24 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 3 



plants (really only 

 green plants in the 

 light) " purify the 

 atmosphere," that is, 

 remove from it the 

 noxious carbon diox- 

 ide released by ani- 

 mals in their respira- 

 tion (and by all com- 

 bustion), and replace 

 it by oxygen essential 

 to animal respiration. 

 Thus is a balance 

 maintained between 

 the two kingdoms. 

 The oxygen released 

 in photosynthesis 

 represents merely an 

 incidental by-product 

 of the process. 



The amount of 

 sugar made in a given 

 time per unit area 

 of leaf has been deter- 

 mined for a number 

 of plants, and shows, 

 as would be expected, 

 much diversity. The 

 average of these fig- 

 ures, however, ex- 

 0&U0, Co - pressed in the nearest 



Fig. 7. — A photosynthometer, by which the 



gas exchange in photosynthesis is quantitatively , . 



tested ; x j. 



Into the chamber containing the leaves a known quantity of carbon dioxide is ad- 

 mitted through the stop-cock from the graduated tube above. After exposure to light, 

 analysis of the gas in the chamber is made by absorption in the graduated tube by aid of 

 the two reagent tubes shown below on the left. The result can be read directly on the grad- 

 uated tube, as shown on the left, where the approximate 28% indicates the oxygen present 

 at the close of an experiment in which 10% of carbon dioxide had been added to the tube. 



