544 LECTURE XX. 



Sachs observed, in the case of Mimosa, that the daily period 

 persisted for four days in darkness, and similar observations 

 have been made by Bert. The movements, however, gradually 

 become irregular, owing to the manifestation of the spon- 

 taneous movements. When the plant is kept in darkness 

 the leaves eventually become rigid, but when continuously 

 illuminated the spontaneous movements continue without 

 any apparent diminution of amplitude. 



We are now in a position to consider the fact, to which 

 allusion was made above, that the spontaneous movements 

 are, as a rule, either arrested or much diminished in amplitude 

 under normal conditions of illumination. The daily period- 

 icity of movement is, as it were, so forcibly impressed on 

 the plant that it leads to the more or less complete oblite- 

 ration of the spontaneous movement. The reason why the 

 lateral leaflets of Desmodium gyrans continue their move- 

 ments actively at all times is that they are apparently not 

 sensitive to variations in the intensity of light, for, as Darwin 

 points out, they do not sleep. 



Variations of temperature act as stimuli, though not very 

 powerfully, in inducing movements. Pfeffer remarked that 

 a sudden change from a temperature of 18 to 20 C. to one 

 of 30 to 31 C. caused the leaflets of Oxalis Acetosella to sink 

 through 80 in the course of half-an-hour. So marked a 

 result was, however, only attained when the effect of the 

 variation of temperature coincided with the phase of the 

 daily period, that is, when it was made in the afternoon or 

 evening. When made in the morning the variation of tem- 

 perature produced a much smaller fall, usually not more 

 than 10 or 15. The same effect was produced by a gradual 

 rise of temperature. According to Millardet a rise of tem- 

 perature causes the main petiole of Mimosa to rise, and 

 a fall causes it to sink : the effect of a variation of tempera- 

 ture in either direction on the leaflets is to cause them to 

 close. 



Some of these motile leaves or leaflets, such as those of 

 the Oxalidaceae and the Leguminosae, notably those of the 

 Sensitive Plant, are sensitive to mechanical, electrical, or 



