PHYSIOLOGY OF TREES. 273 



being thrown off and developed in each year; 

 the first to increase the thickness of the bark, 

 the second to increase the body of wood. Strictly 

 speaking, both these are only excrementitious 

 parts of the plant ; the first as a general pro- 

 tection or covering, the second as a temporary 

 support. In watching the annual growth of a 

 tree, it is only these increments of the stem which 

 are detectable by the eye, and these but imper- 

 fectly till they are displayed into form and sub- 

 stance. The incipient cambium occupies in win- 

 ter so inconsiderable a space on the exterior of the 

 last year's alburnum, that its wonderful compli- 

 cation of parts cannot be distinguished; and 

 when examined, after it has swelled to the thick- 

 ness of one-twelfth of an inch, its fibrous or mem- 

 branous components are so colourless and trans- 

 parent, that neither their forms nor positions can 

 be ascertained. It is this indistinctness of the 

 organs in this stage of their existence which led 

 to the opinion, that the perfect sap generated 

 the organs — a circumstance which, were it true, 

 I believe, has no parallel in Nature. 



T 



