528 PHYSIOLOGY OF GROWTH. 



It is a very interesting fact that plants of Acacia lophanta which 

 have been exposed to normal conditions of illumination, and are 

 then completely removed from the action of the light by being 

 placed in a perfectly dark place {e.g. a cupboard), nevertheless con- 

 tinue to execute the periodical movements of the leaves induced 

 under normal conditions by the daily recurring alternation of light 

 and darkness. A particularly vigorous plant of Acacia lophanta 

 growing in a small flower-pot, and kept in constant darkness for 

 four days, expanded its leaflets in the daytime, and laid them 

 together at night ; the amplitude of the movement, it is true, 

 gradually diminished, and at the end of four days the after-effect 

 movement completely ceased ; the leaflets had become darkness- 

 rigid, and the leaflets of the older leaves were now horizontal in 

 position, while those of the younger leaves were more or less up- 

 wardly directed. When the plant was again exposed to normal 

 conditions of illumination, the leaflets again became phototonic, 

 i.e. they reacted anew to the alternation of day and night. 



If we wish to study accurately the periodic after-effect move- 

 ments executed by leaflets of Acacia plants which have first been 

 exposed to normal conditions of environment, and then kept in 

 constant darkness, we proceed thus, following Pfeffer. We cut 

 out of stiff paper a number of triangles of different but known size 

 of angle. It is sufficient if successive triangles differ from each 

 other by an angle of say ten degrees. The triangles are held 

 between the leaflets in order to determine their inclination towards 

 each other, and if the observations on a particular leaf of an 

 Acacia plant kept in darkness are repeated frequently in the 

 course of the day (say every two hours), we shall obtain fairly 

 accurate information as to the history of the periodic after-effect 

 movements. 1 



See Pfeffer, Die periodischen Bewegunyen der Blattorgane, Leipzig, 1875. 



199. Experiments with Phaseolus multiflorus. 



In the leaves of Phaseolus, as in those of Acacia lophanta, 

 periodic movements are to be observed, which owe their origin 

 to the daily recurring alternations of illumination. The main 

 leafstalk rises in the evening and sinks in the morning, while 

 the three leaflets (see the adjoining illustrations, Figs. 180 and 



