406 



COMPARATIVE ELECTRO-PHYSIOLOGY 



the first effect of light on this sub-tonic tissue was to induce 

 a positive response, followed subsequently by the normal 

 negative. Continuous stimulation is seen later to give rise 

 to oscillatory responses. 



Having thus shown the continuity between the normal 

 negative and positive responses, I give below a record of the 



response to light of a petiole of 

 cauliflower (fig. 240), a specimen 

 which usually, though not always, 

 exhibits galvanometric positivity. 

 Each stimulus, by exposure for 

 five seconds, was in this case 

 applied after an interval of two 

 minutes. It should be mentioned 

 here that the same tissue which 

 gives positive response to the 

 moderate stimulus of light will 

 show the normal negative when 

 subjected to a stronger stimulus, 

 such as the mechanical. 



The effects studied up to the 

 present have consisted of single 

 responses induced by light. But this stimulus also induces 

 multiple response, as we saw in the oscillatory variations 

 of growth in the Crinum lily in fig. 239. The same 

 phenomenon is observed in the case of motile response. 

 For example, I took a plant of Biophytum, in which the leaf- 

 lets are outspread, in the presence of diffuse light, and threw 

 upon it direct sunlight. A series of multiple responses was 

 now induced, under the continuous action of stimulus, a 

 record of which is given in fig. 241. The Up-curves here 

 represent excitatory downward movements, and the down- 

 curves their partial recoveries. Owing to the incomplete 

 character of these recoveries, the leaflets, as the result of a 

 series of such responses, are finally closed downwards. In the 

 outspread diurnal position the lower half of the pulvinus of the 

 leaflet is somewhat more turgid than the upper half, and in 



FIG. 240. Photographic Re- 

 cord of Positive Response 

 of the Petiole of Cauliflower 

 to Light of Five Seconds' 

 Duration applied at Inter- 

 vals of Two Minutes 



