TROPISMS 177 



The fourth diagram shows a curvature produced by uni- 

 lateral application of auxin. After an interval of 35 min- 

 utes, in which no curvature takes place, curvature begins 

 sharply in the upper zones 3 and 4 (zones 1 and 2 being 

 removed by decapitation). It then spreads downwards 

 from zone to zone at an average rate of 10-12 mm. per hour, 

 the rate being somewhat faster in the upper zones. This rate 

 agrees exactly with the transport velocity of auxin found 

 by van der Weij (c/. VI C). At 110 minutes after application 

 of the agar block (= 150 minutes after decapitation) re- 

 generation begins, and immediately the curvatures in the 

 uppermost zones decrease. The lower zones continue to 

 increase in curvature so that the total curvature does not 

 decrease but moves down the coleoptile. The final state 

 of the plant, after 33^ hours, thus shows the curvature 

 located mainly in the base. 



The third diagram (Figure 51) shows the curvature of an 

 intact plant after one-sided illumination with 500 MCS. At 

 first sight it resembles the auxin curvature. There is an 

 interval of 40 minutes, after which curvature begins in 

 several upper zones. The rate of movement of the initial 

 curvature is somewhat faster than the above, about 16 mm. 

 per hour; this agrees with Went's finding (1935) that in the 

 intact plant auxin transport is slightly faster than when 

 decapitated. The other characteristic of this diagram is 

 that the curvatures do not decrease, but continue to increase 

 in all zones for at least 3 hours. This agrees with the fact 

 that the auxin redistribution caused by light persists for 

 many hours (see p. 168). The final state of the plant there- 

 fore shows a curvature throughout. The first diagram shows 

 geotropic curvature of plants placed horizontally for 15 min- 

 utes. The difference may be seen at a glance. The interval 

 before curvature begins is only 20 minutes and the first 

 five zones then all begin to curve simultaneously. This 

 is due to the detection of gravity, with its resulting redis- 

 tribution of auxin, not only by the tip but also by the zones 

 below. In the sixth and lower zones the presentation time is 



