MINUTE STRUCTURE OF LEAVES 



173 



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. 1 



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 FIG* 124. Cross-Section of Stem of Marestail 

 leaves (through their (^) 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. 



