AUXIN TRANSPORT AND POLARITY 99 



this is considered to cause the growth inhibition observed 

 (see Chapter XIII). The inhibition of the growth of roots 

 by auxin he also interprets in terms of two gradients (see 

 IX Z)). There are two major objections to this view. In the 

 first place it is in opposition to the experiments on transport, 

 especially those of van der Weij (VI C). In these, the direc- 

 tion of transport is shown to be totally unaffected by auxin 

 gradients applied from without. Even if the polarity is sus- 

 pended by narcotization, then, on removal of the ether, it is 

 re-established in the original apex-to-base direction, irre- 

 spective of whether this is with or against the applied auxin 

 concentrations. Czaja's view as applied to roots has also 

 been directly disproved by experiment (see IX D) . In the 

 second place, opposing streams of auxin are inconceivable, 

 for an auxin stream has no inertia, by which it might re- 

 tain its direction, and therefore the cell cannot distinguish 

 an auxin molecule which has entered it from above from 

 one which has entered from below. Two such streams in 

 opposite directions would merely amount to a more equal 

 auxin distribution throughout. 



Hitchcock and Zimmerman (1935, 1936) have denied that 

 the transport of auxin is polar at all. They found that when 

 very high concentrations of auxins were applied to the roots 

 of intact plants, some of it was absorbed and moved up- 

 wards, presumabl}^ in the transpiration stream. In the 

 first place, it is very doubtful whether it is justifiable to 

 apply the results of experiments at such unphj^siologically 

 high auxin concentrations to the processes of the normal 

 plant (see also p. 101.) In the second place, it has, of course, 

 been known for a great many years that any substance, 

 even though it be toxic, when it is once in the vessels will 

 be carried with the transpiration stream. The auxins would 

 not be expected to be exceptions to this. Snow (1936) has 

 studied the acropetal (base-to-apex) movement of auxin in 

 coleoptiles and hj^ocotyls, and concludes that the bulk of 

 it takes place in the conducting strands (c/. p. 97). The 

 extensive experiments discussed above, and those on auxin 



