LETTER XIII. 



143 



fibres precisely similar to those which in the former 

 case, it sends down into the soil. But different from 

 what obtains in the Exogen, the Endogen is destitute 

 of Cambium. And this difference seems to go far to 

 explain the respective peculiarities in the buds of these 

 two divisions of trees. But however this may be,; there 

 remains the fact that the bud of the Endogen, in the 

 ordinary course of its development and growth, does 

 send down genuine roots within the trunk of the tree 

 — which roots, in some kinds, as in the Screw Pine, 

 even pierce through the trunk and ultimately reach the 

 ground. And this general fact I add to the evidence 

 already adduced in favour of the assumption that the 

 woody layers formed annually in exogenous trees, — 

 and which comprise the far greater portion of the 

 woody mass both above and below ground, are in 

 point of fact, physiologically and structurally, of the 

 nature of roots. 



12. In maintaining that the woody tissue in the 

 trunk, developed from the Cambium, is of the nature 

 of roots, I have allowed that it is nevertheless formed 

 in situ, or as it lies. It is impossible, however, to 

 overlook the fact, so often and in so many forms 

 brought under your notice, that this tissue is natu- 

 rally evolved not merely in the vertical direction 

 (which all allow), but in such manner that when occa- 

 sion requires, it can (and seemingly without an effort), 

 do that which it as naturally does at all times at its 



