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 bjr ani- 

 mals in their respira- 

 tion (and by all com- 

 bustion), and replace 

 it by oxygen essential 

 to animal respiration. 

 Thus is a l^ilance 

 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 gi-\'en 

 time per unit area 

 of leaf has l.ieen deter- 

 mined for a numfier 

 of plants, and shows, 

 as would be expected, 

 much diversity. The 

 average of these fig- 

 ures, however, ex- 

 B & u CO — pressed in the nearest 



Fig. 7. — A jihoto.syuiliunK'ter, i)y which the 



gasexchaTip;e in photosyiitliesis iy qii;uiti1.ati\'ely . . 



tested ; X j- 



Into the chamber fontaininjz; the Icuvcfi ;l known quMuiily of carbon dioxide is ad- 

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

 analyisiy of tlie g;L4 in tlie chamber Is made by absorption in the graduated tube by aid of 

 the two reagent tubes shown below on the left. The result can [loread dircctlj'on thegrad- 

 .uated tube, -as shown on the lef-t, where theapproximato2S'/i, iruHcates the oxygen present 

 •^t the closetSf an experiment in which 10% of carbon dioxide had been added to the tube. 



