200 TEXTBOOK OF BOTANY 



When a soluble dye is placed in the bottom of a slender glass tube 

 filled with water, the greater concentration of the dye in the bottom of 

 the tube at the beginning of the demonstration is evident by its deeper 

 color there. Gradually, however, the color of the dye becomes evident 

 farther and farther up the tube, and after many months the color is 

 uniform throughout the tube. The dye is now equally concentrated 

 throughout the entire column of water. That is, the number of molecules 

 of dye in comparison to the number of molecules of water would be 

 the same in a cubic centimeter of the solution taken from any part of 

 the tube. 



Evidently for several months before the concentration of the dye 

 became uniform a greater number of molecules of dye moved upward 

 than downward in the tube. Since more molecules of dye are moving in 

 any direction in the region where its concentration is greatest, the dye 

 will diffuse from regions where it is more concentrated toward regions 

 where it is less concentrated. When the concentration of the dye becomes 

 uniform, as many of its molecules will be moving in any direction in 

 any part of the tube as in any other. 



When the above facts are fully appreciated it will be easy to see why 

 carbon dioxide and oxygen diffuse into the green parts of a plant at one 

 time and out of them at another time. Each gas diffuses independently 

 of the other, and solely in relation to its own concentration inside and 

 outside the plant. The same principle holds for the diffusion of water, 

 of a salt, or of any other substance dissolved in the water. For instance, 

 if carbon dioxide is used in photosynthesis in a particular cell, its con- 

 centration within that cell becomes lower than it is outside the cell. 

 Then, according to the laws of diffusion, a greater number of molecules 

 of carbon dioxide will move into that cell than out of it, until its effective 

 concentration is again equalized. 



If we gauge our language h\ our knowledge, we will no longer sa\' 

 that the plant "takes in" and "gives off" carbon dioxide; We will use 

 expressions that imply that the carbon dioxide passes into and out of 

 the plant. To be specific, we may say that it diffuses into and out of 

 the plant. The movement of other materials should be similarly analyzed. 



As our knowledge increases, it becomes more and more evident that a 

 plant is a complex system of materials and processes surrounded by 

 another system of materials and processes that we call the environment; 

 and that the same natural laws are operating in both systems. Processes 

 follow one another as a consequence of their dependent relations in these 



