TROPISMS 159 



subsequently found a somewhat greater difference, the lower 

 side giving 73 per cent of the total auxin.) Hence gravity 

 has no effect on the total auxin production but affects only 

 its distribution between the two sides. Dolk found this 

 difference in distribution to appear within 15 minutes af- 

 ter placing coleoptiles horizontally, and to disappear again 

 within one hour after the horizontal exposure has ceased. 

 In isolated coleoptile cylinders which have been symmetri- 

 cally provided with auxin-agar and placed horizontally, the 

 same distribution takes place and the same ratio, 38 to 62, 

 was obtained. Polar distribution therefore may take place 

 in any zone of the coleoptile. Dolk also measured the curva- 

 ture of different zones of the Avena coleoptile as a function 

 of time, and this brought to light a number of interesting 

 points. Firstly, if the exposure to gravity is short, the 

 curvature of the base is delayed until curvature of the upper 

 zones is well under way, but if the exposure is 30 minutes 

 or more all but the most basal zones begin to curve at about 

 the same time. From this, and other experiments on bases 

 provided with auxin and exposed to gravity, we may con- 

 clude that all zones are able to perceive gravity by redis- 

 tributing their auxin, but that the redistribution in the 

 basal zones only leads to curvature if the exposure has 

 been 30 minutes or longer. In this respect the response is 

 different from that to light (see XH). Secondly, as men- 

 tioned above, the unequal distribution does not last long 

 after the stimulus is removed, and hence the curvatures 

 do not go on increasing for more than 40 minutes after 

 exposure. On the contrary, they begin to decrease from 

 the top down, so that the zone of maximum curvature 

 appears to migrate towards the base; this straightening 

 Dolk explained in terms of the food factor (cf. YD). It 

 thus appears that all the peculiarities of the geotropic 

 reaction are explicable in terms of unequal auxin distribu- 

 tion {cf. also discussion in X H, p. 176). 



Dijkman (1933, 1934) has made a similar study of geo- 

 tropism in Lupinus hypocotyls. Having first established 



