[Chap. VII CELLS AS BIOLOGICAL UNITS 67 



stitutes a medium in which the movement of materials and the 

 chemical and physical changes within the cell readily occur. 

 9. As a result of the chemical changes constantly going on in the 

 cell, electrical energy is released as in a battery, and one end 

 of the cell usually has a higher electric potential than the other. 

 Consequently most cells are polarized, and their opposite ends may 

 behave differently. 

 10. In cell masses of the larger plants electric potentials also develop, 

 and the apex of a plant organ may become electrically negative or 

 positive to the base. 



With these principles in mind, one has a basis necessary for interpret- 

 ing the various extraordinarv reactions of living plants. Today we know 

 only a minor part of the energy and material relations within plant cells, 

 but our present knowledge is certainly sufficient to lead us to doubt the 

 necessity of assuming the presence of mysterious forces to account for 

 the behavior of organisms. 



