228 



LEAVES 



oxygen and greater need for the carrying off of wastes. 

 All this we know, but just what happens when molecules 

 of food release their energy and break down into simpler 



molecules — this 

 still remains a mys- 

 tery. 



Respiration is 

 something like what 

 happens when burn- 

 ing takes place, and 

 yet it is not burn- 

 ing. It takes place 

 at low temperatures 

 and water is the 

 medium in which 

 it occurs. This is 

 not true of burning. 

 As in burning, how- 

 ever, oxygen is used 

 in this process (ex- 

 cept by some bac- 

 teria) and carbon dioxide is given off. This carbon 

 dioxide passes off chiefly through the stomates, but some 

 of it, as you have noted, is given off in solution from the 

 root-hairs. (Carbon dioxide in solution in water becomes 

 carbonic acid, which is formed by the union of a molecule 

 of water with a molecule of carbon dioxide.) 



Fig. 8i. — Cross section of the stem of hornwort 

 {Ceratophyllum), a plant which grows under water. 

 Note the abundant small air passages. Compare 

 with Figure 80. In this plant the vascular bundles 

 are small, but they are not in the center. 



D. Photosynthesis. — The manufacture of food from 

 inorganic materials is the principal function of leaves. 

 There is little question of sharing this function with other 

 parts of the plant, and no question at all as to whether the 



