MOVEMENTS OF IRRITATION. 539 



have recovered, they are sensitive to stimulation. The free fila- 

 ments move when touched, and after executing movements regain 

 their irritability again under favourable external conditions in a 

 few minutes. The movement of the free filaments is due to the 

 fact that the side touched shortens and always first becomes con- 

 cave. 



204. Spontaneous Movements of Variation. 



Spontaneous movements of variation are to be observed in 

 various plants whose leaves are provided with pulvini (Mimosa, 

 Oxalis, Trifolium), and are brought about by fluctuations in the 

 turgor-expansion of the cells of the antagonistic parts of the 

 pulvinus, due to internal causes. In order to acquire information 

 on the subject, we select for examination Trifolium pratense, and 

 cultivate the plants from seed in a flower-pot. When the plants 

 are fairly advanced in development it is best to reject the weakly 

 individuals and retain for observation only a few vigorous speci- 

 mens. 



It has already been mentioned that leaves of Trifolium react 

 very vigorously by movements to change in conditions of illumina- 

 tion. In order to exclude these movements as far as possible, 

 we bring the flower-pot with the Trifolium plants into a dark 

 cupboard, or place over it a cardboard 

 box, and let the plants remain in the 

 dark also on the next day, on which the 

 observations proper are to begin. From 

 time to time, say every half-hour, we de- 

 termine the position of certain of the 

 leaflets, and indeed it is best to fix their 

 position at any time by means of a sketch. 

 Numerous experiments which I made at Flo 184- _ Leaf of De8m o. 

 a temperature of about 18 C. showed me flium gyraus, reduced in size. 



,, fi , a , w - f (After Pfeffer.) 



that the leaflets of Trifolium pratense 



executed in the dark upward and downward movements, the 

 amplitude of which was sometimes less, sometimes more than 

 90. A few hours were necessary for the performance of a com- 

 plete oscillation. 



Very marked autonomous movements of variation are executed 

 by the lateral leaflets of Desmodium gyrans. They describe 

 elliptical paths, whose long axis is approximately parallel with 

 the main leaf-stalk. The temperature minimum for these 



