284 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



day from that assumed at night. Their leaflets approach each other at nighl 

 and the petiole frequently falls. This phenomenon is called nyctitropism 01 

 night movement of the leaves. The falling of mechanically stimulated leaflets 

 of Mimosa is caused by decreased turgidity in the lower half of the pulvinus, but 

 the night movement of Mimosa leaves is the result of increased turgidity in the 

 upper half of the pulvinus. Artificial shading also causes nyctitropic move- 

 ments in leaves. In Fig. 160 two branches of Desmodium gyrans are shown, 

 the leaves of one in the day position and those of the other in the night position. 

 The approach of night causes the leaves to fall and lie against one another. 



