122 



THE LIFE OF THE PLANT 



white ash drops to the bottom of the jar : this is 

 magnesia, a compound of the metal magnesium and 

 oxygen. I repeat the experiment, sinking the wire this 

 time into a jar of carbonic acid ; it must now procure 

 oxygen for itself by separating it from the carbon, which 

 will then be set free. As a matter of fact the wire does 

 not burn so quietly this time, but crackles as if so many 

 weak explosions were taking place, and black soot is 

 deposited on the walls of the glass vessel. This is the 

 free carbon. 



Great stores of carbon are therefore always present 

 in the atmosphere though in an invisible form. 



All water in contact with the air contains carbonic 

 acid, and plants which grow submerged in water are 

 thus able to obtain it. It is by experimenting with the 

 leaves of submerged plants that the interchange of gases 

 between a leaf and its environment is most readily 

 demonstrated. Here are several experiments which 

 can easily be made on any clear sunny morning. 



Let us gather some leaves and place them while 

 fresh under a glass bell, filled up to the top with water 

 and then inverted in another glass 

 vessel (fig. 34). We fill them in a 

 pail of water in which it is easy 

 to sink both bell and vessel. If 

 we use common water, or still better 

 water through which carbonic acid 

 has been passed, and then set the 

 apparatus in the light, we shall 

 ^^^^-^j/^^s^-^y^ soon notice the lower surface of the 



^^^:::: = := :^^ leaves becoming covered with a 



silvery layer of bubbles. A little 

 later a considerable amount of gas 

 will be accumulated in the upper part of the bell, while 

 the surplus water will be displaced into the outer 

 vessel, as is shown in fig. 34. 



Let us repeat this experiment with boiled water, or 



Fig. 34. 



