32 GENERAL BOTANY 



free, and this oxygen passes out into the atmo- 

 sphere again through the stomas ; or, in the case of 

 water plants that live always under water and have 

 no stomas, through the whole surface. 



This can be shown very easily as follows. Get 

 a clean glass jar nearly nil it with clean water, 

 and add a little ordinary soda-water. 



Then put in some healthy water-weed such as you 

 will find at the bottom of any clean tank or channel 

 that is constantly full of water. 



Stand the jar in the sun ; very soon bubbles of 

 gas will form on the leaves and rise up to the 

 surface. This gas can be easily collected in a test- 

 tube full of water inverted over the weed. Or better 

 still by a glass funnel Avith a short piece of rub- 

 ber tube fitting over the tube and closed by a clip. 

 This is let down into the water, with the clip 

 open, till quite submerged and the tube is full of 

 water, then the clip is closed. If the bubbles stop 

 rising add a little more soda-water (which contains 

 carbon dioxide) and they will at once appear again. 



When sufficient gas has been collected (it may 

 take an hour or so), take the test-tube out, or open 

 the clip on the funnel, and introduce into the gas 

 a smouldering piece of wood. It will at once burst 

 into flame, showing that the gas is oxygen. 



The fact that soda-water contains carbon dioxide, and 

 that instead of that gas we get bubbles of oxygen 

 shows very clearly what the action of the plant is. 



Now cover the jar with something that will keep 

 out all the light. The bubbles will at once cease 

 and hardly any gas be collected in the tube. 



