248 PHYSIOLOGY OF TREES. 



are hidden from daily inspection. Examination, 

 by frequent incisions through the bark, give but 

 an imperfect idea of how and when the fibrous 

 organization of the cambium first appears, and 

 whence it is derived. This is a point on which 

 there is at this time much diversity of opinion ; 

 and when the difficulties of having ocular proof 

 of the phenomenon are considered, there is no 

 wonder that physiologists differ so much from 

 each other respecting this occult part of the 

 science. 



Some of the most eminent writers on the sub- 

 ject imagine that the sap is changed, chiefly by 

 the elaborating povvers of the leaves, from a crude, 

 watery state, to what they call mature or perfect 

 sap, and this is conceived to possess a property 

 which is deemed, per se, organisable that is,, 

 when sufficiently abundant and mature, it is 

 transformable into all the membranes, various 

 organs, and components of the plant. One dis- 

 tinguished botanist believes the new accretion to 

 be only a dilatation of the alburnum, while others 

 maintain, that the ligneous matter of the new 



