104 PHYTOHORMONES 



measurements it is never quite clear what kind of potential 

 is being measured. Ramshorn (1934) has opposed the above 

 theory on the ground that in his experiments the growing 

 region was always positive to its surroundings. The meas- 

 urements of Clark (1935), however, show that the apex of 

 Avena is negative to the base. The potentials observed, 

 while they show interesting parallelism with the light-growth 

 reaction, show no parallelism with auxin transport, and the 

 external application to Avena sections of a potential opposite 

 to the inherent one was found to have no effect on the di- 

 rection of auxin transport in the section (Clark, 1937). So 

 far, then, there is no direct evidence to support an electrical 

 theory of the transport, or, indeed, any other theory. This is 

 regrettable because polarity is of great theoretical importance 

 for the explanation both of growth and of correlations. 

 Morphological polarity is essentially only a descriptive term, 

 but it may become analytically approachable through the 

 phenomenon of polar transport of a morphogenetic substance. 



