Occurrence and Role of Growth Hormones 83 



ity away from the root tip does exist (Cholodny, 1934). These differ- 

 ences in polarity have been shown to fit together in an overall polarity 

 gradient in the coleus, from a complete basipetal polarity in the vege- 

 tative apex to a complete acropetal polarity in the root apex, with a 

 gradual gradient between (figure 40). 



Another exception to the strictly polar transport of auxins is to be 

 found in the case of high auxin concentrations. Although polarity of 

 auxin transport is entirely independent of auxin concentrations over 

 the physiological range, higher auxin concentrations will cause a loss 

 of polarity (Snow, 1936). A non-polar movement of auxins will also 

 occur when auxins are applied in such a manner as to enter the 

 transpiration stream (Snow, op. cit.). 



The speed of auxin movement has been measured in several experi- 

 ments and where in general the polar transport mechanism is active, 

 such as in Aveua coleoptiles, the rate is very rapid: 10 to 12 mm. per 

 hour. Where the polar mechanism is less active, such as in roots, the 

 rate is considerably less: 4 mm. per hour (Faber, 1936). These rates 

 are definitely higher than movement by diffusion, which would be 

 approximately 2 mm. per hour. The velocity of auxin transport is 

 imaffected by temperature; however, the amount transported is strictly 

 proportional to temperature. For each 10° C rise in temperature, ap- 

 proximately 3 times as much auxin is transported (van der Weij, 

 1932). The characteristics of auxin transport may be described as 

 follows: 



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 independent 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 constant); if not removed from the end the objects 

 continue to pile up (transport against the gradient). (Went and Thimann, 1937, 

 p. 96.) 



The characteristic polarity of auxin transport is the mechanism 

 by which some of the most striking activities of the growth hormone 

 in plants occur. The amount of auxins formed in any of the aerial 

 portions of the plant will influence the more proximal parts. Thus the 

 auxin produced in leaves moves downward in a polar manner, contrib- 

 uting to the inhibition of lateral buds and the growth patterns of the 

 stem. The auxin produced in any organ serves to prevent abscission 

 of that organ. If the polar transport of the auxin is hindered by 

 wounding, for example, the inhibition of lateral buds may be over- 

 come, and if the hindrance is great enough there may be a formation 

 of roots just above the barrier due to the accumulated auxins, or the 



