CHAPTER V 



A COMPARISON OF CRITERIA AND METHODS USED 

 TO DETERMINE THE TISSUES OF TRANSPORT 



Different investigators have arrived at very divergent 

 conclusions concerning the tissues concerned in solute 

 translocation. This divergence and contradiction seem 

 to be due very largely to the criteria and the methods 

 depended upon to determine the channels of transport. For 

 example, from a comparison of the results obtained from 

 many experiments it is obvious that, when colored solutions 

 or solutions of almost any sort of substance are introduced 

 into cut stems or injected into various tissues, these 

 solutions are carried rapidly, extensively, and almost 

 exclusively in the xylem tissues and it is also obvious that 

 transpiration, directly or indirectly, determines both the 

 direction and the rate of movement of the solutions intro- 

 duced. On the other hand, if the channel for the movement 

 of naturally occurring solutes, those manufactured by 

 the leaves or absorbed by intact and uninjured roots, is 

 sought for, not by artificially introducing materials but 

 by cutting or blocking first one possible channel for trans- 

 port and then another, it is found that these naturally 

 occurring solutes seem unable to move in normal quantities 

 either apically or basally through the water-conducting 

 tissues of the xylem, whereas they seem capable of being 

 carried in approximately normal quantities and at normal 

 rates through the phloem tissues. A critical consideration 

 of these criteria and methods, therefore, seems desirable. 



20. The Movement of Solutes Introduced through Cut 

 Tissues. — Studies of solute movement based upon obser- 

 vations of the movements of solutions artificially introduced 

 into possible conducting tissues are open to serious criti- 

 cism. Thus for the xylem it is practically certain that the 



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