128 THE LIFE OF THE PLANT 



mately equal. We learn from chemistry that when 

 carbon burns in oxygen and carbonic acid is formed a 

 given volume of oxygen forms an equal volume of car- 

 bonic acid. In our experiment, therefore, the carbonic 

 acid is entirely decomposed ; all its oxygen is given off, 

 while all its carbon remains in the plant. 



We can perform this experiment still more satisfac- 

 torily by using some large floating leaves of the water- 

 lily. Water plants have the peculiarity that their 

 stomata are scattered over the upper surface of the 

 lamina which is in contact with the air ; its air cavities 

 are continuous with similar cavities in the petiole. 

 We sink the lamina in water under a piece of glass, 

 which is shown in the figure (fig. 37) behind the 

 vessel, and introduce the stalk into the tube filled 

 with water. Let us commence by using boiled water, 

 i.e. water without any trace of carbonic acid. We 

 do not see anything taking place. We add some 

 mineral water containing carbonic acid, and cover the 

 vessel with a piece of cardboard ; and again we do not 

 observe anything. But the moment we remove the 

 cardboard and expose the apparatus to sunlight, a 

 current of bubbles rushes from the cut end of the petiole. 

 We collect the gas and measure its volume : from what 

 we already know we decide that this gas, which is given 

 off exclusively in presence of carbonic acid and in 

 sunlight, contains oxygen. Let us gently draw the 

 petiole out of the tube, close the latter with a finger, 

 reverse it, and introduce into it a glowing splinter. It 

 burns with a bright flame. We remove the splinter, 

 extinguish the flame, and repeat the experiment 

 ten times over with the same result, and conclude 

 that this is actually a gas rich in oxygen. A leaf 

 exposed to sunlight transforms carbonic acid into 

 oxygen. 



We have so far been experimenting in the presence 

 of sunlight, but this phenomenon of the breaking up of 



