THE MECHANICS OF VARIATION MOVEMENTS 139 



Since transition stimuli may exercise various shock-effects, it is not impossible 

 that in certain plants darkening may excite opposed reactions in the two halves of the 

 pulvinus, or transitory changes may occur without producing any pronounced move- 

 ment or altered rigidity. A slight increase of rigidity appears to be shown by many 

 plants in darkness, but the results which Schwendener l obtained with chloroformed 

 plants are not altogether satisfactory, since the treatment with chloroform slightly 

 increases the rigidity and may exercise other effects as well 2 . The changes of 

 rigidity in Mimosa pudica may be readily followed by working at low temperatures, 

 when the sleep-movements are still performed, but the seismonic irritability is largely 

 suspended. 



The acceleration of growth in darkness is naturally not always alike in all plants 

 or in all parts of these, and the increased activity of growth produced by the with- 

 drawal of light in the convex side of an organ which performs a pronounced nutation 

 curvature in darkness is not of necessity permanent in character, but is in fact usually 

 transitory. As the effect of the stimulus due to the change passes away, the growth 

 assumes the same somewhat enhanced rate in all parts so long as no autonomic 

 modifications ensue. 



Internal factors. It is certain that the modifications of growth pro- 

 duced by light and temperature are not the direct result of changes of 

 turgor, and until the exact way in which these agencies influence growth is 

 known it is impossible to gain any insight into the mode of production of 

 photonastic and thermonastic nutation curvatures. Even in the case of 

 variation movements the increased expansive energy might result from 

 a change in the elasticity of the cell-wall as well as from a rise of turgor. 

 Hilburg 3 was unable to detect any change of turgor in the active pulvinar 

 tissues by plasmolytic methods during photonastic and thermonastic curva- 

 ture, but this might simply be because the changes of turgor are rapidly 

 produced, or are affected by the mode of preparation necessarily adopted. 

 The turgor of the active parenchyma cells sinks after prolonged immersal 

 in water, but not after lying in a solution of potassium nitrate and of a few 

 other salts. Whether this is a question of diffusion, selective absorption, or 

 of some stimulatory action is, however, uncertain, and no light is thrown 

 upon the mechanism of curvature. The geotropic and heliotropic curvatures 

 of pulvini are, however, accompanied by changes of turgor equivalent to about 

 i per cent, solutions of potassium nitrate, according to the same author, so 

 that there appears to be some difference in the mode of production of the 

 variation movement according to the character of the stimulus applied. Even 

 when different mechanisms are in play variation and nutation may co-operate 

 in producing the curvature of a pulvinus, just as geotropism and photonasty 

 may co-operate in certain stems. 



1 Schwendener, Ges. Bot. Mittheil., p. 236. 



2 Cf. Pfeffer, Physiol. Unters., 1873, p. 65. 



3 Hilburg, Unters. a. d. hot. Inst. zu Tubingen, 1881, Bd. I, p. 23. 



