THE METHOD OF MOVEMENT 209 



more normal than, for example, backward flow of dyes 

 through the xylem, but at one time the latter seemed 

 normal to many botanists. Introducing a dye into cut 

 xylem is abnormal in that a foreign substance is intro- 

 duced under conditions that alter or even reverse natural 

 pressure or tension gradients. The introduction of a dye 

 that is taken up by protoplasm may disturb in a comparable 

 manner the normal interfacial relations within the sieve- 

 tube contents. 



The spread of fluorescein through protoplasm, even if it 

 were clearly demonstrated to take place by the mechanism 

 proposed by Van den Honert, does not necessarily signify 

 that sugar and such solutes are carried by the same mech- 

 anism. It is conceivable that cells produce a substance 

 that is active at protoplasmic interfaces and thus causes 

 protoplasmic streaming. This streaming protoplasm may 

 in turn be the vehicle for carrying other solutes, perhaps 

 sugar. The alternative, suggested by Van den Honert, is 

 also possible, that is, that the sugar itself, or some substance 

 into which it is transformed, may be the surface-active 

 material which causes the streaming, and the streaming 

 is then the result of sugar transport and is not the vehicle. 

 A foreign substance like fluorescein, therefore, may be 

 transported because of its activity at the interfaces, or it 

 may be carried in a moving stream of protoplasm activated 

 by some other agent. In a similar way sugar may be active 

 at an interface causing transport, or it may be carried in a 

 stream activated by some other agent. Another alterna- 

 tive may be that fluorescein and certain substances are 

 carried by one of the methods, either active or passive, and 

 sugars and certain other substances by the other method. 

 It is also conceivable that there is a rapid interfacial spread 

 of a substance like fluorescein that is not accompanied by 

 protoplasmic movement. 



The very fact that Schumacher found fluorescein to 

 move out and not into young developing leaves when 

 sugars and other materials were almost certainly moving 

 into them would indicate that sugars are carried by a 



