THE MOVEMENTS AND SENSITIVITY OF PLANTS 315 



while the capacity of the plant to keep its organs perpendicular 

 to the direction of light rays is known as "diaphototropism." As 

 a rule, organs of a dorsiventral structure, such as the leaves, the 

 thallus of the liverworts, are diaphototropic, while axial organs 

 of a radial structure exhibit either positive or negative photo- 

 tropism. 



Not all the organs of a plant are phototropically sensitive. 

 This is a characteristic feature only of the aerial parts of plants. 

 The underground organs, such as the roots and the rhizomes, 

 which normally grow in darkness, very often show no reaction to 



Fig. 124. — Seedlings of oats bending towards light (after Nathansohri). 



light. Some roots, though, as those of mustard and other cruci- 

 fers, are negatively phototropic. 



The capacity of bending toward or away from light is not per- 

 manent for every organ. It depends on the intensity of light. In 

 too strong light positive curvatures frequently change to negative 

 ones. By selecting various intensities of light, it is possible, there- 

 fore, to make the same shoot bend in succession towards and away 

 from the source of light. It is also possible to find a certain inten- 

 sity of light in which the positive effect will be neutralized by the 

 negative, and the plant will then appear to be entirely insensitive 

 to light. 



Phototropism plays an important role in the life of plants. 

 Because of it, leaves, which require light, assume a position that is 



