1] 



GEOTROPISM 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 (WiESNER, '79, p. 196): 



DISTANCE OF SEEDLING FROM FLAME. 



0.25 ineter 

 0.30 " 

 0.75 " 

 1.25 " 



2.50 

 3.00 

 3.75 



(optimum) 



INCLINATION FROM VERTICALITT. 



30 



35 



55 



70 



80 



65 



35 



This table shows that the inclination from vertically 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 FOR VARIOUS SPECIES OF PLANTS 



Phycomyces nitens . . . 90 



Pilobolus crystallinus . . 90 



Vicia sativa 70 



Avena sativa 70 



Phalaris canariensis ... 70 



Linum usitatissimum . . 70 



Brassica napus 70 



Datura stramonium ... 70 



Lepidium sativum . * . 60 



Sinapus alba (plumule) . . 60 



Pisum sativum 60 



Vicia faba 60 



Phaseolus multiflorus . . 60 



Sinapis alba (radicle) . . 50 



Helianthus annus .... 45 



Bicinus communis .... 45 



Cucurbita pepo 40 



