736 PLANT RESPONSE 



stimulus by positive curvature ; whereas in summer strong 

 unilateral stimulation is transversely conducted, and induces 

 negative responsive curvature. 



Heliotropic response in plagiotropic and dorsi-ventral 

 organs. — We have seen that negative response is brought 

 about in a radial organ by induced anisotropy, and transverse 

 conduction of stimulus. Effects fundamentally similar are 

 seen in organs which are characterised by a natural aniso- 

 tropy. A connecting link between this transient induced 

 anisotropy of radial organs, and the permanent anisotropy of 

 a dorsi-ventral pulvinus, is afforded by plagiotropic stems, in 

 which anisotropy has become more or less permanent, owing 

 to the long-continued unilateral action of vertical light. 

 Two different types of response are exhibited by anisotropic 

 organs, depending on their transverse conductivity and on 

 the relative excitabilities of their two sides. In the first of 

 these, transverse conductivity being feeble, vertical illumina- 

 tion remains localised, and induces positive response. This 

 is the true explanation of the so-called diurnal sleep, with 

 upward folding of the leaflets, of Robinia, Erythrina indica, 

 and Clitoria tematea (p. 629). In the second type, the 

 stimulus of vertical illumination is transmitted to the more 

 excitable distal side, inducing concavity of that side, and 

 consequent negative response. The different stages of this 

 effect are well seen in Mimosa, when the stimulus of light 

 acts on the dorsal or upper side of the pulvinus. Here the 

 immediate effect is a positive response or erection of the leaf; 

 and as the stimulus percolates to the distal side, this effect is 

 neutralised and converted into an increasingly negative 

 response. The greatest degree of negativity or fall in nature 

 is thus attained by the cumulative action of the whole day's 

 illumination. Such is the response which is characteristic of 

 the second type. The action of strong vertical light in such 

 cases induces movement downwards. And this is seen in 

 plagiotropic stems like those of Cuciirbita and Iponuva ; in the 

 thallus of Marchantia.) and the midribs of various leaves ; and 

 in the so-called diurnal sleep, with downward folding of the 



