90 CIRCULATION OF SAP. 
at their base a bud from which in like manner addi- 
tional leaves make their appearance. The functions 
of the little plant are now as complete as they are in 
the monster tree. The delicate stem is supplied with 
its capillary tubes, which carry the nourishment from 
the root, and distribute it throughout the different 
parts of the plant. These vessels 
perform the same part to the 
tree as the veins and arteries 
to the human body. One set, 
comparable to the veins, carry 
the sap through the trunk and 
branches to the leaves, where 
it is spread out in the minute 
reticulated net-work on their 
upper surfaces, and like the 
Post-Oak. blood in the lungs is there ex- 
posed to the action of the light and air, absorbing 
from the latter a portion of carbon, and at the same 
time parting with its oxygen. This process purifies 
the sap, and renders it fit for the nourishment of the 
tree, when a new set of vessels, similar in their office 
to the arteries, distribute it to the different parts 
where it is required. This sap, thus purified, con- 
tains all the requisites for the formation of the wood 
and bark of the trunk and branches, and the cuticle 
for the formation of the leaves. 
The effect of light upon the sap is very great; 
those plants which receive the largest amount of sun- 
light have leaves of a deep green. Light is therefore 
an essential element in promoting the healthy growth 
