64: LIFE OF A TREE. 
from the earth, and some soluble secretions which 
it finds in the cells of the stem. Taking thus the 
character of a liquid food, it rises higher and 
higher, enters the leaf-stalk, and then sheds itself 
into the thousand cells of the leaf. The structure 
of the leaf is admirably adapted to effect the 
alteration and conversion of sap into nutriment 
for the plant. It consists of an upper and under 
delicate layer of outer cells, which are very close 
together, so that but little fluid can escape from 
them,—this is called the epzdermis, or skin; 
and between these two layers is a thicker one of 
cells, through which bundles of woody tissue run, 
forming the veins of the leaf. On the under 
surface are a number of minute mouths, which 
form the guardian-entrances to little air-cells 
within the leaf, and are called stomates. ‘These 
little mouths open and close, according to the 
varying necessities of the leaf, sometimes being 
open to allow moisture to escape, at others, open- 
ing to admit the moisture of the dewy night, and 
being closed afterwards. 
Thus it is manifest, that there is the most 
complete provision in this organ—the leaf—for 
exposing to the influence of the air and light, the 
