438 



EFFECT OF LIGHT 



[Cn. XVII 



tropism), the following turn from it (negative phototropism): 

 the hypocotyl of the seedling mistletoe ; the roots of many 

 plants, e.g, Sinapis (Fig. 124), Helianthus, Vicia faba, Zea 

 mais, etc.; stems of some recumbent dicotyledons, e.g. the 

 moneywort ; the root hairs of the prothalli of ferns and hepat- 

 ics ; the tendrils of the vines Vitis and Ampelopsis. As we 

 shall see later, however, the sense of turning is, within limits, 

 dependent upon the intensity of the light. 



Finally, we observe that plants differ greatly in the degree 

 of their phototropism. Thus aquatic plants and non-chloro- 

 phyllaceous phanerogams are only very slightly phototropic 

 (compare HOCHREUTIKER, '96). 



The general phenomena of positive phototropism are seen 

 when a seedling which has been growing in the dark is illu- 

 minated upon one side by a horizontal ray. The tip of the 

 seedling, which is normally constantly "nutating" about the 

 vertical line passing through its axis, now begins to move 

 towards the light side of the vertical. The quickness with 

 which it does so seems to vary with the species and with the 

 intensity of illumination of the plant and other conditions of 

 the environment ; the turning may be evident in 15 minutes * 

 or it may be delayed for several hours. There is apparently a 

 certain, not precisely determined, latent period elapsing be- 

 tween illumination and response. The curvature first appears 

 just behind the tip of the seedling, but later almost the whole 



stem above the ground becomes in- 

 volved, so that after several hours 

 it points straight towards the 

 source of light (Fig. 125). 



The intensity of light necessary 

 to provoke the maximum response 

 varies with the species. WIESNER 

 ('93) especially has made accurate 

 determinations on this subject. 



The unit of measurement is a normal 

 candle (p. 160) burning at a distance 



a 



FIG. 125. Course of phototropic 

 curving of the cotyledon of 

 Avena sativa. a, before illu- 

 mination ; b, after H hours; c, 

 after 3j hours ; d, after 7j hours. 

 (From ROTHERT, '94.) 



* DARWIN ('81, Chapter IX) found with the aid of a microscope that the tip 

 may begin to turn in from 3 to 10 minutes. 



