SPONTANEOUS PERIODIC MOVEMENTS. 599 



accords with our requirements of causality ; but in the processes mentioned that is 

 simply not the case, and it is just this which must impel us to seek for the causes 

 or shocks which produce the movements named. At present, however, we can only 

 offer suggestions in this connection. We may, if need be, imagine that by means 

 of the continually recurring chemical processes in the living parts of plants, 

 changes are produced in the state of the protoplasm, and by this again in the 

 turgescence of the cells, and if the latter are more pronounced sometimes on the one, 

 sometimes on the other side, the parts of the organ concerned must bend alternately 

 to and fro. Now this is of course simply a hypothesis, because we are seeking 

 a cause which renders these phenomena explicable. We have it is true already 

 met with the fact that the excretion of water in decapitated root-stocks 

 exhibits a periodically varying course, which is apparently independent of ex- 

 ternal impulses, and in the same way we found a periodicity in growth in length ; 

 and this presents very great similarity with the periodic movements referred to, 

 i. e. the periodically varying nutations of growing stems, tendrils, and so forth, for 

 which likewise external impulses are wanting, or at least appear to be wanting. We 

 thus find ourselves here on the boundary of a still very obscure domain of science, and 

 in such cases it is often an advantage even to be able to avoid the grosser errors. But 

 such an error would decidedly exist in the assumption that periodic phenomena must 

 necessarily be due to periodically varying causes — an error which, as it almost seems, 

 prevails in the whole literature of this province, although there is by no means a lack 

 of examples showing that, in the domain of mechanical and physical science, periodic 

 movements are produced by constant causes which do not vary periodically. The 

 periodic vibrations up and down of the haulms in a field of corn are due to the 

 pressure of a constant current of wind and the constant elasticity of the haulms, 

 and thus we have the interesting phenomenon of wave-motion in a field of corn. In 

 the same way the periodically varying action of a hydraulic ram is effected by the 

 constant flowing of water into the machine itself, other circumstances also being 

 constant. Nay, even the periodic alternations of day and night and of winter and 

 summer arise from the constant revolution of the earth, and its constant course 

 round the sun. 



It would lead us into very tedious and difficult developments of ideas if I 

 were to attempt to show how periodically varying phenomena must proceed from 

 causes which act constantly; it suffices here to make the fact evident as such, 

 because it protects us from the error of regarding it as necessary to refer the periodic 

 movements of many organs of plants to a periodic variation of their causes. There 

 are, however, also many other likewise periodic changes in the vegetable kingdom, 

 which can be easily referred to definite periodically varying causes : for instance, the 

 so-called sleep-movements of leaves, and the periodic opening and closing of m.any 

 flowers. In the case of the phenomena here under consideration it is not so, 

 however. 



Nevertheless, we are warranted in regarding the so-called spontaneous or 

 independent periodic movements as phenomena of irritability, just as animal phy- 

 siologists place the periodic pulsations of the heart in the series of phenomena of 

 animal irritability. While it is in other cases the first object of research to under- 

 stand the course of a manifestation of irritability from a known external impulse. 



