PRODUCTIVE GARDENING 83 



though slow, interchange between the air in the 

 intercellular spaces and the structure of the pro- 

 toplasm 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 



