120 
UPWARD GROWTH OF THE HEAD, AND [Pakt II. 
A new layer of 
bark is formed 
every year. 
than any foregoing age, I have not a doubt. 
But if any one imagines that we have arrived at 
a competent knowledge of the science, when a 
dozen questions of such vital importance as 
these are open, I think he only shows that he is 
ignorant of the depth of our ignorance. 
A new innermost layer of bark (the new cor¬ 
tical layer) is also formed each year, from the 
descending sap, corresponding with the increased 
girthing of the tree. The old or outward layers 
are stretched outwards, crack, and form the 
rough bark seen on old trunks. The yearling 
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 would 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 circum¬ 
ference of a full-grown tree, supposing it to have 
existed. This growth of the bark may also be 
considered as partaking of the principle of the 
growth by j uxta-position, since the annual new 
