100 



LETTERS ON TREES. 



the agency of the spongioles, furnish no grounds for 

 ascribing vitahty to the old roots.* 



8. Hitherto, then, we have seen nothing in the old 

 stems and roots to lead us to believe that they are 

 possessed of vitality. And I would fain persuade 

 myself that in the facts which have passed under our 

 view, in this and the preceding letter, we have seen 

 enough to satisfy us that they are really dead. And 

 to the considerations already adduced in behalf of this 

 conclusion, I would just add this other. We know 

 that after a time, — altogether indefinite, often not 

 for many centuries, sometimes within a few years, — the 

 earliest formed wood, the heart-wood, as it is called, 

 decays and disappears; and that this change may 

 proceed to such an extent as to destroy a large part 

 of the entire trunk, without in the least degree im- 

 pairing the vegetation going on at the extremities, 

 and on the exterior of the tree. This it is easy to 

 understand on the view we have all along taken 

 of the nature of trees, but very difficult on the sup- 

 position that a tree constitutes but a single or an 



^ " The spongiole is sometimes spoken of as a distinct organ ; 

 but it is nothing more than the growing point of the root, which, 

 with a few exceptions, lengthens only by additions to its extremity. 

 The soft, lax texture of the nev)ly -formed part causes it to possess, 

 in an eminent degree, the power of absorption: but as the fibre con- 

 tinues to grow, and additional tissue is formed at its extremity, that 

 which was formerly the spongiole becomes consolidated into the 

 general structure of the root, and loses almost entirely its peculiar 

 properties.''^ — Dr Carpenter, ibid, p. 652. 



