GROWTH SUBSTANCES FOR PHOTOTROPISM 143 



five types of curvatures, viz., three different positive curva- 

 tures, which are separated from each other by two cHfferent 

 negative curvatures, or indifferent stages. A comparison of the 

 data given in Table 8 with the results of Arisz shows that the 

 second positive curvature of du Buy and Nuernbergk is new, 

 appearing first at about 30,000 to 70,000 meter-candle seconds, 

 i.e., in the range that, according to Arisz and Lundegardh, gives 

 negative curvatures. The first positive curvature of du Buy 

 and Nuernbergk corresponds to the first positiv^e curvature of 

 Arisz, and the third positive curvature of du Buy and Nuernbergk 

 corresponds to the second positive curvature of Arisz. 



The Primary Positively Phototropic Curvature.— Some analyses 

 of the growth processes which occur during the curvatures will 

 now be reviewed. It can be shown experimentally that the first 

 positively phototropic curvature is not connected with a change 

 in the average rate of growth. For such experiments, Cholodny 

 (1930) used a micropotometer. An excised Avena coleoptile from 

 which the primary leaf had been removed was plugged with 

 lanolin at the bottom and placed in the enlarged opening of a 

 capillary tube bent at right angles so that the intake of water 

 could be measured over a period of time. The coleoptile was 

 placed in a saturated moist chamber to check transpiration. It 

 was assumed that all the water taken up under these conditions 

 was used for the volume increase of the coleoptile during growth, 

 and the intake of water, therefore, was used as a measure of the 

 growth rate. It was found that no appreciable change in the 

 rate of growth could be shown in coleoptiles that were unilaterally 

 illuminated with a light value of 500 to 2,000 meter-candle 

 seconds and afterward darkened, although decided phototropic 

 curvatures resulted in the course of 1}4 to 2 hours. Du Buy and 

 Nuernbergk obtained similar results at a later date. In these 

 investigations, the course of growth of both the front and back 

 sides of an Avena coleoptile was recorded during the first positive 

 curvature produced by unilateral illumination of the tip with 

 3.55 ergs at a wave length of 4,360A. for 2 seconds. From the 

 curves it may be seen that the average rate of growth of the 

 coleoptile is not changed during the curvature, so the first posi- 

 tive curvature involves an increase in the rate of growth of the 

 back side and a corresponding decrease in the growth of the 

 lighted side. The increase and decrease in the rate of growth of 



