24 THE WORK OF THE LEAF [cHAP. 



feet long and fill it loosely with freshly gathered leaves 

 of grass or wheat, taking care only to use young, active 

 leaves. Now set up the tube horizontally in the 

 brightest daylight available, and by means of an 

 aspirator slowly draw air, first through the tube, and 

 then through a wash-bottle containing some baryta 

 water. Baryta water is a very delicate test for carbon 

 dioxide, becoming troubled and milky looking with 

 a very small quantity of the gas, but if the light is 

 good there will be no evidence of carbon dioxide in 

 the air that has been drawn through the tube. Then 

 detach the tube and let the ordinary air run through 

 the baryta water ; there is at once a milkiness, showing 

 what must have been the state of the air before it came 

 in contact with the green leaves. This experiment 

 illustrates the fact that green leaves in bright light take 

 the carbon dioxide out of the air, and, as we have seen 

 in the previous experiment, liberate the oxygen con- 

 tained therein. 



By using the same apparatus with petals, bracts, and 

 other parts of a plant which are not green, it can be 

 shown that only the green portions of the plant have 

 this power of absorbing and splitting up carbon dioxide ; 

 in fact, the green colouring matter — the chlorophyll, as 

 it is called — in leaves and stems effects the first 

 necessary step in the process, by absorbing the light 

 which constitutes the motive power. That light is 

 absolutely essential may easily be seen by repeating 

 the last experiment after covering the tube containing 

 the leaves with brown paper. As soon as this has been 

 done the leaves will be found to add carbon dioxide to 

 the air passing through, instead of taking it out ; as we 

 shall see later, all parts of the plant, like germinating 

 seeds, are breathing as long as they are alive, whereby 

 they give off carbon dioxide and absorb oxygen, though 



