53$ LECTURE XX. 



With regard to the action of light, it is true that, as men- 

 tioned above, the spontaneous movements are exhibited 

 when the plant is kept in darkness, but, if the period be 

 prolonged, the movements cease altogether, as Sachs and 

 others have ascertained. Clearly, the tonic influence of 

 light, the state of phototonus, is essential to the movement 

 of these leaves, just as it is essential, as a rule, to the growth 

 of leaves (p. 380). Long continued drought also leads to 

 the arrest of the spontaneous movements. The effect of pro- 

 longed exposure to extremes of heat and cold, to darkness, 

 and to drought, has been investigated by Sachs in the case of 

 Mimosa pudica. Under all these circumstances the spon- 

 taneous movements are arrested, a state of complete immotility 

 being induced. 



Leaving, now, the spontaneous movements of variation, 

 we pass to the consideration of those which are induced 

 by the action of stimuli, and we will begin with those induced 

 by variations in the intensity of light. It may be stated 

 generally that a marked diminution in the intensity causes 

 what is termed a movement of closing, and a marked in- 

 crease, a movement of opening, that is, the assumption of 

 a position in which the leaves are fully expanded. This 

 may be well seen in the case of the Sensitive Plant (Mimosa 

 pudica) ; when it is removed into darkness its leaflets close, 

 and when it is again brought into the light its leaflets open. 

 The effect is not, however, immediate in either case ; in this, 

 as in all other cases of the action of stimuli, there is a latent 

 period, a time, that is, which intervenes between the action 

 of the stimulus and the first indication of its effect. In the 

 Sensitive Plant this period is relatively short, and it is on 

 this account that this plant is especially suitable for the 

 purpose of experiment. A peculiar effect of an increase 

 in the intensity of light on the position of leaves was first 

 observed by Cohn, and subsequently studied by Batalin 

 and by Pfeffer, in the case of Oxalis Acetosella. When a 

 plant with expanded leaves is brought from diffuse daylight 

 into bright sunlight, its leaves fall down, just as they do 

 when the plant is placed in darkness, and they retain that 



