II.] ox YGEN E VOL VED B Y PLANTS 23 



After a few hours of bright daylight the test-tube will be 

 nearly full ; remove it carefully, invert it, and bring into 

 the mouth a chip with a glowing end — the chip bursts 

 into a flame, proving the gas to be oxygen. Now 

 oxygen is one constituent of carbon dioxide, and the 

 experiment is an illustration of the fact that green 

 leaves in the light split up the carbon dioxide with 

 which they are in contact and set free the oxygen as 

 gas, retaining the carbon for themselves. This 

 continual evolution of oxygen by green plants is the 

 great agency which is always renewing the vital part of 

 the atmosphere ; out in the country where the trees and 

 the grass are growing, the amount of carbon dioxide in 

 the air keeps at about 3 volumes in 10,000, being rather 

 less in the summer, when vegetation is active, than in 

 the winter ; in towns the proportion rises to 4, and even 

 in dense streets to 6 and 7 per 10,000. On plants 

 growing in water it is possible to see the bubbles of 

 oxygen as they are produced ; if you look into any 

 ditch or pool on a bright day you will see the bubbles of 

 oxygen entangled in the vegetation, just as you will see 

 them dotted all over the green algal growth which 

 covers the bottom of many streams. The surface of 

 some streams is often covered with little masses of 

 floating scum ; this scum consists of the algal growth 

 that has been lifted off the bottom and buoyed up to 

 the surface by the bubbles of oxygen entangled in it 

 It will be noticed that the scum only begins to float up 

 towards the afternoon and on a sunny day, i.e. after the 

 light has had time to act and bring about a plentiful 

 evolution of oxygen. 



So far, however, we have only demonstrated that the 

 plant adds oxygen to the atmosphere ; we can now show 

 that it will remove carbon dioxide. On a bright day in 

 spring or early summer take a wide glass tube 5 or 6 



