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PLANT PHYSIOLOGY 



particularly conspicuous in plants growing in shady places, e.g., 

 the ivy (Fig. 42). In general, phototropic phenomena are more 

 distinct in plants growing indoors or in the shade than in those 

 grown in open spaces. This is owing partly to the fact that in 

 very strong light phototropic sensitivity is somewhat lowered. 

 It may be caused also by the diurnal and continuous change in 

 the direction of sunlight. Some plants, however, show so rapid 



Fig. 41. — A geranium adjusting its leaves perpendicular to the direction of light 



{after Lubimenko). 



a reaction to an alteration in the direction of light that they 

 follow the motion of the sun. Good examples of such types are 

 the sunflower, Helianthus, and species of Bidens. 



Phototropic sensitivity is in no way connected with the 

 presence of chlorophyll. Sporangia and the fruiting bodies of 

 many colorless fungi also manifest phototropic curvatures (Fig. 

 43). Moreover, etiolated seedlings are always more sensitive 

 to light than green ones. In general, it has been found that the 

 growing of plants in light decreases their phototropic sensitivity 



