THE LIFE OF A PLANT 273 



seeds will grow from the old root. The food of man con- 

 sists largely of grains, which are rich in starch, stored for 

 the nourishment of the young plant that would grow from 

 the seed. Man makes use of materials which the plant 

 would use if it continued to live and reproduce. (LABORA- 

 TORY MANUAL, Exercise XXVIII.) 



308. Respiration. The process of respiration for plants 

 is exactly the same as for animals. It consists of inhaling, or 

 taking oxygen into the interior of the organism for the pur- 

 pose of oxidation; and of exhaling, or expelling the gas re- 

 sulting from oxidation. The necessity of respiration for the 

 plant can be easily shown. Oxygen is necessary for the oxi- 

 dation of foods and tissues, and for the consequent release 

 of energy to be used in movement and growth. Since plants 

 have a lower temperature and less movement than many 

 animals, less oxygen is required to furnish energy, and less 

 carbon dioxide and water vapor are returned to the air. 

 When we recall that starch and its digested product, sugar, 

 contain much carbon and hydrogen, we can readily see why 

 carbon dioxide and water are the substances resulting from 

 the oxidation of these foods, and why therefore they are 

 exhaled. 



309. Excretion and Removal of Oxygen. The term ex- 

 cretion is commonly applied to the removal of waste products 

 of oxidation, such as carbon dioxide and water, which are 

 eliminated because they are of no further use in the cells of 

 the plant. 



In the making of protoplasm, proteid food is chiefly used. 

 If it is oxidized, as is sometimes the case, it forms products 

 not easily excreted, because they cannot be given off as gases, 

 as can water and carbon dioxide. So these waste products, 

 together with certain salts from the soil, which perhaps can- 

 not be .utilized, become stored in leaves, bark, and fruit. As 

 these parts are in time detached, the waste products from 

 proteid are removed from the plant. 



