578 



GROWTH HORMONES 



petally, and (2) that it can occur against a concentration gradient since it 

 accumulates in the lower block. 



According to van der Weij (1934) the rate of auxin transport in oat 

 coleoptiles is about 10- 12 mm. per hour and is almost entirely independent of 

 temperature. The auxin transporting capacity of the coleoptile {amount 

 transmitted per unit time), however, increases rapidly with rise in tempera- 

 ture from 0° C. to about 35-40° C. 



A similar basipetal transport of auxins apparently also occurs in many other 

 plant tissues and organs such as the veins of leaves, petioles, hypocotyls, and 

 stems of various species. In many such organs translocation seemingly takes 



r\ 



- I ' - — ' r "** 



\J 



B 



Fig. 128. Diagram to illustrate basipetal movement of auxin. {A) Agar block 

 containing auxin attached to apical end of segment of oat coleoptile, (B) agar block 

 containing auxin attached to basal end of segment of oat coleoptile. Redrawn from 

 Went (1935). 



place only through the vascular bundles, most probably occurring in the 

 phloem. 



The mechanism of the polar transport of auxins has been subject to much 

 conjecture and considerable experimentation. Etherization stops the transport 

 of auxin, except insofar as it can be accounted for by diffusion, and destroys 

 its polarity. This indicates that living cells are involved in the process, but 

 tells nothing of the manner in which they operate. 



It has been suggested that the polarity in the movement of auxins may 

 be due to differences of electrical potential in the tissues. Apical portions of 

 many plant organs, including coleoptiles, are usually relatively more negative 

 than basal portions (Chap. XXXIV). Since the auxins are acids their active 

 radicals carry a negative charge and would be expected to move towards the 

 more positive basal regions of such organs. It has not, however, been possible 

 to demonstrate any causal relation between the electrical polarity of plant 

 tissues and the direction of auxin transport through them (Clark, 1937). 



