CH. VIIl] OXALIS. 189 



blows are thus applied, the long arm will make corre- 

 spondiQg beats upwards. These can be applied to the 

 leaf of the Mimosa by a pin fixed at the end of the 

 lever so as to strike the pulviuus across its longitudinal 

 axis. The rhythmical blows are applied to the short end 

 of the lever by drops of water falling from a height of 

 30 cm. at the rate of 8 or 10 per minute. 



With this arrangement the leaf falls at first, but 

 as the blows are continued the petiole rises, and after 15 

 minutes is insensible not only to the light blow of the 

 lever (otherwise it would not have risen) but also to a 

 more severe disturbance. 



The method employed is a slight modification of 

 Pfeffer's^. 



(237) Oxalis acetosella. 



In the absence of Mimosa pudica the irritable leaves 

 of Oxalis acetosella or of some of the other trifoliate 

 species such as 0. stricta or corniculata should be studied 2. 

 During the day the three leaflets are spread out hori- 

 zontally; when irritated they' sink downwards and may 

 move through as much as 90°, though when not perfectly 

 irritable or when not strongly stimulated the movement 

 is often much less. They are not nearly so sensitive as 

 the leaves of Mimosa, and a repeated or somewhat pro- 

 longed shaking of the flower-pot is needed to produce a 

 good eSect. Individual leaflets may be stimulated by 

 rubbing the under surface of the pulvinus. They recover 



' Physiologische Untersuchungen, 1873, p. 57. 



^ PfeSer, Physiologische Untersuchungen, 1873, p. 69. 



