MECHANISM OF THE MOVEMENTS. 639 



that the stimulus first produces a change in the molecular structure of the protoplasm, 

 which thereby becomes more capable of filtration, i. e. allows water to escape, 

 whence it penetrates also- through the cell-wall, and consequently the cell 

 becomes smaller. In this manner curvatures, exactly like those here considered, are 

 produced in the similarly motile organs of the leaves of Mimosa and other plants 

 by means of mere vibration. Supported by this analogy then it is probable 

 that we may decide in favour of the second of the two alternatives mentioned 

 above. We may assume as probable that an increase in the intensity of the light 

 makes the protoplasm in the cells of the periodically motile organs a little 

 more cajjable of filtration, so that the already very high pressure of the cell-sap on 

 the wall causes a small quantity of water to filter out and enter the neighbouring parts 

 of the stem or petiole; whereas on darkening, the capacity for filtration of the 

 protoplasm is increased, tfiaking possible a greater turgescence and extension of the 

 cells. 



This is of course, in the meantime, only a conclusion based on analogies ; but it 

 appears to be supported by all that we know otherwise of irritability in the vegetable 

 kingdom, and as to turgescence and the properties of protoplasm. However, I must 

 refer to the next lecture for more particulars. 



In conclusion, before dismissing the subject of daily waking and sleeping, and the 

 paratonic irritable movements of the leaves due to light, so far considered, I may make 

 a few short remarks on a point concerning the mechanism of these movements, which 

 PfefFer first insisted upon and investigated in detail. When, in the case of a com- 

 pound leaf, like that of the Bean, the motile organ is situated at the base of the petiole, 

 the size of the angle between the petiole and the shoot-axis is altered with its move- 

 ments; the entire leaf rises and falls — in the Bean, for example, it rises in the 

 evening and sinks in the morning — in the evening the apex of the angle is smaller, 

 in the morning larger. Simultaneously with these changes, however, the position of 

 the three leaflets at the other end of the leaf-stalk is also changed ; in the evening 

 they all curve downwards, and especially the most anterior leaflet. By this, how- 

 ever, the moment of rotation which the weight of the leaflet on the petiole exerts 

 as a lever on the lower motile organ becomes smaller. This favours the erection of 

 the whole leaf-stalk, or the diminution of the angle between it and the shoot-axis, and 

 the reverse must occur on the access of the light in the morning ; by the direct 

 stimulus of the light the chief organ of the leaf-stalk not only becomes curved 

 downwards, but allso a little more flaccid, and the weight of the three leaflets 

 causes a further depression, and in addition the leaflets now become expanded 

 and thereby exert a greater moment of rotation on the motile organ, and this 

 also acts again in the same sense that the leaf-stalk falls somewhat more than 

 would have happened simply by the stimulus of its motile organ by light. In 

 this case, then, the changes of position of the leaflets in connection with their sleep- 

 movements, acting on the petiole as a lever, produce a similar effect to that of the 

 paratonic stimulations on the lower motile organs of the leaf-stalk itself. In the 

 Mimosa, on the contrary, Pfeffer showed that the sleep-movements of the two or 

 four secondary petioles at the end of the main stalk produce lever-actions on the 

 latter which influence the paratonic movements of the large motile organ at the 

 base of the leaf-stalk in the opposite sense ; since, on darkening, the secondary 



