CRITERIA AND METHODS 131 



"stimulators," or to an interference in the transmission 

 of "stimuli." But a much simpler and more straight- 

 forward explanation that fits the facts and does not require 

 special assumptions is, that solute movement, in both 

 directions, takes place chiefly through the phloem tissues. 

 Therefore, when these tissues are cut, the normal movement 

 of these solutes is interfered with, and growth changes 

 result. I do not deny the possibility that some stimulus 

 may be transmitted through the phloem or other tissues, 

 or that hormones may be transported through them, or 

 that some such stimulus or hormone may influence growth 

 and solute distribution. The evidence seems clear, how- 

 ever, that certain solutes are carried chiefly through the 

 phloem and that the distribution of these solutes may be 

 the factor determining the nature and extent of the growth. 

 That growth may in turn influence the distribution of 

 solutes is also clear, and the possible interrelation of the 

 two is discussed in Sees. 41 and 42. 



SUMMARY 



20. It is not safe to interpret movements of solutions that are introduced 

 into cut tissues as indicative of normal movements. Any one or more of 

 the following phenomena: the direction of movement, rate of movement, 

 type of substance moving (liquid, gas, or solute), and method of movement 

 (that is, the forces concerned) may be different from what was naturally 

 taking place before the substance was injected. Movements of solutions 

 and gases that have been observed in the xylem of cut or mjected stems, 

 therefore, may be highly misleading and cannot safely be interpreted as 

 indicating normal movements. For similar reasons, a failure to obtain 

 flow through the phloem tissue, by methods that are successful in causing 

 flow through the xylem, need not preclude the possibility of rapid movement 

 through these tissues. 



21. When intact plants are placed in certain solutions, especiallj^ solu- 

 tions of eosin and of lithium salts, it has been found that these solutes are 

 rapidly carried through the xylem. The eosin probably travels through 

 the xylem because it is toxic and injures the living cells which might other- 

 wise prevent its entrance into the transpiration stream. The use of lithium 

 is criticized on the grounds that it may alter the permeability of the cells, 

 that it has been used as a quahtative test not as a quantitative test, and 

 that its behavior may be different from that of most of the normally occur- 

 ring solutes. 



22. Tests for deposits of dyes or specific elements in cells or on walls of 

 certain tissues have been taken as proof of transport through that region. 

 Some have cited this as proof of transport through the lumen of the cells 



