98 PHYTOHORMONES 



terms of the lateral transport of auxin (see X C), it might 

 be thought that this ^'geotonic effect" is due to the action 

 of gravity on the longitudinal transport of auxin. Pfaeltzer 

 (1934), however, was unable to find any indication of an 

 effect of gravity, acting in the longitudinal axis of the plant, 

 on auxin transport. Its polarity is therefore completely 

 independent of gravity. 



The effect of light on the transport has been studied by 

 du Buy (1933). In 1926 Went had concluded, from the 

 effect of light on auxin curvatures, that light temporarily 

 decreases the rate of auxin transport, and a temporary de- 

 crease in curvature for half an hour after illumination was 

 confirmed by van Overbeek (1936a). The exact explanation 

 of this phenomenon is, however, difficult in the present 

 state of our knowledge. The experiments of du Buy were 

 measurements of the effect of lateral or terminal illumina- 

 tion on the amount of auxin transported through coleoptile 

 sections. His data, however, are scarcely sufficient to sup- 

 port the rather far-reaching conclusions which he draws 

 from them, and one can only say with certainty that light 

 appears to have no effect on auxin transport. The earlier 

 experiments of van Overbeek (1933) on Raphanus hypo- 

 cotyls also failed to show any effect of light on the transport, 

 and the same conclusion may be drawn from Boysen Jensen's 

 experiments (1933) with A vena. 



We may therefore conclude that the polarity of auxin 

 transport is determined by some property inherent in the 

 living cells, and difficult to influence from without. 



D. Other Views on the Transport of Auxin 



Recently Czaja (1931, 1935o) has suggested that the 

 polarity is not the cause of, but is due to, the movement of 

 auxin. Thus he believes that the apex-to-base gradient of 

 auxin, caused by its production at the apex, is the cause of 

 the polarity in the plant. If high concentrations of auxin 

 be applied to the side of the plant the gradient from outside 

 inwards is at right angles to the longitudinal gradient and 



