246 PHYSIOLOGY OF TREES. 



which they are only temporarily attached ; they 

 also having pre-existence before development. 

 Indeed, it is most material to remark, in this 

 place, the important fact, that every part of a 

 plant displayed in the course of time previously 

 exists in embryo ; vegetable life being only the 

 expansion of incipient organization which is gra- 

 dually excited into form and amplitude by the 

 stimuli of heat, air, and water. 



The above description of a shoot from a seed, 

 is applicable to every shoot afterwards produced 

 from a bud. 



If we examine a transverse section of a two- 

 year old seedling, we shall find that the pith 

 maintains its place, though somewhat reduced in 

 diameter : that the cambium of last year has now 

 become perfect wood, but remaining of the same 

 size and figure it had at the end of the first sum- 

 mer; that a new circle of alburnum has been 

 formed on the outside, involving the first ; and 

 that this is defended by two films of bark, the 

 inner one having been added at the same time 

 with the new alburnum. This is the process that 



