582 PLANT RESPONSE 



sensibilities of different organs, but can all be shown to form 

 only special instances of a single fundamental effect. And this 

 fundamental effect is the same in growing, in stationary, in 

 radial, and in anisotropic organs. In describing the respon- 

 sive actions induced by light, it will, however, be necessary to 

 distinguish the direction of movement, in relation to the 

 stimulating light, by clearly defined terms. I shall therefore 

 designate all movements, of whatever organs, toxvards light 

 as positively heliotropic, and all movements away from light 

 of whatever organs, as negatively heliotropic. 



Darwin's theory of modified circumnutation. — Before 

 describing the response, it is necessary to say a few words 

 regarding Darwin's view of heliotropic movements as a 

 modified form of circumnutation. According to this, the 

 already existing movement (of circumnutation) had only to 

 be increased in some one direction, and lessened or stopped 

 in others, in order to become heliotropic or ap-heliotropic, as 

 the case might be ; l but, in order to prove conclusively that 

 heliotropic curvatures were caused by the modification of the 

 pre-existing movements, it would be necessary to show that 

 they did not take place in those organs from which circum- 

 nutation was absent, and this would constitute the crucial 

 test of the theory. The difficulty of obtaining such a speci- 

 men is, however, so great, that Darwin, although he noted 

 the point, was unable to apply the test. 



I have shown elsewhere that circumnutation is only a 

 particular manifestation of that multiple or autonomous 

 response of plants which is due to an excess of energy, 

 previously absorbed. In order to obtain a plant-organ com- 

 pletely at standstill, therefore, it would be necessary to find 

 a specimen in which there was no such excess of latent 

 energy. The fact that circumnutation was absent could then 

 be ascertained by means of the high magnification obtainable 

 from the recording apparatus which I have already employed. 

 Another difficulty lay in the fact that the use of light, how- 

 ever feeble, for purposes of observation, would be apt of itself 



1 Darwin, The Movements of Plants, p. 449. 



