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PLANT PHYSIOLOGY AND ECOLOGY 



109. Phototropism. Seedlings exposed to light from one side 

 alone regularly bend in the direction of the light. This reaction 

 is perceptible even with low light intensities. It occurs in adult 

 plants as well as in young ones, but it is usually more noticeable 

 in the latter. In most cases the stimulus is received by the tip 

 of the stem, though in some plants the cotyledons are the part 

 stimulated. In either event the stimulus is transmitted to the 

 zone in which growth is most rapid, and the response first 



Fig. 25. Rosettes of a saxifrage, Teleonyx jcnnesii, growing under an 

 overhanging rock. The leaves are turned squarely to the north. 



becomes evident there. Stems that bend toward the light are 

 termed prophototropic, while roots and those stems that bend from 

 the light are spoken of as aphototropic. Horizontal leaves endeavor 

 to maintain a position at right angles to the incident light, and are 

 consequently termed diaphototropic. All phototropic movements 

 have a common purpose in that they serve to place and keep the 

 leaf surfaces in the most favorable position with respect to light. 

 A few plants such as the sunflower often respond to the direct rays 

 of the sun by turning the crown in the direction of the latter. 

 These plants may be termed heliotropic. The leaves of certain 

 plants, chiefly leguminous ones, seem to possess a similar property. 



