RESPIRATION 



295 



to the weight of the plants, it has been computed that for ordinary 

 house plants about 300 sq. m. of leaf surface are required to equal 

 the amount of carbon dioxide given off by a person in the same 

 time. This would lead to the conclusion that plants in a sleeping 

 room are not harmful and that one can continue to sleep in forests 

 without danger of asphyxiation. 



Respiration and Photosynthesis. — Respiration is most easily 

 detected in opening flower buds and germinating seeds, not only 

 because the plant is very active at that time, but also because 

 where chlorophyll-bearing tissues are concerned, it is always 

 difficult to separate fully respiration and photosynthesis and to 

 measure the products given off in the one operation which are 

 immediately consumed in the other. Photosynthesis uses carbon 

 dioxide and water giving off oxygen, while in respiration the re- 

 verse process takes place. In daylight the photosynthetic activity 

 is so much more vigorous than the respiratory that the latter is 

 completely masked. All the carbon dioxide given off by respira- 

 tion is used up in photosynthesis and still more is brought in from 

 the outside, with the result that the plant increases in dry weight. 

 It is for this reason that one sometimes finds in elementary books 

 and popular papers the statement that animals take in oxygen 

 and give off carbon dioxide in their respiratory processes while 

 plants do just the reverse, taking in carbon dioxide and giving 

 off oxygen. This is, of course, erroneous. Plants respire as well 

 as animals, but in addition they possess the power of photosyn- 

 thesis which animals lack, so that under the conditions which 

 favor photosynthesis the respiration is hard to detect. Both 

 plants and animals give carbon dioxide to the air by respiration, 

 but only plants have the power to return the carbon dioxide from 

 the inorganic to the organic world. 



The comparison between photosynthesis and respiration may 

 be more easily seen if the pertinent contrasting facts are tabulated: 



Photosynthesis 



1. Only in green plants. 



2. Only in the cells with chlorophyll. 



3. Only in light. 



4. Constructs food. 



5. Increases dry weight. 



6. Releases oxygen. 



7. Requires water and carbon dioxide. 



8. Changes kinetic energy of light to 



potential energy of food. 



Respiration 



In all plants and animals. 

 In all living cells. 

 In both light and darkness. 

 Destroys food. 

 Decreases dry weight. 

 Absorbs oxygen. 



Releases water and carbon dioxide. 

 Changes potential energy of food to 

 kinetic energy. 



