STIMULATION AND ITS RESULTS 408 
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 
26 * 
