96 



PHYTOHORMONES 



which diffusion could cause. At 0°, however, the character 

 of the process changes, the capacity becoming partly de- 

 termined by the length of the section, although its direction 

 is still polar (''polar diffusion"). With increasing concen- 

 tration of auxin in the donating block the absolute amount 

 transported through a given section increases, but not 

 quite in proportion, so that the percentage transported 

 decreases. 



He later found (1934) that the polarity of the transport 

 is so strict as to be completely independent of an external 

 auxin gradient, auxin being actually carried from a lower 

 to a higher concentration (see Table VII). The increase in 



TABLE VII 



Auxin Transport through 1 mm. Coleoptile Cylinders. Concentration 

 100 = 14.6° ± 0.5°. (After van der Weij, 1934) 



auxin concentration in the receiving block may be seen to 

 equal exactly the decrease in auxin in the donating block. 

 The best simile for the transport is that of objects along a 

 moving band; the band goes at constant speed, so that the 

 number of objects arriving at the end per unit time is in- 

 dependent of the length (capacity independent of length of 

 section) ; the time required for the first object to reach the 

 end is proportional to the length of the band (velocity con- 

 stant) ; if not removed from the end the objects continue 

 to pile up (transport against the gradient). 



If the coleoptile sections are placed in low concentrations 

 of ether vapor, the polarity is suspended and the transport 



