pfeffer's experiments. 637 



Both were then placed at the window, where the leaves of the rigid plant again 

 maintained their position unchanged, whereas the previously closed leaves of the 

 normal plant opened again in an hour, the sky being cloudy. In the evening of this 

 day (about 5 o'clock) the lower six leaves still remained stiffly open, the upper ones 

 (8 and 9) closed : next day the periodic movement was completely reinstated. The 

 plant had taken no harm, and is now vegetating actively.' 



In the year 1870 Paul Bert, as Pyrame de Candolle had already done at the 

 beginning of this century, exposed Mimosas to continuous illumination with lamps 

 during the night and ordinary daylight during the day, and found that the extent 

 of the movement of the principal leaf-stalk gradually diminished, but subsequently, 

 under the influence of the daily variation of light, was again restored. The effect of 

 continuous illumination was also investigated in detail by Pfeffer, who employed 

 for this purpose two Argand gas-lamps, which, however, were likewise employed 

 only for illumination during the night, the plants being exposed to diffused bright 

 daylight during the day. Pfeffer expresses himself as follows respecting an investi- 

 gation made in this way with Acacia lophantha : — 



*In continuous illumination the amplitude of the daily periodic movements 

 gradually decreases, and if no autonomous movements affect the experimental 

 objects, the leaves at length become motionless, but are at the same time perfectly 

 sensitive paratonically. In an experiment made with Acacia lophantha, a small pot 

 plant bearing four leaves was kept in diffused light on 13th June, 1873, and 

 illuminated in the evening. On this day the leaflets closed completely ; on the 

 next almost completely; on June 15th the amplitude of the movement of each 

 leaflet was about 70°; on the i6th June 15-35°; ^^ the 17th June 5-20°; and 

 then on June 19th, variations in the brightness being avoided as much as 

 possible, the amplitude was at any rate less than 5°. The periodic movements 

 had thus practically ceased, the leaflets on the older leaves being expanded nearly 

 flat, those on young leaves being inclined towards one another as much as 130°.' 



Plants which have become motionless in the light are just as sensitive to 

 darkness as are plants which have been submitted in the usual way to the daily 

 variation of illumination. 



' In the leaves of the above-named plants,' he continues, ' no autonomous 

 movements can be demonstrated with certainty, but where such exist they continue 

 even when the iliumination is constant, and apparently do not lose in amplitude. 

 The autonomous movements of the leaflets of Trifolium prate7ise are very considerable; 

 the terminal leaflet may accomplish a movement of 30-120 degrees in the course 

 of 1^-4 hours. If such a plant, previously exposed to daily variation of light, is illumi- 

 nated in the evening and thenceforth kept continuously in the Hght, no further 

 closing movement corresponding to the daily period was to be observed even on the 

 following evening, apparently because it was obscured by the autonomous move- 

 ments, which went on with an amplitude of as much as 100 degrees, and with 

 a rhythm of about two hours. These autonomous movements continued also un- 

 changed while the plant was kept constantly illuminated for two days longer. Au- 

 tonomous movements of less amplitude and shorter persistence occur in the 

 primary leaf of Hcdysanim gyrans, in which occasionally an amplitude of vibration 

 of only 8 degrees in a period of ten to thirty seconds is observed. If this plant 



