26 MOVEMENTS OF CURVATURE 



be discussed later. At present we have merely to deal with the fact that 

 certain spontaneous movements are produced by the autogenic utilization 

 of external factors for directive purposes. 



This applies to ephemeral as well as to periodic movements, which can 

 be produced in plants as well as in animals, although hardly in so striking 

 a form as the respiratory movements or the pulsation of the heart in mam- 

 mals l . All organic life is a repetition in the individual of the course of 

 development of the parents, and we have mechanical instances of rhythm 

 in clocks, and in the movements of planets under the action of constant 

 external conditions. Similarly the rhythmic beat of the interrupter of an 

 induction machine is dependent upon an external agency (gravity) when the 

 interrupter falls back by its own weight, but solely upon the inherent 

 properties of the mechanism when the break is effected by an elastic 

 spring. 



Each motile organ possesses a considerable degree of independence 

 as regards the inception and performance of movement. Thus similar and 

 dissimilar organs of a plant may perform various movements simultaneously, 

 and even the leaflets on the same leaf of Oxalis, Trifolium^ or Mimosa may 

 move in different directions at the same time. This can be very strikingly 

 shown by shading the pulvini of some of the leaflets, while the remainder 

 are exposed to bright sunlight so that they fold up. On now exposing to 

 slightly weaker general illumination the expanded leaflets fold up, while the 

 folded ones partially re-expand 2 . In addition, similar organs of a plant do 

 not always respond alike to the same stimulus, owing to autogenic modi- 

 fication of the responding organs. The resulting movement is in fact 

 always due to the conjoint action of external and internal factors, some- 

 times the latter and sometimes the former predominating 3 . 



It is often the case that an organ performs a spontaneous curvature 

 and assumes a new direction of growth as the result of a change of its 

 geotropic irritability, the external conditions remaining unaltered. The 

 part played by gravity in such cases is readily ascertained by the aid of 

 the klinostat, and in fact a large number but not all of the autogenic 

 tropic movements performed by plants require the aid of gravity. When 

 a factor such as light undergoes continual change as regards direction 

 and intensity, observations in nature often suffice to determine the part 

 it plays in a particular movement, but it is only under light of constant 

 intensity and direction that a satisfactory decision can be made as to 

 whether the stimulus of light is involved in a particular autogenic move- 

 ment. That periodic movements may occur under such conditions is 



1 On the production of rhythm by periodic changes in the external conditions cf. Darwin and 

 Pertz, Annals of Botany, 1892, Vol. vi, p. 245. 



a Ewart, The Effects of Tropical Insolation, Annals of Botany, Vol. XII, 1898, p. 448. 

 3 Cf. Pfeffer, Periodische Bewegungen, 1875, pp. 35, 153. 



