206 BRUCKE'S EXPERIMENT. [CH. vm 



(237) Oxalis: Brucke's experiment 1 . 



Oxalis acetosella provides convenient material for the 

 repetition of Briicke's classical experiment, by which it may 

 be shown that the rigidity of the pulvinus diminishes after 

 stimulation. Pfeffer's method 2 is to fix the stalk of the 

 leaf in a bottle of water by means of a bored cork, which 

 is cut into a cone at its upper end so that the cork may 

 come close up to the pulvinus and yet allow the leaflets 

 room to fall. We prefer to cement the petiole to a 

 vertical pin fixed into an ordinary flat-topped cork. A 

 needle-point to act as an index is fixed at the heart-shaped 

 extremity of the leaflet so as to form a continuation of 

 the midrib. According to our observations the needle 

 should be fairly heavy or the excursions of the leaflet are 

 not large enough. A C-shaped piece is cut out of card, 

 which is made to serve as a graduated arc, and it 

 is fixed to the bottle so that the position of the leaflet can 

 be recorded by means of the graduations. The first 

 reading must be taken when the leaf is in its normal 

 position, the bottle being held so that the leaflet is 

 horizontal. The bottle is now turned over so that the 

 leaflet is still horizontal but upside down, and a second 

 reading is taken. If the pulvinus were absolutely rigid 

 the first and second readings would coincide ; as it is 

 they differ by 5 15. The leaf being replaced in the 

 first position, the pulvinus must be irritated by rubbing it 

 on the lower surface, or by a blow or two on the leaflet. 



1 Miiller's Archiv fur Anatomic und Pliysiologie, 1848, p. 434. 



2 loc. cit. p. 74. 



