MOVEMENTS OF IRRITATION. 527 



apex and leaves on the one hand, and the way in which the 

 development of the buds proceeds, on the other. 



1 See Sachs, Lectures on Plant Physiology. 



2 See Voehting, Die Bewegungen der Bliithen und Friichte, Bonn, 1882. 

 8 See Gobel, Botan. Zeitung, 1880, pp. 771 and 807. 



V. MOVEMENTS OF VARIATION. 

 198. Experiments with Acacia lophanta. 



Movements of variation, for the most part effected by means of 

 special pulvini, are characteristic of many plant structures. They 

 are due partly to internal causes, partly to the influence of external 

 conditions (variations in illumination, shocks, etc.), but all these 

 conditions merely induce, as we shall see further on, changes of 

 turgor-expansion in the cells of the tissue complexes from which 

 the movements proceed, and this is of special importance. The 

 following experiments, firstly with Acacia lophanta, will make us 

 accurately acquainted with these remarkable movements. 



The leaflets of the compound leaves of Acacia lophanta, a plant 

 which can be raised in flower-pots from seed, are expanded hori- 

 zontally, when the plant is exposed to bright diffuse daylight. In 

 the evening the leaflets lay themselves together in an upward 

 direction, spreading out again on the following day, under the in- 

 fluence of the stimulus of light. We can, however, compel the 

 leaflets to assume the night- position even during the day, if we 

 take the plants into a dark place. After the lapse of from half an 

 hour to an hour the -leaflets have laid themselves together, but 

 they expand again if the plants are anew exposed to diffuse light. 

 The movements of the Acacia leaflets are the result of the change 

 in the condition of illumination, for they are also exhibited, as can 

 easily be determined, if the temperature remains constant while 

 the observations are being conducted. 



I was able to determine that the leaflets of Acacia lophanta also 

 took up a position similar to the darkness position when the plants 

 were exposed to direct sunlight. An Acacia with expanded leaves 

 was placed in direct sunlight under a bell-glass. The leaflets at 

 once laid themselves together ; they again spread out horizontally, 

 when the plant, without the removal of the bell-glass, was exposed 

 to bright diffused daylight. 



