CH. VIIl] OXALIS. 205 



the other end of the thread being attached to the pen- 

 dulum of a metronome. In this way a rhythmical series 

 of shocks are applied. 



(236) 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 1 ; 

 0. rosea is remarkably sensitive. During the day the 

 three leaflets are spread out horizontally ; 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 prolonged shaking of the 

 flower-pot is needed to produce a good effect. Individual 

 leaflets may be stimulated by rubbing the under-surface 

 of the pulvinus. They recover slowly, as much as three- 

 quarters of an hour or an hour being sometimes necessary. 

 No transmission of stimulus from one leaflet to the next 

 has as far as we know been observed. If leaves are placed 

 in alcohol in the expanded and also in the contracted 

 condition they retain their respective positions, according 

 to Pfeffer, and if median longitudinal sections are made, 

 the disappearance of the furrows on the upper side and 

 their increase on the lower surface of the pulvinus can be 

 seen in the contracted as compared with the expanded 

 leaves 2 . 



1 Pfeffer's Physiologische Untersuchungen, 1873, p. 69. 



2 See Pfeffer, loc. cit. p. 70 : see also his figures 5 and 6. 



