190 bkucke's experiment. [ch. viii 



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'- 



(238) Oooalis: Brucke's experiment''. 



Oxalis provides convenient material for the repetition 

 of Brucke's classical experiment, by which it may be 

 shown that the rigidity of the pulvinus diminishes after 

 stimulation. PfefFer's method' 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 move. 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 big 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 



' See Pfefter, loc. cit. p. 70. See also his figures 5 and 6. 



2 Muller's ArchivfUr Anatomie und Physiologie, 1848, p. 434. 



3 loc. cit. p. 74. 



