140 



CHAP. XIII. DIA-PHOTOTROPISM 



to that in the shoot, hght does not always produce this 

 opposite reaction ; for though some roots turn away from 

 light, others move towards it. 



The difficulty encountered in obtaining a direct record 

 of the phototropic curvature of the root was great. It was 

 overcome by the use of a sensitive recorder, and by the 

 employment of roots which possessed a moderate amount of 

 rigidity, so that the induced curvature could exert a sufficient 

 pull on the recording lever. One of the most suitable plants 

 is Ipomoea reptans, which floats on ponds, the roots normally 

 growing vertically downw^ards. The aerial roots of certain 

 plants were also found suitable for the investigation. 



The Root-Recorder 



The Recorder (fig. y^) consists of a very light writing lever 

 W, the axis carrying an aluminium wheel of a small diameter. 



W 



Fig. 78. The Root-Recorder. {See text.) 



A string S, appropriately attached to the wheel, is tied to 

 the tip of the root T, the growing region G being a httle 

 above the tip. The stem St is fixed on a piece of cork by 

 the pin P, the root projecting vertically downwards. The 

 plant is suitably mounted in a rectangular trough of mica. 



