THE CAUSES OF MOVEMENT u 



sensitive to this stimulus. The ectoplasmic membrane may in fact be specially 

 sensitive to orienting stimuli, but it is uncertain whether stimuli inducing move- 

 ment in the chloroplastids are perceived in the chloroplast itself, and it is very 

 doubtful whether the eye-spot of zoospores is an organ specially adapted for the 

 perception of light. 



The process of sensation is not revealed by the movements or changes in the 

 protoplast which result from or accompany stimulation. Thus the movement of 

 a swarm-spore towards light, or the local accumulation of the cytoplasm or chloro- 

 plastids produced by tropic stimuli, afford no insight into the processes of perception 

 and induction. In many cases local accumulations of the protoplasm form the purely 

 mechanical result of a realized curvature, but in others preparatory processes of this 

 nature may precede or accompany the actual perception of a stimulus. 



Historical. From the beginning of the nineteenth century attempts have been 

 made to explain the causes and mechanism not only of the rapid movements of 

 Mimosa pudica, but also of heliotropic and other growth curvatures. It was naturally 

 only at a somewhat later date that the smaller and less known motile organisms were 

 also drawn into consideration. At first it was attempted to explain the movement 

 as being the direct mechanical result of the exciting stimulus. Thus the partial 

 etiolation of the shaded side of a stem, or the modification of the elasticity of the 

 cell-walls by the direct action of light, were considered to be the causes of heliotropic 

 curvature, while geotropism was supposed to result from the plastic curvature of the 

 root or of the growing apex under its own weight, or to the unequal distribution 

 of food-materials of different densities brought about by the action of gravity. 



The true nature of these complicated manifestations of irritability was therefore 

 not recognized, although Dutrochet 1 in 1824 expressed the opinion that light and 

 gravity were only the inducing causes of heliotropic and geotropic curvatures, and not 

 the direct mechanical agencies in producing them. This author, however, can hardly 

 have thoroughly comprehended the phenomena in question, since at a later date he 

 arrives at direct contradictions to his original principles 2 . Even in the brilliant 

 Experimental Physiology of S^chs 3 the mechanical explanation of the slower growth 

 movements retains the upper hand. Pfeffer in 1877 * pointed out that the move- 

 ments were in all cases the responses of irritable structures to stimuli, and brought 

 the subject up to our present standpoint. The researches of Darwin were of the 

 utmost value in this connexion since they showed that the processes of perception, 

 induction, and movement might take place some distance apart 5 . 



Darwin 6 considered all curvatures to be modified forms of circumnutation, but 



1 Dutrochet, Rech. s. la structure intime d. animaux et d. vegetaux, 1824, pp. 107, 117, 

 130, &c. 



3 Dutrochet, Me"m. anat. et physiol. d. vegetaux etc., 1837. 



3 Sachs, Experimentalphysiologie, 1865. 



4 Pfeffer, Osmot. Unters., 1877, p. 202; Pfeffer, Pflanzenphysiologie, 1881, Bd. I, p. 3; Bd. II, 

 pp. 117, 178, 286, 327 u. s. w. Sachs, Vorlesung iiber Pflanzenphysiologie, 1882, p. 71 7, then pointed 

 out the general character of irritability, but was wrong in supposing that for every irritable response 

 a labile condition is essential. Cf. also Pfeffer, Die Reizbarkeit d. Pflanzen, 1893, p. 10 (Reprint 

 from the Verb. d. Ges. detitscher Naturf. u. Aerzte zu Nurnberg). 



5 Darwin, Insectivorous Plants, 1875 ; The Power of Movement in Plants, 1880. 



6 Darwin, The Power of Movement in Plants. Darwin himself doubted whether the movements 



