THE METHOD OF MOVEMENT 203 



Furthermore, since the lowering of the surface tension is 

 dependent on the concentration of the interface-active 

 substance, the rate of transport is a function of concentra- 

 tion gradient. This also is in agreement with transloca- 

 tion along a concentration or diffusion gradient. It is 

 tentatively suggested by Van den Honert that the interface 

 boundary between protoplasm of the sieve tubes and the 

 vacuole might serve as the boundary between two immis- 

 cible liquid phases, where any substance lowering the 

 interfacial tension would tend to spread along this interface 

 until concentration differences are equalized. He says, 

 "This should of course be accompanied by protoplasmic 

 streaming which could, however, be the consequence 

 instead of the agent of the transport. The primary cause 

 of the transport would be only the concentration-difference 

 of the substance to be transported." 



It has long been recognized that surface tension phe- 

 nomena might account for the observed streaming of 

 protoplasm (see especially Ewart, 1903; Tiegs, 1928) and 

 that the streaming may account for the transport of 

 materials. It is therefore highly interesting that Van den 

 Honert suggests that protoplasmic streaming is the result 

 of transport and not the vehicle itself. If transport is 

 brought about by spread in an interfacial film, it is not 

 clear why this moving film should result in a streaming of 

 the more viscous protoplasm without causing an even 

 greater flow of the more liquid watery phase. The latter, 

 however, appears stationary (Ewart, 1903; Scarth, 1927). 

 It is conceivable that the surface film of protoplasm itself 

 may move faster than other layers (see Sec. 34a) and may 

 then be the vehicle of transport. In such an event there 

 is no real difference between the mechanism proposed 

 by Van den Honert and the protoplasmic streaming 

 hypothesis. 



Even accepting his suggestion that the more rapid inter- 

 facial movement carries surface-active materials, it is at 

 the same time conceivable that the visibly moving proto- 

 plasm itself also carries some of the transported substances. 



