11 



FiGUKK 5. — Leaflets of Erythrina showhig position in extreme sunlight ; a, 

 when leaf midrib is vertical ; b, when it is horizontal. Taken about 

 11 :30 A. M. 



that iiioi'c tiiiH' wniild he (•(Hisiiiiicd. Rcasitns for such a belief are 

 at least two. One is the absence of sueh a powerful stimulus 

 as direct sunligrht. A second is the lack of urgent need for 

 fjuickly altering the ])Osition. since diffuse daylight or darkness 

 are in no way dangerous to the chlorophyll. The cause for the 

 return to the expanded condition may be either the absence of 

 the exciting influence of direct sunlight; or. in case of exposure 

 to diffuse daylight, t** the desire on the part of the leaflt'ts to 

 be at right angles to the light rays; or. if exposed to darkness, 

 to a desire to assume the sleep position. If the first alternative 

 were ti'ue. it would presuppose a neutral position into which the 

 leaflets would fall as soon as the exciting stimulus were removed. 

 Just what this neutral jtosition is. is uncertain. There is consid- 

 erable doubt as to whether such a position exists. The so-called 



