272 PHYSIOLOGY OF TREES. 



has already been adverted to, page 252, and no 

 one who is aware that the vessels of the roots are 

 at all times replete with sap can possibly imagine 

 that they have capacity for such a surcharge. 



In recurring again to the most obscure part of 

 the above sketch of vegetable life and constitu- 

 tional development, a few additional observations 

 may conclude this section of the book. 



The arrangement of the structure has been 

 set forth under the proper names of pith, wood, 

 and bark. These are the three principal con- 

 stituents of the stem or trunk ; and which are all 

 in a state of gradual change, by annual accretion 

 or transformation ; and, while undergoing a 

 change, the processes receive temporary dis- 

 tinctions for the better describing the parts. 

 Thus the last year's layer of bark is called the 

 liber ; and the last year's ring of wood is called 

 alburnum: but these, during their growth, are 

 both designated cambium. This cambium is 

 the seat of life ; is composed of an indefinite 

 number of layers of bark and wood ; one of each 



