POSITIVE HELIOTROPISM 583 



to give sufficient stimulus to initiate multiple responses 

 which had not been present before. This might have 

 appeared incredible had I not, in the course of my experi- 

 ments, had reason to know how extraordinarily sensitive 

 plants may become to the influence of light, instances of 

 which will be found in the investigations presently to be 

 described. In view, indeed, of the vitiation of results which 

 might be caused in this way, I was compelled to devise 

 special means for obviating the use of any light whatsoever 

 for the observation of responsive curvature. 



Response of the terminal leaflet of Desmodium. — In 

 order, then, to determine the important question of whether or 

 not light will produce heliotropic movement in a plant devoid 

 of circumnutation, I acted on the idea that the withdrawal of 

 superfluous energy was the essential preliminary condition, 

 and chose for my purpose, as already said, the large terminal 

 leaflet of a specimen of Desmodium gyrans in which all 

 movement had come to a stop — the plant having been 

 exhausted by flowering, and by the unfavourableness of the 

 season, which was winter. And further, in order that there 

 should be no storage of energy derived from light, I kept 

 this plant for one day in a dark room. To eliminate the 

 necessity of using light for purposes of observation, I attached 

 the leaflet by cocoon fibre to the arm of the Optic Lever 

 Recorder. The plant itself was enclosed in a dark box, and 

 thus protected from any access of light. Through a trap- 

 door in the box, light for stimulation could be thrown down 

 on the leaf at the desired moment. The preliminary absence 

 of any autonomous movement in the plant was seen by the 

 quiescence of the spot of light reflected from the mirror 

 attached to the recording lever. 



I now subjected the terminal leaflet of the selected speci- 

 men to light from a candle, this being thrown down on the 

 leaflet vertically by means of a mirror, the effective distance 

 of the candle from the leaf being twenty centimetres. The 

 leaflet, which had previously been quiescent, began to respond 

 after a latent period of ten seconds, and during the course of 



