THE LUNGS OF PLANTS. 65 
juices poured from the stem into its countless 
cells; and we shall find that both light and air 
exercise a wonderful effect upon it, and upon the 
whole system of the plant. If we were asked 
What is the leaf to a plant? it would be per- 
haps hard to give a better answer than—Both 
its stomach and its lung,—that is, both the organ 
for digesting food, and the organ for breathing 
air. Dr. Lindley writes: ‘ It is not improbable 
that the cells of a leaf which form the upper 
stratum (or layer), perform a function analogous 
to that of the stomach in animals, digesting the 
crude matter they receive from the stem; and that 
the lower stratum takes up the matter so altered, 
and submits it to the action of the atmosphere, 
which must enter the leaf purely by means 
of the stomates.” Thus the upper layer is the 
stomach, and the under layer the lung of the 
plant. When we consider the usual position 
of leaves, one surface always looking to the 
sky, and the other toward the ground, we can 
readily imagine that this is the case; and that 
light acts principally upon the upper surface, 
while air acts chiefly on the under. In the Aus- 
tralian woods there is a singular exception to 
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