AUXIN TRANSPORT AND POLARITY 91 



tween the transport of auxin along the broad and narrow 

 sides of the coleoptile could be found ^ ; the discrepant re- 

 sults of Laibach and Kornmann may perhaps be explained 

 by their use of too low a humidity, which causes pronounced 

 drying of the tissue outside the vascular bundles. At all 

 events, transport can take place through the parenchj^ma, 

 the cells of which are strongly elongated in the direction 

 of transport. Lateral transport of auxin across the coleoptile 

 must be slight ; if it were not, curvature could not so easily 

 be produced by applying auxin to one side. Direct e\'idence 

 that the movement of auxin follows the direction of elonga- 

 tion of the parenchyma is provided by some experiments of 

 Tammes (1931). He found that when a permanent torsion 

 is brought about in the upper part of an Avena coleoptile, 

 the phototropic or geotropic stimulus is no longer trans- 

 mitted longitudinally, but follows the direction of the twdsted 

 cells. The displacement of the plane of phototropic curva- 

 ture out of the direction of the light w^as proportional to 

 the amount of torsion imposed on the upper part of the 

 coleoptile. 



The path of transport of auxin in other organs may be 

 different from that in the coleoptile. In the Nicotiana leaf 

 auxin moves almost exclusively through the vascular bun- 

 dles, or at least through the veins (Avery, 1935). Similar 

 results have been obtained in Malva leaves (u). Corre- 

 spondingly, Cooper (1936) showed that in Citrus stems the 

 movement is through the phloem, where it follows the same 

 rules as the translocation of carbohj'drates. On the other 

 hand, Hitchcock and Zimmerman (1936) found that auxin 

 may be transported with the transpiration stream; this, as 

 will be shown in section D, only occurs when excessively 

 high auxin concentrations are applied. 



B. Polarity in General 



The earliest investigators of correlation found that such 

 correlations as were studied, particularly shoot and root 



* The bundles are on the narrow side — see Chapter III. 



