PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 627 



of daylight by 10 A.M. the stem is seen to have taken up a 

 strongly curved position, with concavity of the shaded side. 

 This process is seen from the figure to have been progressive, 

 but on the approach of darkness there was recovery, the 

 return being considerable at 7 P.M., and still greater at 

 10 P.M. It may be stated here that such recoveries are never 

 quite complete, part of the curvature being fixed by growth. 

 Thus in a naturally growing plagiotropic stem, the portion 

 which was one day slightly lifted above the ground is on the 

 next, owing to this residual effect, made to lie closely against 

 it, so that by the action of light the growing stem is pressed 

 progressively closer to the earth. 



Daily periodic movement of plagiotropic stem. — From 

 this demonstration of light-curvature and recovery, it will be 

 seen that the free organ has a daily periodicity in virtue of 

 which, in the daytime, it moves gradually downwards, and 

 at night, owing to recovery, upwards. In this plagiotropic 

 stem, then, we see, as will be shown later, the first induction 

 of that nyctitropic movement which is more strikingly dis- 

 played in dorsi-ventral leaves. 



Responsive movement of pulvinated organs. — The 

 demonstration which I have just made of the peculiar 

 responses of anisotropic organs to the stimulus of light, as 

 exemplified in the case of plagiotropic stems, will be found 

 still more strikingly applicable to pronouncedly dorsi-ventral 

 organs like pulvini. In the last-named instance, indeed, as 

 motility is great, the investigation has the advantage of con- 

 cerning itself with responsive effects which take place very 

 quickly. 



From what has already been said, we shall be prepared 

 to meet with two different types of responses, according as 

 the transverse conductivity of the organ is feeble or consider- 

 able. In the former of these cases, under unilateral stimu- 

 lation, there is no internal diffusion of stimulus, and the side 

 acted on, whether above or below, will respond by concavity ; 

 but in the latter case — that is to say, where the transverse 

 conductivity of the tissue is great — it will be the more excit- 



