Polarity 141 



polarity. Simon (1920) has suggested that the polar character of regen- 

 eration in leaves is related to the basipetal movement of carbohydrates. 



How much the direction of flow in the phloem is due to polar behavior 

 in the strict sense and how much to other factors is not clear, but Schu- 

 macher (1933, 1936) has shown a polar flow of fluorescein there, basipetal 

 in the petiole and in various directions in the stem. It seems clear that in 

 most vegetative stems the flow of nutrients in the phloem is predomi- 

 nantly basal. Sax (1956 and earlier papers) removed a ring of bark in a 

 young tree and then replaced it in an inverted position. Under these con- 

 ditions phloem transport is markedly checked and the tree is much re- 

 duced in growth. This effect is not permanent, however, because the new 

 bark regenerated at the seam permits phloem transport upward. 



The clearest case of physiological polarity and the one most thoroughly 

 studied is that of the flow of auxin (p. 384). In the Avena coleoptile it has 

 been shown that auxin normally is produced at the tip and moves toward 

 its base. If the coleoptile is cut off, decapitated, and auxin applied at 

 the morphological apex it will move toward the other end, whether the 

 coleoptile is normally oriented or inverted. If auxin is applied to the 

 morphologically basal end, however, it will not move toward the tip even 

 if the coleoptile is inverted and the auxin is placed at the end now upper- 

 most. Auxin flow here is therefore strictly polar. The cause of this 

 polarity is not clear, for there is no histological difference with which 

 it is correlated. It seems to be characteristic of auxin transport gen- 

 erally, for this substance, commonly produced in buds, moves down- 

 ward from them but not upward. Jacobs and others (p. 384), however, 

 report that auxin may sometimes move acropetally, especially in weak 

 concentrations. 



Went ( 1941 ) has shown that the auxin flow continues to be morpho- 

 logically basipetal in inverted cuttings of marigold but that after a time, 

 presumably following the production of new and reoriented vascular 

 bundles, the flow is reversed and auxin now moves downward toward the 

 new root system. He suggested that auxin polarity is electrical in char- 

 acter, but this idea has encountered some difficulties ( p. 360 ) . 



The significance of auxin polarity for many problems of plant develop- 

 ment is great since this substance is so intimately related to both stimula- 

 tion and inhibition of growth and to so many specific growth reactions, 

 such as the initiation of shoots and roots (p. 390). 



It is tempting to explain all structural polarity in the plant as due to this 

 polar flow of auxin, but here again it may be that both are the result of 

 some more deeply seated factor. No satisfactory solution of the problem 

 has yet been found. It is surely a remarkable fact that a simple, relatively 

 undifferentiated parenchyma cell of the oat coleoptile will allow auxin to 

 pass through it in only one direction. An understanding of the mechanisms 



