COUESE OF THE SAP. 



PT. II. 



formed sap, coiTespoiiding with the increased girthing of the 



every year. 



tree. The old or outward layers are stretched out- 

 wards, crack, and form the rough bark seen on old 

 trunks. The yearhng shoot has but one layer of bark, 

 besides the outer cuticle, 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 layers 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 Avould be very disproportioned to 

 that of the later ones. If the circumference of the bark 

 of the seedling oak were half an inch, it would make 

 a poor show when rent and divided over the outer cir- 

 cumference of a full-grown tree, supposing it to have 

 existed. This growth of the bark may also be con- 

 sidered as partaking of the principle of the growth by 

 juxta-position, since the annual new layer is a distinct 

 coating or deposit of new growth on the inside of the 

 bark, and not a growth or increase of parts already 

 formed. It is from the downward sap, since in branches 

 which are ringed it ceases to be deposited below tlie 

 rings, but is continued annually above the rings. 



De Candolle makes a distinction between the outer 

 skin or covering of the leaf and annual shoot and that 

 of all other parts of the tree. He calls the outer cover- 

 ing of the leaf and annual shoot the cuticle^ and that 

 of the rest of the tree the epidermis. There is certainly 

 a difference between living skin and bark and dead 

 skin and bark, and it might be as well if they had 



