STIMULATION AND ITS EESULTS 395 



If we now return to the study of the rhythmic changes 

 which we have seen to be essentially characteristic of 

 vegetable protoplasm, we see that while rhythm is no doubt 

 inherent in plants it lends itself especially to such changes 

 as are caused by stimulation. It is indeed this feature 

 which is especially brought out by the various responses 

 made to changes in the environment. While it occurs with 

 some regularity when conditions are kept constant, it is 

 easily affected by external causes. The effect of continuous 

 darkness, or of too great cold, or other abnormal conditions, 

 is that the rhythmic movements are made irregular and 

 ultimately stop. In many cases differences in the degree of 

 illumination during the day affect the readiness with which 

 the nyctitropic movements of the leaves are brought about. 

 After a day of brilliant sunshine they set in more quickly 

 than after one of dull light. 



These movements may show indeed a secondary in- 

 duced rhythm superposed upon a normal one. The move- 

 ments of heliotropism, geotropism, &c. may be looked 

 upon as instances of this. We have seen that they are 

 based upon the ordinary movement of circumnutation, and 

 are in fact exaggerations of it. As the latter is generally 

 a manifestation of a rhythm of turgidity in the cells affected 

 we have in them a case in point. In other cases the 

 tendency to rhythmic change can be demonstrated by the 

 production of an altogether artificial rhythm induced by 

 submitting the plant to intermittent stimulation. F. Darwin 

 and Pertz have described a very interesting experiment of 

 this nature. A plant was fixed to a spindle placed hori- 

 zontally, in a modification of the klinostat, and was by an 

 arrangement of clockwork made to undergo a semi-revolution 

 at intervals of thirty minutes. The force of gravity thus 

 exerted its effect upon alternate sides for this interval of 

 time, so that each side of the stem became slightly convex 

 apogeotropically in turn. After a period of exposure upon 

 the instrument the clockwork was stopped. Instead of the 

 side which was then undermost increasing its convexity 



