158 TROPIC MOVEMENTS 



phototropic and thermotropic irritabilities. In addition, the blue and red 

 rays may awaken two different tropic reactions, which co-operate in 

 producing the position assumed in mixed light. 



An orienting stimulus exerts a double action when it induces 

 dorsiventrality in a radial tropic organ, and this induced dorsiventrality 

 may result ultimately in the assumption of a permanently plagiotropic 

 position, as in the thallus of Marchantia. The same thing applies to the 

 prothallus of the Fern, although here the unilateral illumination only 

 induces a labile dorsiventrality. Labile or stable hyponastic or epinastic 

 tendencies may also be induced in connexion with the labile or stable 

 dorsiventrality, and the appearance of the latter may awaken or modify 

 special tropic or nastic powers of response. 



Even when no dorsiventrality is induced, a single agency may exert 

 two dissimilar tropic reactions, as, for instance, in the case of the radial 

 plagiotropic branches of trees, in which the action of gravity appears to 

 excite an epitropic tendency to curvature on the upper side, and a 

 hypotropic one on the under side. Both responses may be regarded as 

 geotropic curvatures, whether they are indirectly or directed excited, or 

 whether the epitropic response follows as a counter-action to the induced 

 tendency to hypotropic curvature. Differences in the times of reaction 

 and induction merely show that dissimilar stimulatory actions are involved, 

 and afford no argument against both being geotropic responses. Every 

 tropic reaction may indeed involve epinasty or hyponasty, unless we 

 elect to restrict these terms to curvatures produced by diffuse stimuli. 

 The hypotropic reaction of the branch does, in fact, appear and disappear 

 more rapidly than the epitropic one on the upper side, so that the existence 

 of the two dissimilar tendencies is readily detected, whereas this would be 

 impossible if the times of induction and the duration of the after-effect 

 were alike in both cases. 



Naturally no curvature results if the antagonistic stimulatory actions 

 balance, and the same applies when the direction of the stimulus alters, 

 provided that the opposing reactions increase or decrease in the same 

 proportion. This must actually be the case in such branches as continue 

 to grow in a new direction forcibly impressed upon them ; for if their altered 

 position in regard to the perpendicular caused unequal geotropic responses 

 to be given by the upper and under sides, the natural result would be to 

 produce a curvature of the branch to its original line of growth where the 

 geotropic actions balanced. Dissimilar tropic agencies or reactions may 

 also antagonize each other, and in the case of an organism which is 

 positively chemotactic and negatively osmotactic to a particular substance, 

 a position of equilibrium is reached at a definite point in the zones of 

 diffusion, owing to the fact that the negative osmotactic action increases 

 more rapidly than the positively chemotactic action with increasing 



