36 



LETTERS ON TREES. 



between them, parting them asunder. And this they 

 do, as represented in figure B. And these latter, 

 again, in direct relation to one another for one season, 

 are themselves in their turn parted after the like 

 fashion by the root and bark of the third year's plant. 

 And so on. Thus, the most external of the annual 

 layers of bark corresponds to the most internal of the 

 annual layers of wood. And were the several layers 

 of bark distinguishable, or rather could they be sepa- 

 rated from one another, each would be found to cor- 

 respond in length with the plant in connection with 

 which it grew, and the whole to become individually 

 longer from without inwards, as shewn in figure C. 



9. From this disposition and wedgelike action of the 

 wood and bark, it comes to pass that year by year the 

 older layers of wood are pressed inwards, and more 

 closely compacted ; while the older layers of bark are 

 pushed outwards, and become in process of time so 

 distended or stretched, that, unable to yield otherwise 

 to the pressure from within, they rend. And this 

 occurs in a great variety of ways — by exfoliation in 

 some (as in the Sycamore or Plane, and in the Birch),, 

 and by fissure in most (as in the Fir, the Oak, and 

 very remarkably in the Acacia) — each particular kind 

 of tree having its own, and, it may be added, its dis- 

 tinctive manner of rending. 



10. Again : The bud at the top of the terminal 

 shoot, the embryo of the plant of next year, is to 



