MECHANISM OF SPONTANEOUS PERIODIC MOVEMENTS. 
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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 Brücke 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 Oxalts, 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. Sei. Phys. et Nat. de Bordeaux, 1866, Bd. VIII. 
