78 EXPERIMENTAL PLANT PHYSIOLOGY 



A control experiment is set up in which the peas are 

 replaced by cotton-wool or sawdust. 



Observations. — On reading the thermometer it is 

 found that the temperature of the germinating peas is 

 higher than that of the control experiment. 



Inference. — The breathing of plants causes the tem- 

 perature of the surrounding air to rise. 



Health Value of Plants Indoors. — It has now been 

 proved that plants, like ourselves, continually take in 

 oxygen and give out carbon-dioxide. This may at first 

 sight seem to contradict what was said on page 68 as to 

 the health value of plants in the house. 



It is evident that the breathing of the plant helps to 

 make the air "stuffy," just as our own breathing does. 

 But during the day the green plant takes in carbon- 

 dioxide as food and gives out oxygen and thus helps to 

 purify the air; the amount of carbon-dioxide absorbed 

 as food far exceeds that expired in the breathing process. 

 Thus a purifying of the air is effected by the presence of 

 the plant. But this advantage lasts only during the 

 day. As the evening approaches the feeding process 

 slackens and finally stops, but the breathing never 

 ceases, and so, during the night, plants, like ourselves, 

 only vitiate the air, and should therefore be removed 

 from the rooms in which we sleep. 



The Significance of the Breathing Process. — The true 

 meaning that underlies the need for continuous breath- 

 ing in all living things can now be explained. 



Something of the process has been learnt. It has 

 been found that all living things, plants as well as 

 animals, breathe constantly as long as they continue to 

 live (Experiment 47) ; and, further, that death follows 

 when breathing ceases (Experiment 48) . So far as the 

 interchange of gases is concerned, breathing consists in 

 the taking in of oxygen and the giving out of carbon- 

 dioxide; but this is no explanation of the fact that 

 breathing is essential to life. 



As long as a plant or an animal lives, it is constantly 



