48 PLANT PHYSIOLOGY 



and enclosed within the permeable membranes which bound 

 the eells of plants and the atmospheric air outside will 

 cause diffusion. This will tend to make the composition the 

 same within and without the plant. But since every living 

 active cell is constantly respiring, taking in oxygen and 

 giving out carbon-dioxide, a difference in composition be- 

 tween the mixture of gases contained in the plant-body and 

 the atmosphere is constantly brought about. As constant 

 will be the diffusion currents, one inward consisting of oxy- 

 gen molecules, one outward consisting of carbon-dioxide 

 molecules. These currents, tending to restore the uniformity'- 

 in composition, but never accomplishing this so long as the 

 plant respires, prevent the undue accumulation of carbon- 

 dioxide and maintain an adequate supply of oxygen. 



Only in green plants, and in the green cells of these, is 

 there any even apparent exception to this simple physical 

 law. If, however, any cells which liberate carbon-dioxide in 

 respiration also absorb and combine it again in food-manu- 

 facture, the diffusion currents to and from these cells will 

 be made up of different molecules from those composing the 

 diffusion currents maintained by other cells. In the green 

 cells the process and the products of food-manufacture are 

 the reverse of respiration, and the diffusion currents necesr 

 sarily correspond. The green cells respire as constantly and 

 at least as actively as other cells containing no chlorophyll ; 

 but while they are manufacturing food, they absorb and use 

 more than all the carbon-dioxide which they exhale. Hence, 

 in experimenting upon the respiration of green plants or of 

 parts containing chlorophyll, it is necessary to stop the 

 opposite and constructive process. 



The absorption of carbon-dioxide gas is accomplished by 

 means of the diffusion currents of this gas set and kept in 

 motion by the combination of carbon-dioxide with water 

 in the production of food. The absorption takes place 

 through the walls of all the chlorophyll-containing cells of 

 the lower plants. These are all small and for the most 

 part aquatics. In Fucus and other large algae periodically 

 exposed to the air, there is a difference in the absorption to 

 correspond with the division of labor among the tissues. 



