AUXIN TRANSPORT AND POLARITY 



95 



or capacity of the auxin transport, in units per section per 

 hour. The velocity is about 10-12 mm. per hour in Avena 

 sections at 25°, and is almost completely independent of 

 temperature (Figure 38 A). The capacity, or amount trans- 

 ported per unit time, however, depends on temperature, 

 increasing rapidly from 0° to reach an optimum at 35-40°. 



10% 



16% 



12% 



4% 



2 mm. 



I mm. inverse 



2 m m.^inv erse 



10 20 30 40 50 60 70 60 90 100 min. 



Fig. 38. A, auxin transported as per cent of the amount applied (ordinate) 

 through 2 mm. sections of Avena coleoptiles as a function of time (abscissa) 

 and temperature. Since all curves cross the x axis at about 10 minutes, the 

 transport velocity is unaffected by temperature. B, amounts of auxin trans- 

 ported (ordinate) at different temperatures (abscissa); curve Bi pure diffusion, 

 curve B2 transport through coleoptile sections. C, auxin transported through 

 1 mm. and 2 mm. coleoptile sections: crosses, from apex to base; circles, in- 

 verse. (All from van der Weij, Rec. trav. hot. neerl. 29: 379-496, 1932.) 



Between 0° and 30° the Qio is close to 3, i.e. that of a chem- 

 ical process (see Figure 38 B). Further, at physiological 

 temperatures the capacity is independent of the length of 

 the section. Since the capacity of true diffusion falls off 

 with the square of the distance, the transport process must 

 be fundamentally different from that of diffusion. Under 

 van der Weij's conditions, the concentration of auxin in 

 the receiving block became equal to that in the donating 

 block in 1-2 hours, — a rate of transport many times that 



