4 MOVEMENT 



periodic movements resulting from internal non-homogeneous stimuli ex- 

 changes may be called autotropic. An autonomic movement resulting 

 from a change of the geotropic irritability affords, however, an undoubted 

 instance of tropic stimulation and would not be shown on a klinostat. 



Further, the induction of a labile dorsi ventral ity by unilateral illumina- 

 tion produces the conditions for a photonastic reaction, and it is by no 

 means easy to resolve the combination into its component factors. Indeed, 

 all geotropic or phototropic curvatures may be regarded as the result of 

 epinastic or hyponastic properties induced temporarily by the unilateral 

 action of gravity or light. 



There is, however, no necessity for rigid restriction in discussing these 

 phenomena. Thus, the tropic movements of tendrils may be treated 

 together with other adaptations for climbing, while various 'nastic' re- 

 actions will be first mentioned in connexion with the tropic orienting 

 movements. Furthermore, the mechanisms for dehiscence and active 

 dispersal are of economic importance, but of special character, and often 

 not vital phenomena. 



SECTION 2. The Causes of Movement. 



All these movements are produced in response to stimuli of either 

 internal or external origin. The first indication of a motile irritability is 

 afforded by the realized movement, which forms at the same time an 

 evidence of a power of perception. No movement is possible, however, if 

 a block or gap occurs in the chain of processes intervening between 

 perception and response. 



In plants whose sensory and motor parts are some distance apart, the 

 destruction of the sensory organ, or a break in the path of the stimulus, 

 prevents response. Similarly no movement is possible if the responding 

 organ has lost its power of movement, so that parts which have ceased 

 to grow lose their motility, if they are only capable of growth-curvatures. 

 A power of perceiving stimuli might, however, still be present, although the 

 power of reacting to them appears to be absent. 



In many such cases the processes of perception and induction appear 

 to take place as before, as is evidenced by the fact that the rapidity and 

 amount of response decrease when growth is enfeebled, but stop usually 

 only when growth ceases and may again become perceptible if it is 

 reawakened. 



The power of movement in adult organs depends largely upon their 

 structure and upon the properties of their cell-walls. Thus a fall of turgor 

 which produces a pronounced shortening in the stamens of Centaur e a or 

 a curvature in the pulvinus of Mimosa does not cause any perceptible 

 change of shape or curvature in the filament of a Spirogyra^ or in the 

 branch of a tree. 



