398 The Movements of Plants 



Darwin's view is plainly stated on pp. 3 4 of the Power of 

 Movement. Speaking of circumnutation he says, "In this universally 

 present movement we have the basis or groundwork for the acquire- 

 ment, according to the requirements of the plant, of the most 

 diversified movements." He then points out that curvatures such 

 as those towards the light or towards the centre of the earth 

 can be shown to be exaggerations of circumnutation in the given 

 directions. He finally points out that the difficulty of conceiving 

 how the capacities of bending in definite directions were acquired 

 is diminished by his conception. "We know that there is always 

 movement in progress, and its amplitude, or direction, or both, have 

 only to be modified for the good of the plant in relation with internal 

 or external stimuli." 



It may at once be allowed that the view here given has not been 

 accepted by physiologists. The bare fact that circumnutation is a 

 general property of plants (other than climbing species) is not 

 generally rejected. But the botanical world is no nearer to be- 

 lieving in the theory of reaction built on it. 



If we compare the movements of plants with those of the lower 

 animals we find a certain resemblance between the two. Accord- 

 ing to Jennings 1 a Paramoecium constantly tends to swerve towards 

 the aboral side of its body owing to certain peculiarities in the set 

 and power of its cilia. But the tendency to swim in a circle, thus 

 produced, is neutralised by the rotation of the creature about its 

 longitudinal axis. Thus the direction of the swerves in relation to 

 the path of the organism is always changing, with the result that the 

 creature moves in what approximates to a straight line, being how- 

 ever actually a spiral about the general line of progress. This 

 method of motion is strikingly like the circumnutation of a plant, 

 the apex of which also describes a spiral about the general line of 

 growth. A rooted plant obviously cannot rotate on its axis, but the 

 regular series of curvatures of which its growth consists correspond 

 to the aberrations of Paramcecium distributed regularly about its 

 course by means of rotation 2 . Just as a plant changes its direction 

 of growth by an exaggeration of one of the curvature-elements of 

 which circumnutation consists, so does a Paramcecium change its 

 course by the accentuation of one of the deviations of which its 

 path is built. Jennings has shown that the infusoria, etc., react to 

 stimuli by what is known as the " method of trial." If an organism 



1 H. S. Jennings, The Behavior of the Lower Animals. Columbia U. Press, N.Y. 

 1906. 



2 In my address to the Biological Section of the British Association at Cardiff (1891) I 

 have attempted to show the connection between circumnutation and rectipetality, i.e. the 

 innate capacity of growing in a straight line. 



