STARCH FORMATION THE GASES. 63 



quickly thrusting the glowing end of the splinter in with the 

 other hand. If properly 

 handled, the splinter will 

 flame again. If it is neces- 

 sary to keep the appa- 

 ratus standing for more p . g ^ 



than one day it is Well The splinter lights again in the presence of 



oxygen gas. 



to add fresh water in the 



place of most of the water in the jar. Do not use leaves of land 

 plants in this experiment, since the bubbles which rise when these 

 leaves are placed in water are not evidence that this process is 

 taking place. 



129. Oxygen given off by green land plants also. If we should extend 

 our experiments to land plants we should find that oxygen is given off 'by 

 them under these conditions of light. Land plants, however, will not do 

 this when they are immersed in water, but it is necessary to set up rather 

 complicated apparatus and to make analyses of the gases at the beginning 

 and at the close of the experiments. This has been done, however, in a suffi- 

 ciently large number of cases so that we know that all green plants in the 

 sunlight, if temperature and other conditions are favorable, give off oxygen. 



130. Absorption of carbon dioxide. We have next to inquire 

 where the oxygen comes from which is given off by green plants 

 when exposed to the sunlight, and also to learn something more 

 of the conditions necessary for the process. We know that 

 water which has been for some time exposed to the air and soil, 

 and has been agitated, like running water of streams, or the 

 water of springs, has mixed with it a considerable quantity of 

 oxygen and carbon dioxide. , 



If we boil spring water or hydrant water which comes from 

 a stream containing oxygen and carbon dioxide, for about 20 

 minutes, these gases are driven off. We should set this aside 

 where it will not be agitated, until it has cooled sufficiently to 

 receive plants without injury. Let us now place some spirogyra 

 or vaucheria, and elodea, or other green water plant, in this 

 boiled water and set the vessel in the bright sunlight under the 

 same conditions which were employed in the experiments for the 

 evolution of oxygen. No oxygen is given off. 



