[Chap. XXII THE MOVEMENT OF MATERIALS IN PLANTS 207 



of protoplasm to water and solutes. Among them are variations of tem- 

 perature and light, or the presence of chloroform, ether, alcohol, saponin, 

 and the ions of various salts. 



The influence of external factors upon the permeability of protoplasm 

 may be demonstrated easily by placing small pieces of the red garden 

 beet in water and exposing them to any one of a number of factors. 

 The anthocyanin is in solution in the vacuoles of the cells. If some factor 

 increases the permeability of protoplasm beyond a certain point, this 

 red pigment diffuses out of the cells. The influence of a factor that in- 

 creases the pemieability of protoplasm mav be counteracted by another 

 factor that decreases it. For instance, if sodium chloride is added, the 

 red pigment begins to diffuse out of the cells; but if a little calcium 

 chloride is also added the influence of sodium chloride is annulled. The 

 permeability of protoplasm at any instant in a living cell is apparently an 

 equilibrium dependent upon the interaction of many factors. 



Summary. The movement of materials into, out of, and within plants 

 occurs by xarious combinations of phvsical processes. All investigations 

 indicate that these processes occur, not because plants exert some pe- 

 culiar vital force, but because these processes are universal properties 

 of matter. Their interactions in simple physical systems may be accurately 

 measured, but the plant is such a complex system of structures and 

 processes that it is difficult to detect these interactions in all their details. 

 We begin by recognizing the forces involved and then try to relate 

 them to our present knowledge of plants. Among these forces we must 

 recognize the mutual attractions among molecules of gases, liquids, and 

 solids; surface tension; molecular motion and diffusion; and electrical 

 forces. Certainly we shall no longer think that substances move into and 

 out of plants because they are good or bad for the plant, or because the 

 plant needs or does not need them. Also we shall be less inclined to 

 think and say that the plant "takes in" and "gives off" these substances. 



The movement of materials into, out of, and within plants by diffusion 

 is much easier to perceive and is better understood than movement by 

 other means. Moreover, several of the common observable results of this 

 diffusion are not difficult to analyze. We have therefore given most atten- 

 tion to diffusion. A substance diffuses from a region where it is more 

 concentrated toward a region where it is less concentrated, regardless of 

 the concentration or the diffusion of any other substance with which it 

 may be mixed. This is the result merely of the relative number of 

 moving molecules. There are many more molecules of a substance mov- 



