THE LEAF 



215 



That leaves absorb carbonic acid gas, we shall learn from a 

 simple experiment. Take a bunch of fresh green leaves of a 

 water-plant^ say Utricularia, 

 and place it under a funnel in 

 a vessel tilled with fresh spring 

 water. Over the mouth of the 

 funnel place a test-tube filled 

 with water. Then expose the 

 apparatus to strong sunlight. 

 After a very short time you will 

 see bubbles arise from the leaves 

 which are collected at the top 

 of the test-tube, as is shown in 

 the figure. When all the water 

 in the test-tube has been dis- 

 placed, we shall close it with the 

 thumb, take it out and introduce 

 a glowing chip, which will at 

 once burn very actively: the tube 

 contained oxygen gas. This gas ^^ 

 was evolved by the leaves under§ 

 the water. They absorbed the ^S 

 carbonic acid gas, dissolved in 

 fresh spring Avater, and retained 

 only the carbon of it, setting 

 free the oxygen. That this ex- 

 planation is correct, i. e., that the oxygen set free is derived from 

 the decomposition of the carbonic acid gas and not from the 

 water, may be proved by continuing our experiment. The evolu- 

 tion of oxygen gas will become less and less until it ceases al- 

 together. This can only be due to the exhaustion of carbonic 

 acid in the water. 



We know now that plants possess the iwiver of afjsorbing carbonic 

 acid gas through their leaves and of deriving from this gas the 

 carbon which they require for the construction of their body. 



(b) Only the Green Parts of Plants absorb Carbonic Acid. — It 

 must, however, be borne in mind that not all plants have the 



Fig. 197. — Absorption of carbonic acid 



gas and setting free of oxygen by 



a water-plant. 



