384 Morphogenetic Factors 



cells it does not move in all directions but has a definitely polar flow, 

 going from the more apical regions of the plant downward ( p. 141 ) . This 

 was first observed in the oat coleoptile, where auxin is formed at the tip 

 and then passes basipetally, regardless of how the axis is oriented. If a 

 portion of the coleoptile is cut off and inverted and auxin is then applied 

 to the end now uppermost, it will not move downward, though auxin 

 placed at the other end will move up, in the original polar direction. 

 Such polar transport is also shown in the stem axis, in which auxin 

 moves down through the phloem. Inverted stems, after some time and 

 apparently after the development of some new vascular tissues, may gain 

 the ability to transport auxin in the opposite direction from the original 

 one (Went, 1941). 



Auxin transport may not invariably be polar, for Jacobs (1954) has 

 reported that, if relatively weak concentrations are used instead of the 

 strong ones commonly employed in experiments, there is a good deal of 

 upward translocation. In a young internode of Coleus he found that 

 about one-third as much auxin moved upward as downward. He also ob- 

 served that in young bean hypocotyls, although auxin transport was al- 

 ways basipetal, the ability to transport it at all was lacking in very young 

 seedlings but increased as they grew older. It was greatest in the upper 

 portion of the hypocotyl and decreased toward the base. Oserkowsky 

 ( 1942 ) concluded that where auxin moves only basipetally it is carried 

 in living cells but that transport in both directions may result from 

 diffusion through dead cells or cell walls. Leopold and Guernsey (1953fl) 

 showed that in Coleus the flow was clearly basipetal in the shoot and 

 in the opposite direction in the root ( Fig. 18-8 ) and that flowering stems 

 transport auxin in both directions. Haupt (1956), however, reports that 

 polar transport of auxin is as clear in the floral structures he studied as it 

 is in vegetative shoots. Niedergang-Kamien and Skoog ( 1956 ) were able 

 to reduce or inhibit polar flow by triiodobenzoic acid (Fig. 18-9) and 

 suggest that the reported effect of this substance on growth correlations 

 is due to this fact. 



Nevertheless, polar transport of auxin seems to be a general phenome- 

 non and is obviously of much morphogenetic importance since it under- 

 lies the marked structural differentiation between the two ends of the 

 plant axis. It is probably involved in the polar character of regeneration 

 (p. 119) and in many other developmental events. 



Auxin flow may show a transverse as well as a longitudinal polarity, 

 notably in geotropic movements. The accumulation of auxin in the 

 lower half of a horizontally placed axis, although doubtless a response to 

 the stimulus of gravity, is not simply a downward diffusion but is made 

 possible by differential and unidirectional changes in the permeability 

 of the cells. Such transverse polarities have been emphasized by de 



