MOVEMENTS OF IRRITATION. 535 



movements cease, and the leaves are no longer able to react even 

 to shock or contact. In observations, however, on plants of 

 Mimosa placed in darkness, it is to be noted that leaves of different 

 ages do not behave in the same manner. When the daily periodic 

 after-effect movements are no longer taking place, the darkness- 

 rigid leaves do not appear in the normal night position, but, on 

 the contrary, the main leaf-stalk in such leaves is nearly horizontal 

 in position, and the leaflets are outspread. If a plant is now 

 for a short time illuminated, and then again brought into the 

 dark, it still does not react to the change of illumination, and to 

 shock or contact; its leaflets do not go together; the plant is 

 still darkness-rigid. The phototonic condition only returns after 

 long-continued illumination, and then only does the Mimosa regain 

 its capacity of reacting to change of illumination; later still it 

 becomes sensitive also to shock or oonta.ct. I kept one plant of 

 Mimosa pudica, a, in the dark, at a temperature of about 20 C., 

 from August 16 to half-past seven in the evening of August 21. 

 On the evening of August 21, all the leaves up to the two youngest 

 were completely darkness-rigid. The plant a was now, with 

 another, 6, which had been exposed to normal conditions of 

 illumination, placed in front of a window. On August 22 the 

 light was allowed to act on the two plants from sunrise till 10 

 o'clock; they were then both placed in the dark, but while the 

 leaves of 6 closed up, those of a remained outspread. The leaves 

 of the plant a also did not yet react to shock or contact; 

 they were still darkness-rigid. After being in the dark for half 

 an hour, both plants were brought back into the light again. 

 On the evening of the 22nd August some of the leaflets of the 

 plant a laid themselves together, and on the 23rd August irrita- 

 bility to contact returned, and also a very energetic closing move- 

 ment of the leaflets took place in the evening. Many leaflets of 

 the plant a, however, during the last days of the research became 

 yellow and fell. 



If plants of Trifolium pratense are observed in the open, or if 

 we keep under observation plants raised from seed in flower-pots, 

 we shall find that the leaflets, which during the day are outspread, 

 lay themselves together in an upward direction in the evening. The 

 leaflets of Oxalis Acetosella, on the contrary, fold together down- 

 wards in the evening. 



1 As to the causes of these complicated phenomena, see Pfeffer, Die period- 

 isclien Bewegungen, etc., 1875, p. 74. 



