Growth Substances 



385 



Haan (1936), who reexamined the fact earlier noted by Noll that when 

 primary roots are bent, lateral roots grow chiefly from their convex sides 

 (Fig. 18-10). He observed that in such cases the root primordia, push- 

 ing out through the cortex, bend toward what is at that point the convex 

 side of the root (Fig. 18-11), and he interprets this as the result of a 

 transverse polar gradient in a root-forming hormone. Such transverse 

 reactions of plants are extensively discussed by Borgstrom (1939), who 

 shows their importance in many structural and physiological characters 

 of plants. 



TIP 



2 0.4 06 08 1.0 

 CONC TIBA IN MG./L 



Fig. 18-8. Relative amount of basipetal Fig. 18-9. Effect of 2,3,5-triiodobenzoic 

 transport of auxin in various regions of acid on polar transport of auxin. Graph 

 the vegetative and of the flowering axis showing reduction in amount of auxin 



of Coleus ( From Leopold and Guernsey. ) 



transported basipetally in sunflower epi- 

 cotyl cylinders which had been pre- 

 treated for 2/2 hours with various concen- 

 trations of TIBA. (From Niedergang- 

 Kamien and Skoog. ) 



No satisfactory explanation of polar auxin transport is available (p. 361). 

 The suggestion of Went (1932) that electrophoretic diffusion might ac- 

 count for it has been shown by Clark ( 1938 ) to be doubtful, since the 

 polar transport of auxin can be abolished by the application of sodium 

 glycocholate without producing any change in the electrical polarity. 

 Schrank, however, in a series of papers ( 1957 and others ) presents evi- 

 dence that electrical polarity is the essential basis for the polar transport 

 of auxin. The question of auxin transport is part of the more general one 

 as to why many substances move about the plant in certain directions 

 more than in others. Auxin is doubtless important in the development of 

 growth patterns, and an understanding of the mechanism of its differen- 

 tial movement would clear up many morphogenetic problems. 



