32 LEAVES ABSORB CARBONIC ACID DURING DAYLIGHT. 



it can afterwards be found. How active must every 

 little mouth on the leaf be at such a time ! 



a. The effect of the carbonic acid thus absorbed, is 

 to hasten the growth of the plant by furnishing part 

 of the material from which its stalks, stems, leaves, 

 etc., are composed. But it may be asked, is the whole 

 of the carbonic acid used, or only a part? We re- 

 member that it is composed of two substances, oxygen 

 and carbon; are both of these, or only one, retained? 



b. It is not dithcult for the reader to satisfy himself 

 on this point. If the leaves of a flourishing growing 

 plant be immersed in an inverted vessel full of water, 

 and exposed to the rays of the sun, little bubbles of 

 air will gradually begin to form, and to increase in 

 size imtil they rise and collect in the upper part of the 

 vessel. If fresh branches be occasionally placed in 

 the water, and the operation thus continued for a time, 

 enough air will be collected for purposes of experi- 

 ment. It will then be found that this air, which has 

 thus escaped from the surface of the leaves, shows all 

 of the properties which were described under oxygen. 

 It is in fact pure oxygen, thus showing that the carbon 

 of the carbonic acid is retained in the plant to con- 

 stitute a portion of its bulk, while the oxygen goes off 

 through the pores of the leaf. The pores in the under 

 side of the leaf usually effect the absorption, the de- 

 composition goes on in the interior, and the oxygen is 

 given off through the pores on the upper part. These 

 pores have for their office to give off, while that of the 

 others is to receive. Some plants will live for a long 

 time if the under surface of the leaves is kept con- 

 stantly wet; if the upper only be wet, the plant soon 

 dies. If either surface be varnished, so as to stop the 

 pores, great injury results. 



During daylight the leaves are constantly absorbing 

 carbonic acid, and giving off oxygen; but as soon as 



