LETTER XVI. 



169 



ments made by him, elsewhere, in his Principles of 

 General and Comparative Physiology, Dr Carpenter's 

 whole view of the subject under discussion may, I 

 think, be thus fairly stated : — 



First, The buds and the Cambium, together with 

 the leaves and the wood and roots which proceed from 

 them, if not also the flowers and fruit, are merely an 

 extension (on its free side) of the general cellular 

 basis, and the transformation of this basis into certain 

 definite structures. And this extension and transfor- 

 mation are processes which, but for certain periodic 

 checks put to them, would go on uninterruptedly. 

 But for these, there would neither be the breaks which 

 occur in them during the winter, nor would their per- 

 manent produce — the wood — present those lines of 

 demarcation which we see in the so-called " annual 

 layers or rings.* But for those periodic checks, all 

 our trees would be evergreens ; there would be an 

 unbroken succession in the formation and shedding of 

 the leaves, and flowers, and fruits : and the wood 

 would constitute one entire mass of ligneous and cellu- 

 lar tissue, — The buds and the Cambium are merely 



continuous products " of the general cellular basis, 

 evolved at the ends of the medullary rays in the case 

 of the former, and over the whole exterior surface of 

 the woody stem in the case of the latter ; and it is 

 from those checks alone, and from no other cause, 



* Ibid, p. 790. 



