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 Avith 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 Avorkl 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^ a Paramoecium constantly tends to swerve towards 

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

 and poAver of its cilia. But the tendency to SAvim in a circle, thus 

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

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

 the path of the organism is ahvays changing, Avith the result that the 

 creature moves in AA'hat 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 Avhich also describes a spiral about the general line of 

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

 regular series of curvatures of Avhich its groAvth consists correspond 

 to the aberrations of Paramoecium distributed regularly about its 

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

 of groAvth by an exaggeration of one of the curAature-elements of 

 Avhich circumnutation consists, so does a Paramcecium change its 

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

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

 stimuli by Avliat is knoAvn as the " method of trial." If an organism 



' H. S. Jennings, The Behavior of the Loxcer Animals. Columbia U. Press, N.Y. 

 1900. 



' In my address to the Biological Section of the British Association at Cardiff (1801) I 

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

 innate capacity of growing in a straight line. 



I 



