CHAPTER ELEVEN 



PHYSICAL PROCESSES INVOLVED IN THE MOVEMENT OF 

 MATERIALS IN PLANTS 



Since the earliest times students of plants have been trying to 

 find out just how gases, water, and minerals get into plant cells 

 and how they pass from one cell to another ; also how the soluble 

 foods move from one organ to another. Not all of these ques- 

 tions have been satisfactorily answered, but there are certain 

 physical processes that at least help to explain all of them. We 

 must not be misled into thinking that the plant does all these 

 things because, being ahve, it can take in substances, move them 

 where they are needed, and throw off those that are not needed. 

 All investigations indicate that these processes take place, not 

 because plants exert some peculiar vital force, but because plant 

 cells possess those particular physical and chemical properties 

 and structural arrangements which, even in a non-living piece of 

 physical apparatus, induce these same processes. Although we 

 cannot now imitate perfectly all the processes concerned in the 

 movement of materials within the plant, it is fair to predict that 

 we shall be able to do so in the not distant future. 



Solution. All substances, whether gaseous, liquid, or sohd, 

 must be dissolved in water before they can pass into, or out of, a 

 plant cell. By solution is meant the dividing of a substance into 

 invisible particles that distribute themselves throughout a liquid. 

 Carbon dioxide, for example, occurs in the air as a gas. Its 

 particles strike the water surface of the cell and go into solution. 

 Mineral substances coming in contact with the water in the soil 

 do the same thing, and it is only after they are dissolved that they 

 enter the root. Sugar, likewise, can move out of or into a cell 

 only when it is dissolved in the water of the cell. Solution is the 

 first of four physical processes which are important in the move- 

 ment of substances in plants. 



Diffusion. If a small dish of ether is exposed in a room, in a 

 few minutes the odor of the ether may be noticed in all parts of 



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