that might be taken to my theory, and that I there 

 promised to consider these exceptions in future letters. 

 I purpose now to redeem that promise. 



2. One exception which I mentioned bears on a 

 much agitated question in vegetable physiology — the 

 nature and mode of formation of the woody layers 

 annually formed in exogenous trees. M. Du Petit- 

 Thouars holds, as you are aware, that the layer in 

 question is none other than the roots of the buds, and 

 grows if not actually yet virtually downwards from 

 these to the soil. This view, I need scarcely say, 

 accords entirely with the argument pursued in that 

 letter — viz. that the shoots emanating from the buds 

 constitute severally perfect and independent plants, 

 and as such are furnished with roots. 



3. M. Mirbel, however, and otners, maintain that 



