FUNCTIONS OF LEAVES tts 
their life. For, like animals, plants get the energy with 
which they do the work of assimilation, growth, reproduc- 
tion, and performing their movements from the oxidation 
of such combustible substances as oil, starch, and sugar.! 
The amount of oxy- 
gen absorbed and of car- 
bonic acid given off is, 
however, so trifling com- 
pared with the amount 
of each gas passing in 
the opposite direction, 
while starch-making is 
going on in sunlight, 
that under such circum- 
stances it is difficult to 
observe the occurrence 
of respiration. In ordi- 
nary leafy plants the Fie. 124. — Cross-Section of Stem of Marestail 
leaves ( through their (Hippuris) with Air-Passages, a. 
stomata) are the principal organs for absorption of air, but 
much air passes into the plant through the lenticels of 
the bark. 
In partly submerged aquatics especial provisions are 
found for carrying the air absorbed by the leaves down to 
the submerged parts. This is accomplished in pond lilies 
by ventilating tubes which traverse the leaf-stalks length- 
wise. In many cases such channels run up and down the 
stem (Fig. 124). 
1 The necessity of an air supply about the roots of the plant may be shown 
by filling the pot or jar in which the hydrangea was grown for the transpi- 
ration experiment perfectly full of water and noting the subsequent appear- 
ance of the plant at periods twelve to twenty-four hours apart. 
