2 CHAP. I. INTRODUCTORY 



the positive movement is towards, and the negative away 

 from, the stimulus. Strong sunHght brings about a para- 

 hehotropic movement, often described as the ' Midday sleep,' 

 in which the apices of leaves or leaflets turn either towards 

 or away from the source of illumination. In the leaflet of 

 Averrhoa Caramhola the movement is downward, which- 

 ever side is illuminated with strong light ; in the leaflet 

 of Mimosa the movement under similar circumstances is 

 in the opposite direction. Such photonastic movements, 

 apparently independent of the directive action of light, 

 have been regarded as phenomena unrelated to photo- 

 tropic reaction, due to a different kind of irritability with 

 a different mode of response. Pfeffer,^ after enumerating 

 the apparently anomalous effects induced by light, which is 

 only one out of numerous factors in operation, dwells on the 

 inadequacy of the various explanations that have been 

 advanced, and concludes by saying that ' the precise 

 character of the stimulatory action of light has yet to be 

 determined. When we say that an organ curves towards 

 a source of illumination because of its heliotropic irritability, 

 we are simply expressing an ascertained fact in a con- 

 veniently abbreviated form, without explaining why such 

 curvature is possible or how it is produced.' 



Greater difficulties are encountered in explaining the 

 effects of other modes of stimulation. An attempt is made 

 in this volume to discover and explain the common and 

 fundamental reaction which underlies the various responses 

 given by growing organs under all modes of unilateral 

 stimulation which induce tropic curvature. The tropic 

 movements are essentially due to unequal growth induced 

 in the two opposed sides of the organ. What now are 

 the characteristic modifications of growth at the proximal 

 and distal sides of the organ under various modes of 

 stimulation ? Do they induce reactions essentially similar 

 or specifically different ? 



1 Pfeffer, Physiology of Plants, trans, by A. J. Ewart, vol. ii., p. 74 

 (Clarendon Press). 



