STIMULATION AND ITS RESULTS 403 



and modify their growth accordingly. Almost all show a 

 peculiar relationship to their substratum, stems growing 

 out from it and roots into it in a direction at right angles 

 to the surface. This can be seen by cultivating them so 

 that they do not emerge in the normal direction, but from 

 the side of a cube of earth. They do not long maintain 

 this direction, as they speedily feel the influence of light 

 and gravity. If, however, appropriate means are adopted 

 to eliminate these, the growth is always at right angles to 

 the surface of the soil in which they live. 



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 induced 

 rhythm superposed upon a normal one. The movements 

 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 ten- 

 dency to rhythmic change can be demonstrated by the 

 production of an altogether artificial rhythm induced by 

 submitting the plant to intermittent stimulation. F. Darwin 



