HYDRONASTIC MOVEMENTS 119 



The former action alone is sufficient to explain the decrease in or cessation of the 

 power of reaction as the result of injecting the intercellular spaces with water '. 

 The growth and development of many plants are strongly affected by submersion in 

 water, and hence it is not surprising that, according to Hansgirg, certain flowers 

 which remain closed under water open when placed in air saturated with moisture, 

 although the turgidity remains at its maximal limit. In addition, the leaves of 

 Callitriche assume different positions in moist air to what they do in water 2 . 



SECTION 26. Conjoint Effects. 



Changes of temperature and of turgor always influence to a greater or 

 less extent the progress and amplitude of the photonastic daily movements, 

 either owing to their influence upon the power of response, or to their 

 awakening special thermonastic or hydronastic reactions. In addition, the 

 induced after-effects may cause a periodic repetition of the movements, and 

 this tendency acts in the same way as that to a movement of autonomic origin. 

 The simplest series of combinations is given when only the illumination 

 varies, the other conditions remaining constant, so that the daily movements 

 are due to the co-operation of the photonastic responses to changes of 

 illumination with the periodic after-effects which, under normal conditions, 

 follow approximately the same rhythm. 



The degree to which the directly induced closing or opening movements 

 exceed that due to the after-effect of previous stimulation will depend 

 upon the readiness of the plant's photonastic response, and upon the intensity 

 of the after-effect. Naturally, however, the full possible movement may 

 not be shown when the different responding organs press against one 

 another. Both the after-effect and the original photonastic response involve 

 oscillations about the ultimate position of equilibrium, and hence action 

 excites reaction. It depends upon the time period of the after-effect of the 

 photonastic reaction whether the maximum movement is attained imme- 

 diately after sunrise or sunfall, or later on in the day or night, and also 

 whether the opening and closing of flowers is rapidly or slowly induced. 



On cloudy days the photonastic reactions are feebler than usual at 

 morning and evening, so that the amplitude of the daily movements is con- 

 siderably reduced when the after-effects are less active than the direct 

 photonastic reactions. Naturally also, a plant placed in darkness from 

 morning onwards will perform less pronounced sleep -movements, or will 

 take longer to produce them, than one illuminated during the day and 

 hence strongly stimulated by the failure of the light in the evening. These 

 and similar consequences follow naturally from the facts put forward by 



1 Pfeffer, Physiol. Unters., 1873, PP- 75, 9^, 188. 



2 Frank, Cohn's Beitrage zur Biologic, 1872, Bd. I, Heft 2, p. 80. 



