686 PLANT RESPONSE 



a remarkable sight in the early mornings to see all the six, 

 with their upper halves bent over equally to the east, and in 

 the evenings equally to the west. One curious phenomenon 

 connected with this consisted in the fact that not only was 

 the nightly recovery completed by I A.M., but the upper part 

 of the shoot was already carried over to the east, just as we 

 found the Mimosa leaf to be erected to the highest position by 

 midnight in consequence of the presence of internal energy. 



Analysis of constituent impulses causing nyctitropic 

 movement. — Nyctitropic movements are thus brought about 

 by two different periodic factors, themselves induced by the 

 periodic action of light and darkness. These periodic forces 

 acting on the pulvinus are : 



1. The differential heliotropic effect on the pulvinus 

 itself. By the stimulus of light, externally and internally 

 diffused, the dorsi-ventral leaf is progressively depressed 

 during the day. The reverse process takes place at night, 

 by means of natural recovery, aided by internal energy, 

 which gives an impulse opposite to that of external stimulus. 



2. A periodic inflow and outflow of water taking place 

 in the plant as a whole, by the recurrent action of light and 

 darkness. This, acting on the dorsi-ventral pulvinus, causes 

 periodic movement of the petiole. 



3. Both these periodic forces are concordant in their 

 action on the pulvinated organ, and give rise to periodic 

 movements of large amplitude. 



Summary 



The nyctitropic movement of such leaves as that of 

 Mimosa is believed to be distinguished from heliotropic 

 action proper by the facts that (i) it takes place in a definite 

 plane, and (2) it is caused, not by light as a constant force, 

 but by its variation of intensity, the fall of the petiole in the 

 evening being thus ascribed to the on-coming of darkness. 



With regard to the first of these points, I have shown 

 that, even under heliotropic action proper, all dorsi-ventral 

 organs move in a definite plane ; and as regards the second, 



