IRRITABILITY. 537 



leaflet of Hedysarum, leaves of Mimosa pudica, of Trifolium 

 incarnatum and pratense, of Oxalis Acetosella, and others. 

 But the movements are not to be readily observed under 

 ordinary conditions, inasmuch as they are masked by the 

 movements which these leaves execute, as will be subsequently 

 described, under the influence of those variations in the in- 

 tensity of light which normally take place in the course of the 

 twenty-four hours. The spontaneous movements can only be 

 made manifest, either by keeping the plant for some time in 

 darkness, or by exposing it continuously, as de Candolle, Bert, 

 and Pfeffer have done, to artificial light of uniform intensity. 

 Again, it appears from Darwin's researches, that the cotyledons 

 of many plants belonging to these Natural Orders possess 

 pulvini, and it is therefore probable that they too perform 

 spontaneous periodic movements of variation. 



As illustrations of these spontaneous movements the following obser- 

 vations of Pfeffer's may be given. When exposed to continuous illumi- 

 nation a terminal leaflet of Trifolium pratense executed movements 

 through from 30 to 120 in periods of from \\ to 4 hours. The terminal 

 leaflet of Hedysarum (Desmodium) performs more rapid but less extensive 

 movements : it has been observed to move through 8 in from 10 to 30 

 seconds. 



The performance of spontaneous movements of variation 

 is dependent upon a combination of favourable external 

 conditions. These conditions are, a suitable temperature, 

 exposure to light, a supply of free oxygen, and a supply of 

 water. With regard to the relation between the activity of 

 movement and the temperature, it is doubtless essentially the 

 same as in the case of all other functions : that there are, 

 namely, minimum and maximum temperatures at which 

 the movement is arrested, and between these an optimum 

 temperature at which it is most active. For example, ac- 

 cording to Kabsch, the lateral leaflets of Desmodium do not 

 move at temperatures below 22 C. : at 28 to 30 C. the cycle 

 of movement occupies about four minutes; at 35 C. it 

 occupies from 85 to 90 seconds : at higher temperatures, 

 probably, though the observations were not made, the 

 movement would be less active, and finally would be arrested. 



