LUTHER BURBANK 



cellular spaces and the structure of the protoplasm 

 within the cells. 



This interchange includes the absorption of 

 oxygen and the giving out of carbonic acid on the 

 part of the plant cell, which is precisely the same 

 thing that occurs in the functioning of the cells 

 in the tissues of an animal. In point of fact the 

 essential properties of protoplasm are the same, 

 whether that protoplasm is found in the tissues 

 of a plant or in the tissues of a man. 



Plants, like animals, in breathing take in oxy- 

 gen and exhale carbonic acid gas. 



PLANT CELLS AND ANIMAL CELLS 



This fact, as was said, has not been clearly 

 understood until somewhat recently. 



The phenomenon of the absorption of oxygen 

 and the exhalation of carbonic acid has been ob- 

 scured in the case of the plant by the further fact 

 that the plant leaf absorbs constantly from the air 

 during the daytime, under the influence of light, 

 a relatively large quantity of carbonic acid gas 

 from the minute quantity in the air, so that the 

 net result is that it takes up from the air more 

 carbonic acid than it exhales. 



It was only by studying the plant in the dark, 

 when the elaborate processes through which it 

 utilizes the excess of carbonic acid are in abey- 

 ance, that the fact of the close analogy between 



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