78 COMPOUND ORGANS OP PLANTS. 



together with the assimilation of the" carbon and the expira- 

 tion of the oxygen, constitutes what may be truly denominated 

 vegetable breathing or respiration. 



These results take place only when the plant is exposed to 

 the direct rays of the sun. Recent experiments have shown 

 that the process ceases when the sun is behind the clouds, and 

 that not only during the night, but even under the influence 

 of diifused daylight, the exhalation of oxygen is stopped. 



The exhalation of oxygen gas from the leaves of plants is 

 the only provision that we know of for keeping up its supply 

 in the atmosphere. The prevailing chemical tendencies are to 

 take oxygen from the air. Were it not for the copious sup- 

 plies of this gas poured into the atmosphere from the pores of 

 plants, animal life could not exist. Hence the perfect adap- 

 tation of the two kingdoms of nature, each removing from the 

 atmosphere what would be noxious to the other, each yielding 

 to the atmosphere what is essential to the life of the other. 



To show that plants give out oxygen in sunshine. Fill a 

 jar with water, and invert it in a vessel containing the same 

 fluid. Introduce beneath the jar a sprig of mint, or any other 

 living plant. After a while bubbles of gas will collect on the 

 leaves and ascend to the top of the jar, displacing the water. 

 If the air thus collected be tested, it will be found to be pure 

 oxygen gas. If the vessel be placed in the shade, the bubbles 

 of gas will disappear from the leaves. 



The leaves are not the only organs of vegetable respiration. 

 The young branches, the scales, in a word, all the herbaceous 

 and green parts of plants act on the atmosphere in a similar 

 manner to the leaves. They take in carbonic acid from the 

 atmosphere, assimilate the carbon, and give out the oxygen. 



Form of Leaves. — It has been stated that the leaf of a plant 



