PHOTONASTIC CURVATURES 379 



movement will further be shown to be distorted by the 

 lateral action of li^jrht. 



PHOTOTROPIC RESPONSE OF ANISOTROPIC ORGANS. 



The different sides of a radial organ, such as the young 

 stem of Mimosa, are equally excitable. The response to 

 unilateral light of moderate intensity is therefore positive ; 

 owing to equal excitabilities of the two sides the response 

 of the opposite sides are alike. Diffuse stimulation there- 

 fore induces no resultant curvature. If, however, the plant is 

 allowed to form a creeping habit, the excitabilities of the 

 dorsal and ventral sides will no longer remain the same. 

 Thus in the creeping stem of Mimosa the lower or the 

 shaded side is, generally speaking, found to be the more 

 excitable. In fact such anisotropic stem of Mimosa acts 

 somewhat like the pulvinus of the same plant. Diffuse 

 stimulation induces, in both, a concavity of the more 

 excitable lower half with the down movement of the leaf 

 or the stem. 



Experiment 141. — I took four creeping stems of Mimosa 

 in vigorous condition and tied them in such a manner that 

 their free ends should be vertical. The shaded sides of the 

 four specimens were so turned that each faced a different 

 point of the compass — east, west, north and south. Subject- 

 ed thus to dift'use stimulus of light from the sky, they all 

 executed curvatures. The specimen whose under side faced 

 the east, became bent towards the east ; the same happened 

 to those which faced north, south, and west, that is to 

 say they curved towards the north, south, and west respec- 

 tively (Fig. 140). The fundamental action by which all 

 these were determined was the induced concavity of the 

 under or normally shaded sitle, which was the more excitable 



