84 The Living Plant 



very much higher), a preponderance that is obviously so great 

 as to over-balance not only the respiration of the leaves, but of all 

 the remainder of the plant besides, and not for daytime alone, 

 but also for night. Therefore, day and night together, the green 

 plant absorbs much more carbon dioxide than it releases and re- 

 leases much more oxygen than it absorbs. It vitiates the air by 

 its respiration, but in the long run purifies it still more by its 

 photosynthesis. 



Before leaving this part of our subject, we should look a little 

 more closely into the relations of the two processes within the 



FIG. 29. Diagrammatic sections across leaves, to illustrate the movements of gases in 

 and out of the same during, a, light, c, darkness, and b, the balance period between. 

 The squares are carbon dioxide, the triangles are oxygen, and the arrows show the 

 direction of movement. 



lighted green leaf, a subject diagrammatically illustrated by the 

 accompanying figures (figure 29). At night all of the carbon 

 dioxide given off by the respiration of the living cells into the air 

 passages, makes its way along these and through the stomata 

 to the atmosphere outside, (figure 29, c). In the daytime any 

 carbon dioxide given off by the respiration of the protoplasm is 

 absorbed by the chlorophyll grains in the same cells, but as this 

 supply is wholly insufficient, a constant stream of that gas passes 

 in from the atmosphere through the stomata and along the pas- 

 sages to the different cells, where it is absorbed by the chlorophyll 

 grains; simultaneously a part of the oxygen given off by the 

 chlorophyll grains is absorbed by the protoplasm of the same cells 

 for their respiration, while the very large surplus is sent into the 



