50 



is also formed each year, from the de- 

 scending sap, corresponding with the in- 

 creased diameter of the tree. The old or 

 outward lavers are stretched outwards, 

 crack, and form the rough bark seen on 

 old trunks. The yearling shoot has but 

 one layer of bark, the two-year old shoot 

 two, and so on ; and each shoot may be 

 said to have as many layers of bark, as 

 well as as many laj'ers of wood, as it 

 is years old. But with regard to the 

 layers of bark, besides the sloughing 

 off, the circumference of the earlier 

 layers would be very disproportioned to 

 that of the later ones. If the circum- 

 ference of the bark of the seedling oak 

 was half an inch, it would make a poor 

 show on the outer circumference of a 

 full-grown tree, supposing it to have 

 existed. This growth of the bark may 

 also be considered as partaking of the 

 ])rinciple of the growth by juxta-position, 

 since the annual new laver is a distinct 



