STARCH formation:.— THE GASES. 



63 



Fig. 67. 

 The splinter lights again in the presence of 

 ixygen gas. 



quickly thrusting the glowing end of the sphnter in with tlic 

 other hand. If properly 

 handled, the sphnter will 

 flame again. If it is neces- 

 sary to keep the appa- 

 ratus standing for more 

 than one day it is well 

 to add fresh water in the 

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

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

 leayes are placed in water are not eyidence that this process is 

 taking place. 



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

 our c.xprriments to land plants we should find that oxygen is given off by 

 them under these conditions of light. I^and plants, ho\\'ever, \\-\\\ 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 stiffi- 

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

 sunlight, if temperature and other conditions are fa\-orabIe, gi\"e off ox\'gen. 



130. Absorption of carbon dioxide. — \\'e haye next to inquire 

 where the oxygen comes from ^^'hich is giyen off by green plants 

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

 of the conditions necessary for the process. \"\'e know that 

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

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

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

 oxygen and carbon dioxide. 



If w& boil spring water or hydrant ^vater which comes from 

 a stream containing oxygen and carbon dio.xide, for about 20 

 minutes, these gases are driven oft". 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 spirog}Ta 

 or yaucheria, 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 

 eyolution of oxygen. No o.xygen is given off. 



