CIRCULATION OF SAP. 91 
of trees, as it will be observed that the absorption of 
carbon, and the giving out of the oxygen becomes 
less active as the light is diminished, while during 
the night the contrary process occurs, the oxygen be- 
ing absorbed, and the carbon released. 
Thus it is by the assistance of 
these vessels that the little oak- 
plant becomes a perfect tree in 
miniature, and continues to increase 
in size and strength from year to 
year, until the woodman levels with 
his axe the sturdy trunk that has 
defied the storms of a century. Let 
us here examine the stump that re- 
mains, and we will observe a num- 
ber of concentric rings commencing 
at the bark, and running around  Chestnut-Oak. 
the tree, one inside of the other, until they reach the 
‘eentre. (See figure of Exogenous Wood on a pre- 
vious page.) Hach ring indicates one year’s growth, 
a new layer of wood being depo- 
sited every season immediately 
under the bark. The age of 
any tree may be determined 
with considerable accuracy by 
counting these rings. 
Some species of Oaks retain 
their foliage during winter, as 
the Live-oak. This tree in- 
habits the Southern States, 
where it occasionally grows to cet 
