624 PLANT RESrONSE 



autumn, as observed by Darwin, passes into the exhibition of 

 a strongly negative heliotropic condition in summer. 



Responses of plagiotropic stems. — It is found that 

 under feeble diffuse illumination the Gourd-plant iCuciirbitd) 

 for example, grows erect. In the open, however, if, on 

 account of its own weight, or by the action of the wind, it 

 is once made to bend, the stem is brought into a position 

 where its upper surface is constantly acted on by strong 

 light, whilst its lower is shaded. By the continuous action 

 of the stimulus of light the upper surface is now rendered 

 less excitable, and a permanent anisotropy (plagiotropism) 

 is induced, such as we saw transiently exhibited in the 

 hypocotyl of Sinapis (p. 609) under the short experimental 

 exposure of a specimen to intense unilateral illumination. It 

 has already been shown in Chapter VII. that such plagio- 

 tropic stems are more excitable on their lower or shaded 

 side, and that under diffuse stimulation response is by con- 

 cavity of that side. Hence a recumbent plagiotropic stem 

 of this description, acted on by strong vertical light, which 

 becomes internally diffused, will always exhibit concavity of 

 the lower surface, in consequence of which it will be closely 

 pressed against the ground. 



Such a plagiotropic organ, then, acted upon dorsally, 

 shows a concavity of the ventral surface, or a negative helio- 

 tropic effect ; but if the ventral, or normally shaded, side 

 be itself acted upon directly by light, the result will still be 

 the concavity of the lower or more sensitive surface — that is 

 to say, a positive heliotropic effect. In the former case, then, 

 we have an example of the differential response of an aniso- 

 tropic organ to diffuse stimulation, by concavity of the more 

 excitable ; and in the latter, the direct contraction and con- 

 cavity of the surface acted upon, which happens in this case 

 to be also the more excitable. This will explain why, in the 

 midribs of leaves, in plagiotropic shoots of Lysiniachia, and 

 in the thallus of Marchantia, we always observe the concavity 

 of the shaded and more excitable side, in response to the 

 action of strong light, whether it is applied from above or below. 



