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TEXTBOOK OF PLANT PHYSIOLOGY 



most favorable for the utilization of this source of energy. They 

 usually spread perpendicularly to the direction of the incident rays 

 (Fig. 125). Owing to phototropic movements, the leaves of 

 many plants form the so-called leaf mosaic, which is particularly 

 conspicuous in shady plants, for example, the ivy (Fig. 126). In 

 general, phototropic phenomena are more distinct with plants 

 growing indoors or in the shade than in those grown in open spaces. 



Fig. 125. — A geranium adjusting its leaves perpendicular to the direction of light 



(after Lubimenko). 



This is due 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 direction of sunlight. Some 

 plants, however, show so rapid 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 and bur marigold {Bidens). 

 Phototropic sensitivity is in no way connected with the pres- 

 ence of chlorophyll. Sporangia and the fruit bodies of many fungi 

 also manifest phototropic curvatures. Moreover, etiolated seed- 



