THE WORK OF STEMS 



283 



mud. In the case of plants which live entirely sub- 

 merged it would seem that they must get their air 

 from the water. Does water contain air? We have 

 already gained some evidence on this point in our 

 previous experiments. Place some tap- or spring - 

 water in the air-pump, and exhaust. Are air -bubbles 

 formed! Boil some of the water for half an hour, 

 allow it to cool, place it in the air-pump, and exhaust. 

 Are bubbles formed to the same extent as before ? 

 Careful tests have shown that water may dissolve a 

 large amount of air. It is this dissolved air in the 

 water on which totally submerged 

 plants depend for tlieir supply. 



In order to grow water-plants 

 successfully in aquaria, it is fre- 

 quently necessary to furnish them 

 with a constant supply of air. This 

 may be done by means of the ap- 

 paratus shown in Fig. 158. It con- 

 sists of a long, vertical glass tube 

 about one-eighth inch in diameter, 

 widened a little at the top. When 

 a drop of water falls into this 

 from the siphon it will, if it 

 strikes in the center, fill the 



158. Apparatus for supplying air in a 

 tube and then fall dOWn, Carry- constant stream to an aquarium. 



ing the air before it (i. e., it acts like a piston). On 

 arriving at the T-tube connection, the air passes over 



