PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 625 



*P-H. 



{a) Mimosa. — This fact, that in plagiotropic stems under 

 the diffuse stimulation of light it is the more excitable 

 shaded side which becomes concave, I have been able to 

 demonstrate by numerous other experiments ; for example, 

 taking four creeping stems of Mimosa, I tied them in such a 

 manner that their free ends should be vertical. The shaded 

 sides of the four specimens were turned so as to face each a 

 different point of the compass 

 — east, west, north, and south. 

 Subjected thus to the diffuse 

 stimulation of light from the 

 sky, they all executed curva- 

 tures. The specimen whose 

 under side faced the east be- 

 came bent towards the east. 

 The same happened to those 

 which faced north, south, and 

 west — that is to say, they 

 became curved towards the 

 north, south, and west re- 

 spectively. The fundamental 

 responsive action by which 

 all these were determined 

 was the induced concavity of 

 the under, or normally shaded, 

 side, which is the more excit- 

 able. 



{b) Ipomcea. — Another ex- 

 ample was that of the creeping 

 stem of Ipomiva. This I tied 

 up vertically with its end free, at 10 A.M., the normally 

 shaded side being represented in the diagram to the left 

 (fig. 249). For purposes of the record I placed behind 

 it a piece of paper, on which its different positions were 

 traced from time to time. It will be seen that by 1 p.m 

 it had become considerably curved, the normally shaded 

 side being concave. This concavity had become still more 



s s 



FlG. 251. Response to Diurnal Light 

 and Darkness of Plagiotropic Stem 

 of Iponnva held vertical 



The curvature is seen to increase with 

 the progress of day, the shaded side 

 to the left becoming more and more 

 concave. The dotted figures repre- 

 sent positions of gradual recovery 

 at night. 



