MIMOSA 



53 



another danger to the plant, consisting in an 

 extremely rapid radiation of heat and accom- 

 panying loss of water. This threatened in- 

 jury is again avoided by deflection of the 

 blades from the horizontal to the perpen- 

 dicular. The movement begins at or near 

 sunset and the laminae slowly rise until their 

 upper surfaces are appressed. The evapora- 

 ting surface is thus reduced exactly one-half, 

 while the radiation of heat is much less from 

 a vertical plane than a horizontal one. Ref- 

 erence to the accompanying illustrations will 

 show that minor movements have occurred. 

 A movement of the pulvinus at thebaseof the 

 petiole results in placing that member hori- 

 zontally. 

 The ''night," ''sleep" or nyctitropic move- 

 ments occur daily while 

 those to avoid the ef- 

 fects of the hot sun are 

 exhibited only when a 

 certain intensity is at- 

 tained. The regular 

 daily recurrence of the 

 conditions which cause 

 the nyctitropic move- 

 ments has fixed them 

 so firmly in the plant 

 that they occur at 

 ^'''" ^" of'mmo^a.''''^''''' rhythmic intervals re- 



Reaction 

 to shock^ etc* 



