134 INTAKE AND DISTRIBUTION OF GASES 



the cell-wall. Likewise gases that are to be given off from the 

 cells must in solution diosmose through the cell-wall and break 

 away from the superficial film of water into the intercellular 

 space or outer air. The dry walls of dead cells are much less 

 permeable to gases than those of the living ones soaked with 

 water. 



Motive Power in the Distribution of Gases throughout 

 Plants. — Gases pass by diffusion in and out through the sto- 

 mata and lenticels, and throughout the intercellular spaces. 

 By this process the atmosphere does not move as a whole but 

 each component gas moves independently in obedience to the 

 laws of diffusion according to 'which the movement is from the 

 place of greater concentration of a particular gas to that of 

 less concentration. So that if carbon dioxide is being used in 

 photosynthesis by any tissue, or oxygen is being employed in 

 respiration, more of these gases will flow to that tissue by diffu- 

 sion. Or if respiration is pouring carbon dioxide into the inter- 

 cellular spaces, or photosynthesis is loading them with oxygen, 

 these gases will flow toward the exterior and pour out through 

 stomata and lenticels. Interchange of gases in this way is con- 

 stant so long as their concentration inside and outside the plant 

 body is different. In the daytime there would be a flow of oxygen 

 from the photosynthetic tissues toward the other parts that 

 are breathing, as well as toward the exterior, and a flow of 

 carbon dioxide toward the photosynthetic tissues from those 

 that are giving it off in respiration, as well as from the exterior. 

 In parts of plants destitute of chloroplasts, and hence incapable 

 of photosynthesis, there would be no evolution of oxygen, but 

 on the contrary there would be a continual production of carbon 

 dioxide by respiration, and this gas would flow thence toward 

 the photosynthetic tissues and toward the exterior. When 

 darkness falls no part can longer photosynthesize while all parts 

 continue to respire, and therefore a flow of oxygen more exclu- 

 sively from the exterior to all parts sets in, and the return flow 

 of carbon dioxide becomes more exclusively toward the exterior, 

 since the photosynthetic tissues can no longer make use of it. 



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