CHAPTER XXII 



PHYSICAL PROCESSES INVOLVED IN THE MOVEMENT OF 

 MATERIALS IN PLANTS 



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The movement of materials into and out of plants, and from place to 

 place within the plant, was often mentioned in previous chapters with- 

 out reference to the manner of movement or to the energy necessary 

 for it. 



The reason for this omission will soon be apparent. Energy is always 

 involved in the movement of material from one position to another. The 

 immediate source of this energy may be in the moving object, or it may 

 be in the environment of the object. For a clear concept of the move- 

 ment of materials into, out of, and within a plant it is necessary, there- 

 fore, that one distinguish whether the source of the energy which causes 

 the movement is within the moving material, within the plant, or in 

 some other external agency. It will be much easier to attempt this dis- 

 tinction now by viewing in retrospect some of the processes already 

 discussed and considering some additional ones. It is sometimes diffi- 

 cult, even impossible today, to make this distinction; but the major 

 source of energy in many plant processes is not difficult to recognize. 



Intrinsic and extrinsic sources of energy. Perhaps a crude analogy may 

 clarify this problem of recognizing different sources of energy. The 

 movement of a floating log in a river is dependent upon a source of 

 energy obviously outside the log. The movement of a man swimming in 

 the river results from a combination of the energy that moves the log 

 plus a source of energy within the man. By utilizing this second source 

 of energy the man may move upstream. The movement of a boat in the 

 river may be traced to the same source of energy that accounts for the 

 movement of the log, plus another external source of energy that may be 

 traced to the motor attached to it, or to the man rowing it. 



The movement of materials into, out of, and within plants, like that 

 of the objects in the river, depends upon several diverse sources of 

 energy. The plant may have no active part in the movement. We must 

 not be misled into thinking that the plant, being alive, can take in sub- 



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