Chap. III.] DOWNWARD GROWTH OF THE ROOT. 
121 
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 rung it ceases to be deposited below 
the 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 covering of the leaf and 
annual shoot the cuticle , and that of the rest of 
the tree the epidermis . There is certainly a dif¬ 
ference between living skin and bark and dead 
skin and bark, and it might be as well if they 
had different names; but if we give the name of 
cuticle to the outer covering of the living bark, 
it will be found, with its green under layer of 
parenchyma ,—the green, porous, spongy layer, 
which is also called the “ herbaceous envelope,’’— 
to extend over a much larger space of our forest- 
trees than De Candolle assigns to it. There is 
nothing in which even the same sort of trees 
differ more than in this respect. According to 
growth, soil, exposure, &c., the cuticle exists to 
a very indefinite period; and it would be hard 
to say where cuticle ceased and epidermis began. 
