498 PLANT RESPONSE 



these would be the sudden magnetic pull which would cause 

 a movement of the flower away from the pressed side. This 

 effect would be instantaneous, and quickly come to a stop. 

 The second would be the excitatory effect on the specimen 

 of the unilateral irritation caused by the pressure of the 

 iron particles. If this response, like response in general, 

 were to take the form of a contraction, a curvature concave 

 to the source of stimulus would be produced, by which the 

 flower should be seen to bend towards the stimulated side. 

 This stimulatory effect, unlike the mechanical disturbance, 

 would go on increasing with time. In this way it is easy to 

 demonstrate that unilateral irritation of pressure of particles 

 induces contraction, which in growing organs retards the 



normal rate of growth, 

 the side acted upon 

 thus becoming concave 

 (fig. 206). It should 

 also be borne in mind 

 that in consequence of 



Ftc. 206. Record of R^ponsfve Curvature this responsive con- 

 induced in Criniim under Experimental traction some of the 

 Conditions shown in fig. 205. Magnifica- pvnp ii Pf 4 watpr fi nf 4c 



tion twenty-five times expelled water nncis 



its way to the opposite 

 side, thus increasing its turgidity, with consequent acceleration 

 of rate of growth and convexity of that side. The experi- 

 mental proof of this latter phenomenon will be given in the next 

 chapter. Moreover, in consequence of the curvature induced 

 by this contraction of the excited side, the further side will 

 be stretched, and tension is, as we know, an agent tending to 

 increase the rate of growth. Thus we see that in this case 

 the contraction of the excited side is the active factor, and the 

 convexity of the further side is the secondary or subsidiary 

 effect in the growth-curvature. 



If then such stimulation by unilateral weight-effect, whether 

 statolithic or hydrostatic, be the cause of the apogeotropic 

 curvature induced by gravitation, it follows that the active 

 factor in the process lies in the contraction of the upper side, 



