MECHANISM OF SPONTANEOUS PERIODIC MOVEMENTS. 895 



in the rigidity of the part, and therefore with an increase in its turgidity; while 

 in that caused by irritation there is a decrease in all these, as Briicke was the first to 

 show in the case of Mimosa. In the leaves of Phaseolus which are not irritable 

 to concussion Pfeffer also found an increase of rigidity in the nocturnal position. 

 Conversely the diurnal position caused by the action of light or an increase in its 

 intensity is the result of a diminution of the rigidity or turgidity. The effect pro- 

 duced upon the turgidity of motile organs by variations in the intensity of light 

 causes upward and downward curvatures, since the variation of the turgidity of the 

 one side of the organ is more considerable and occurs more rapidly than that of 

 the other. A rise of temperature, on the contrary, which affects the motile part 

 directly, is, according to Pfeffer, associated, in Oxalis, and in a less degree in 

 Phaseolus^ with increase of rigidity, and therefore also of turgidity, and causes a 

 movement towards the nocturnal position, and hence a stronger turgidity of the 

 upper side. When, on the other hand, an increase in the intensity of the light and 

 a rise of temperature act on a contractile organ at the same time, its curvature is 

 a resultant of the two changes ; according as the one or the other preponderates, 

 the leaf approaches more nearly the diurnal or the nocturnal position. Beyond this 

 we have less certain knowledge as to the action of variations of temperature, than 

 we have with reference to light. 



Pfeffer, who has been especially engaged for a considerable time in the study of the 

 mechanism of the movements produced by stimulation, has supplied me with the follow- 

 ing : — ' The tendency to expand is increased by darkness equally in both the antagonistic 

 halves, and in the tissues of the organ generally : light has the contrary effect, and the one 

 half always reacts more powerfully than the other. The expansive force increases more 

 rapidly in the half which is becoming convex, but it may become more considerable in 

 the other half: it is for this reason that every movement produced by removal into 

 darkness is followed after a time by a movement in the opposite direction, tending to the 

 resumption of the position of equilibrium. It is quite certain that darkness not merely 

 produces a closure but that it has a persistent effect, just as the movement imparted to 

 a pendulum persists for a time whilst the amplitude of the oscillations is rapidly 

 diminishing.' 



Bert ^ showed that if a Mimosa be continuously exposed to light for five days, the 

 amplitude of its periodic movements diminished considerably, whereas the irritability 

 increased. Pfeffer also found that continuous illumination for a period of one or 

 more days arrested the daily periodic movements of Acacia lophantha. If a plant so 

 treated be placed in the dark, closure takes place and then opening, and in continuous 

 darkness for one or more days opening and closing repeatedly alternate. When the 

 plant had been exposed to a strong light before being placed in the dark, the interval 

 between the movements of opening and closing was about 24 hours. This is also the 

 case if the plant be placed in the dark in the morning, so that the first closure takes place 

 during the day. Pfeffer regards these phenomena (like those of motile flowers and 

 of growing leaves) to be due to the persistent effect of the previous alternation of day 

 and night; they are not to be confounded with the spontaneous periodic movements 

 of these plants, for these latter continue when the former have disappeared, and the 

 interval at which they occur is shorter. 



Sect. 31. — Mechanism of spontaneous periodic Movements. The ex- 

 istence of spontaneous periodic movements which are not directly produced by the 



1 Bert, Mem. de I'Acad. d. Sci. Phys. et Nat. de Bordeaux, 1866, Bd. VIII. 



