HOW THE PLANT LIVES 117 



2c. Oxygen 



193. Oxygen is essential to all of the life pro- 

 cesses in the plant, as well as to the animal. For 

 perfect germination oxygen is required, and this 

 gas diffuses into and is used by all living or grow- 

 ing plant organs. The stomata of leaves and 

 shoots are mechanisms insuring an adequate 

 supply for these parts. Entering these stomata, 

 it is readily diffused throughout the neighboring 

 cells and tissues. 



194. Oxygen is then constantly "absorbed," 

 and associated with this absorption is the giving 

 off of carbon dioxid. This appropriation of 

 oxygen and escape of carbon dioxid are results 

 of respiration, a process equivalent in its pur- 

 pose and results to respiration in animals. Young 

 growing plants absorb an amount of oxygen about 

 equal to their volume, in from twenty -four to 

 thirty- six hours. Germinating seeds absorb oxy- 

 gen, and give off ordinarily about an equal quan- 

 tity of carbon dioxid. 



195. Germinating seeds, opening flower buds, 

 parts of plants that have been injured, and cer- 

 tain organs in which decay is imminent, respire 

 more rapidly than other parts. Respiration prac- 

 tically represents molecular change and the re- 

 lease of energy in the living substance. 



196. Oxygen is also taken in through the 

 roots. Land plants, whose roots are deprived of 



