ACTION OF THE NERVOUS SYSTEM. 75 



therefore, suitable plants are exposed to the changes 

 of day and night, — the other conditions remaining 

 constant, — if the growth movements of the limbs 

 be measured, it will be found that the growth at 

 night is considerably greater than during the day. 

 Now it is remarkable as Baranetzky has found, that 

 these daily periods of growth do not disappear as 

 soon as the plant is placed in a constant darkness, 

 but on the contrary may be observed for a long 

 time as after-effects. 



" By observing the position of the leaves of 

 mimosa piidica and acacia lophanta, it is found that 

 in the day-time they are spread out, but on the 

 approach of darkness they fold their upper surfaces 

 together. The alternating movements which come 

 here into consideration, have their cause in the 

 change in the conditions of illumination. If vigor- 

 ously developed specimens of mimosa or acacia be 

 brought into constant darkness, the alternating 

 movements continue for days (by my experiments 

 four or five days) ; the leaves are folded together 

 at night-time and spread out in the day-time. 

 Pfeffer shut off all light from specimens of mimosa 

 in the day-time, but illuminated them artificially 

 all through the night. After a long time, the plants 

 under experiment were brought into constant dark- 

 ness ; then they spread their leaves at evening, 



