CHAPTER XLV 



ON PHOTONASTIC PHENOMENA AND ON DIURNAL SLEEP 



Photonasty and para-heliotropism — Response of Tropicolum majits — Responses 

 of plagiotropic stems : {a) Mimosa — (/>) Ipomasa—(c) Cucurbita — Daily periodic 

 movements of plagiotropic stems — Responsive movements of pulvinated organs 

 — Pulvinated organs showing positive heliotropic movement : (a) Response of 

 terminal leaflet of Desmodiiim—{b) Response of leaflet of Robinia — (c) Re- 

 sponsive movements of leaflets of Erythrina indica and Clitoria tematea — The 

 negative heliotropic type of response : (a) Response of pulvinus of Mimosa — 

 (b) Diurnal sleep of Oxa/is—(c) Diurnal sleep of Biophytum —Directive versus 

 non-directive action of light — General view of responsive curvatures induced 

 in different organs by unilateral application of light. 



I SHALL now enter upon the investigation of a very large 

 class of phenomena, brought about by the action of light, 

 which have hitherto been regarded not only as obscure, but 

 also as totally unrelated to each other. These phenomena 

 may be described in a general way as the exhibition of the 

 differential effects of light on anisotropic or dorsi-ventral 

 organs. Such effects may again be divided for convenience 

 into two classes, according as they are exhibited either by 

 growing or by mature and pulvinated organs. The respon- 

 sive curvature in the former of these cases, due to the 

 differential growth induced by light, we shall designate as 

 photonastic. It must be understood that there is in reality, 

 as we shall see, no fundamental difference between the 

 responsive curvatures induced in growing organs, whether 

 radial or dorsi-ventral, and those in pulvinated organs ; but 

 it is nevertheless suitable for the purposes of this investigation 

 to treat them under separate headings. 



Photonasty and para-heliotropism. — In experimenting 

 with dorsi-ventral shoots, De Vries found that when light 

 was applied to the lower or normally shaded surface of, for 



