100 PHYTOHORMONES 



correlations in Chapters XI, XII, and XIII show that the 

 normal transport of auxin is through living tissues and it is 

 this transport which is polar. Hitchcock and Zimmerman 

 have completely failed to grasp the importance of the rela- 

 tion between the polarity of auxin transport and the already 

 well-established polarity of growth, organ formation, and 

 transmission of stimulus. 



Laibach and Fischnich (1936) have shown that if a Coleus 

 leaf be split so that the lower part of the blade be connected 

 to the midrib only through lamina! tissue above it, auxin 

 applied to this lower part will still reach the midrib and thus 

 the petiole; its path of transport must therefore be at first 

 acropetal in the leaf-blade. They thus oppose Avery's (1935) 

 conclusion that auxin transport in the leaf is a strictly polar 

 phenomenon. However, in Avery's experiments the polarity 

 was determined in leaves with veins, while in Laibach and 

 Fischnich's work the veins were cut, so that the possibility 

 remains that auxin transport in the larger veins is polar, 

 but in small veins and mesophyll not. Here, too, it must 

 be emphasized that the lack of polarity is only detected 

 when unphysiologically high auxin concentrations are used. 



Both Cholodny (1935) and Pohl (1935, 1936) claim that 

 auxin moves from the seed upwards into the coleoptile, 

 especially in the early stages of germination. As we know 

 from Skoog's experiments (1937), this, however, is not 

 auxin, but an auxin precursor, which is converted into auxin 

 in the coleoptile tip, and also, probably, is convertible into 

 auxin in the seed itself (cf. IV B). It is impossible to obtain 

 the slightest amount of auxin from the apical cut surface 

 of a coleoptile or mesocotyl stump when connected with the 

 seed, even in the earliest stages of germination (u) . Further, 

 Thimann (1936a) has shown that application of indole- 

 acetic acid to the scutellum of de-seeded plants does not 

 accelerate the growth of the coleoptile. 



Many of the conflicting conclusions regarding polarity of 

 auxin transport are brought into line by the assumption 

 that auxin transport in the shoot is perfectly polar as long 



