368 PLANT PHYSIOLOGY 



for geotropic stimuli. Just what the plant perceives is also an 

 open question. Some have believed that the inequality of illumina- 

 tion is what is perceived while others have held that the direction 

 was the important effective element. Still others have sug- 

 gested that the plant is able to perceive the difference in pres- 

 sure on the two sides of the stem. Since this difference in light 

 pressure is almost infinitesimal, it would seem to be out of the 

 question, and nothing is to be gained by the addition of meta- 

 physical speculations. 



The relation of hormones to growth has also been discussed 

 (Chap. XVII). Not only is the downward flow of growth hormones 

 checked on the illuminated side with the result that this side 

 grows less, but this result is also favored by the fact that the 

 permeability of the illuminated side seems to be decreased some- 

 what by the light (Blaauw). The result is that (1) the cells on 

 that side are less turgid and (2) the growth-inhibiting hormone 

 can diffuse into these cells more easily. The checking of growth 

 on the illuminated side and its promotion on the opposite side are 

 thus seen to be decidedly complex phenomena. 



During the development of the plant, the sensitiveness to light 

 may change. The flower stalks of Kenilworth ivy (Linaria) are at 

 first positively phototropic, but later, when the seeds have ripened, 

 the phototropism becomes negative, pushing the fruits into crev- 

 ices where the seeds can germinate. In other cases a plant may 

 be positively heliotropic in weak light and negatively so in strong 

 light. In the oat plant (Avena) such variations with intensity 

 are very pronounced. 



The amount of time required to cause bending varies inversely 

 with the intensity of the light. Thus a light of 26,500 candle 

 power required only 0.001 of a second, while light of 0.000439 

 candle power required 13 hours to bring about the same degree 

 of curvature. Violet light is most effective in causing photo- 

 tropic bending and yellow light the least, with the red intermediate 

 (Fig. 28). Similar results were obtained with Pilobolus, with 

 which it was also noted that the presentation time multiplied by 

 the square root of the wave frequency is nearly a constant. Sen- 

 sitiveness generally diminishes with age and also varies with the 

 temperature and other external conditions. 



Roots are negatively phototropic, but the direction of their 

 growth is determined more by gravity than by light. Leaves 



