GROWTH OF PLANTS 



151 



ventral organs such as leaves or the thallus of the liverworts are 

 diaphototropic, while axial organs of a radial structure generally 

 exhibit either positive or negative phototropism. 



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 roots and rhizomes, which 

 normally grow in darkness, very often do not react to light. 

 Some roots, however, like those of mustard and other crucifers, 

 are negatively phototropic. 



The capacity of bending toward or away from light is not 

 permanent in every organ. It depends on the intensity of light. 



Fig. 40. — Seedlings of oats bending toward light (after Nathansohn). 



In too strong light, positive curvatures frequently change to 

 negative ones. By selecting various intensities of light, it is 

 possible, therefore, to make the same shoot bend in succession 

 toward and then away from the source of light. It is also possible 

 to find a certain intensity of light in which the positive effect will 

 be neutralized by the negative; the plant will then appear to be 

 insensitive to light. 



Phototropism plays an important part in the life of plants. 

 On account of it, leaves, which require light, assume the position 

 that is most favorable for the utilization of this source of energy. 

 They usually spread perpendicular to the direction of the inci- 

 dent rays (Fig. 41). Owing to phototropic movements, the 

 leaves of many plants form the so-called ''leaf mosaic," which is 



