84 MANUAL OF NATURE STUDY. 



united in the way mentioned above, are together 

 called carbon dioxide and when dissolved in water it 

 is called carbonic acid. It is carbonic acid that es- 

 capes from our lungs. We have no more use for this 

 acid gas, and hence thro w it out with our breath. The 

 plants are glad to take it up. This they do as just 

 stated, by breathing it through the stomata of their 

 leaves. The plant-substance factory is not in 

 operation at night, so carbonic acid gas is allowed 

 to escape to the atmosphere until morning. 

 Throughout the day, if the factory is running at 

 full speed, carbon dioxide is separated as rapidly as 

 it arrives, the oxygen returning to the air for more 

 carbon, securing which, it makes another visit to 

 the factory ; and thus the process goes on through- 

 out the entire plant growth. 



The vapor that we collected on the inner surface 

 of the tumbler in our experiment with the 

 green leaves a while ago was given off by the 

 leaves through the stomata. This process of 

 giving ofE vapor is called transpiration. Again 

 experiment with the plant a few days by putting 

 some in a dark place, others in a room of medium 

 light, and others in a strong light for the same 

 length of time. Note the difiference in color and 

 in growth. Account for the difference. Take 

 another body of plants and separate it into groups 



