528 TRANSFORMATION OF ENERGY 



with movements induced by stimuli previously studied, this comparison does 

 not by any means lead us to the root of the matter. We cannot prove it, but 

 we think it is extremely likely, that autonomous movements are also induced, 

 but by internal rather than external stimuli. It will now be our task to 

 attempt to gain some acquaintance with these autonomous movements. As 

 already noted, we may distinguish them into variation movements and nutation 

 movements, according to the media by which they are carried out. 



When speaking of nyctitropic movements of articulations we had occasion 

 to note that these periodic oscillations continued for a long time in darkness 

 when the temperature was kept constant, with almost the usual daily rhythm ; 

 in that case we were dealing with after-effects, which must not be confounded 

 with autonomous movements. These oscillations are very prominent in Mimosa 

 and Acacia, but they are not manifested by all leaves provided with pulvini. 

 When we study a plant of clover kept in darkness, we may observe very marked 

 to and fro oscillations in the leaflets which, however, show no relations to daily 

 periods (PFEFFER, 1875). This is a case of genuine autonomous movement 

 which is indeed itself autonomously periodic. These movements occur in light 

 also, though frequently masked, owing to the greater effect of the paratonic 

 (nyctitropic) movements. In Averrhoa bilimbi (a member of the Oxalidaceae) 

 these movements may be seen very clearly. When temperature and illumination 

 are kept constant the pinnate leaves of this plant con- 

 tinually perform backward and forward oscillations (DAR- 

 WIN, 1881), suddenly drooping and then slowly rising 

 again. [Very remarkable autonomous movements are 

 also exhibited by Oxalis hedysaroides (MoLiscn, 1904).] 

 PFEFFER'S (1875) researches have shown that while 

 the autonomous movements are in progress, just as in 

 the case of after-affects, the resistance to flexion of the 

 pulvinus remains unaltered. We may, therefore, assume 

 that the expansive force of the cells on the concave side 

 of the articulation decreases proportionally as it increases 

 on the convex side. 



After PFEFFER from DET- The leaves of Averrhoa and of the majority of nycti- 



tropic plants perform simple autonomous pendulum oscil- 

 lations, but in the well-known Desmodium gyrans (DAR- 

 WIN, 1881, p. 304) the movements are more complicated still. The leaves of 

 this plant (Fig. 161) are tripartite. The terminal leaflet is large and performs 

 well marked nyctitropic as well as less noticeable autonomous movements, while 

 the two smaller lateral leaflets, on the contrary, show no nyctitropic movements, 

 but do show autonomous oscillations, which at a certain temperature (minimum 

 22 C. ; optimum, 40 C.) are so rapid that they may be readily followed with the 

 naked eye. The whole backward and forward movement is complete in about 

 ij minutes. The alteration in the expansion of the pulvinus does not take place 

 alternately, first on one side and then on the other, but it proceeds in a circular 

 manner, one longitudinal area after the other being affected. The result of that 

 is that the tip of each leaflet describes approximately an ellipse whose long axis 

 is parallel with the main petiole. The movement is, however, not uniform but 

 is jerky in character, and on the whole more rapid downward than upwards. 

 The jerks are especially prominent if the efforts to move on the part of the leaf 

 are prevented for a long time by external resistance, so leading to tissue tensions. 

 According to STAHL (1897), such tensions, arising by inhibiting movement in the 

 terminal leaflet, as they become equalized, lead to vibrations and hence to 

 increased transpiration in the terminal leaflet. Whether other autonomous 

 variation movements also have a biological significance may be left undecided. 

 In the flowers of certain Orchidaceae and Stylidiaceae we also meet with many 



