THE DIFFERENT FORMS OF MOVEMENT 3 



tion 1 . To invent names for each variety of movement such as gamotropic, 

 carpotropic, and the like, as Hansgirg has done 2 , aids nothing in eluci- 

 dating the phenomena in question 3 . 



Curvatures produced by diffused stimuli are aitionastic, those pro- 

 duced by unilateral stimuli are orienting or tropic. The latter to which 

 geotropism, heliotropism, and the other tropic movements belong, are the 

 result of a sense of discrimination and have this in common, that the 

 responding organ assumes a definite position in regard to the direction of 

 the stimulus 4 . 



Both radial and dorsiventral organs respond in this way, whereas 

 a curvature can be produced by a diffuse stimulus in dorsiventral (and 

 anisotropic) organs in which the opposite halves respond by unequal 

 growth and elongation. 



The movements of Mimosa pudica produced by a blow, as well as the 

 contraction of stamens of Cynareae, also take place in a definite direction 

 predetermined by the structure of the organ. The curvatures produced 

 by diffuse stimuli are termed ' nastic,' while by epinasty, hyponasty, and 

 paranasty, the sides are indicated which elongate on stimulation. 



These distinctions only apply to special types of movement between 

 which transitions occur, not only because the two forms of movement may 

 take place at the same time, but also because the same movement may be 

 regarded as ' nastic ' or as tropic, according to the point of view adopted. 

 Thus a curvature which we term ' nastic ' is primarily the result of tropic 

 stimulation, which is awakened in spite of the homogeneous external 

 conditions, owing to the dissimilar physiological properties of the sides of 

 the curving organ. This would be the case if the formation of pigment or of 

 a more opaque cuticle hindered the penetration of light on one side, as well 

 as when one side was smeared with Indian ink. In the same way, a local 

 increase in the permeability of the cuticle might cause stronger transpira- 

 tion on that side, and thus produce hydrotropic stimulation and curvature. 

 Further, equal contact on all sides of a physiologically radial tendril will 

 produce a curvature if a protective layer is interposed on one side so that 

 the stimulation on that side is less pronounced. 



Autogenic curvatures which are produced under constant and homo- 

 geneous external conditions may be termed autonastic, and the single or 



1 Cf. A. P. de Candolle, Physiologie des plantes, a German translation by Roper, 1883, Bd. II, 

 p. 552 ; Dutrochet, Mem. anat. et physiol. d. vegetaux et d'animaux, Bruxelles, 1837, p. 225. 



2 Physiol. Unters., 1893, p. 966. 



3 Pfeffer, Die period. Bewegungen d. Blattorgane, 1875, p. 2. 



4 Oltmanns (Flora, 1892, p. 206) suggests the term 'Photometry' to indicate the power of the 

 plant to respond to light. [The term is inadvisable, since in correspondence with its physical usage 

 it would suggest that plants detect and respond to the intensity of the light alone and not to its 

 direction.] 



B 2 



