§1] 



GEOTKOPISM AND PHOTOTROPISM 



471 



the light. As a result of this combined response the plant 

 comes to occupy an oblique position. 



The exact angle assumed by the plant is variable. It varies- 

 in a given species with the intensity of the light, and, the in- 

 tensity of the light remaining constant, the angle varies with 

 the species. For example, seedlings of Lepidium sativum sub- 

 jected to a unilateral horizontal illumination for 48 hours 

 showed at different intensities of the light the following- 

 inclinations from the vertical (Wiesnbr, '79, p. 196) : — 



This table shows that the inclination from verticality in- 

 creases with the intensity of illumination to a certain maximum 

 degree, beyond which it diminishes again. This maximum 

 inclination is called the phototropic limiting angle. 



]More extended determinations of the limiting angle have 

 been made by Czapek ('95), who finds that it is the same for 

 a given intensity of horizontal light whether the plant is verti- 

 cal or horizontal, having its apex directed towards the source 

 of light. 



TABLE LVII 

 Giving the Phototropic Limiting Angle fok Various Species of Plants- 



Phycomyces nitens . 

 Pilobolus crystallinus 

 Vicia sativa .... 

 Avena sativa . . 



Phalaris canariensis . 

 Linum usitatissimum 

 Brassica napus . . . 

 Datura stramonium . 

 Lepidium sativum 



90° 



90 



70 



70 



70 



70 



70 



70 



60 



Sinapus alba (plumule) 

 Pisum sativum . . . 

 Vicia faba . . . 

 Phaseolus multiflorus 

 Sinapis alba (radicle) 

 Helianthus annus . . 

 Eicinus communis 

 Cucurbita pepo . . . 



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