334 Mr. G. J. Romanes. [Nov. IS, 



during the second half of the experiment this cap was removed, and 

 the pot turned round, so as to expose the previously protected 

 seedlings to the influence of the light. The principal results thus 

 obtained, and frequently corroborated, were as follows. 



I. Even having regard to the fact that for equal strengths of a 

 stimulus excitable tissues are more responsive in proportion to the 

 suddenness of the stimulus (or in a kind of inverse proportion to the 

 duration of the stimulus), the heliotropic effects of such flashing 

 stimulation as is above described proved to be much greater than 

 might have been antecedently expected. This was shown to be 'the 

 case whether the effects were estimated by the rapidity with which 

 the seedlings began to bend after the flashing stimulation was begun, 

 or by that with which they continued to bend until attaining a hori- 

 zontal line of growth, i.e., bending to a right angle. Thus, at a 

 temperature of 70 Fahr., and in a moist camera, vigorously growing 

 seedlings begin to bend towards the electric sparks ten minutes after 

 the latter begin to pass, and will bend through 45 in as many 

 minutes; frequently they bend through another 45 in as many 

 minutes more. This is a more rapid rate of bending than can be 

 produced in the same pot of seedlings when the previously protected 

 side is uncovered and exposed for similar durations of time, either to 

 constant sunlight, or to constant diffused daylight. This is the case 

 even if the sparks (or flashes) succeed one another at intervals of 

 only two seconds. 



II. It would thus appear that the heliotropic influence of electric 

 sparks (or flashes) is greater than can be produced by any other 

 source of illumination. But, in order to test this point more con- 

 clusively, I tried the experiment of exposing one half-pot of seedlings 

 in one camera to the constant light of a Swan burner, and another 

 half-pot of similar seedlings, in another camera, placed at the same 

 distance from the same source of light, but provided with a flash 

 shutter working at the rate of two seconds intervals. The amount of 

 bending in similar times having been noted, the pots were then 

 exchanged, and their previously protected halves exposed to the 

 constant and the flashing light respectively. In both cases, the 

 rapidity with which the bending commenced, and the extent to 

 which it proceeded in a given time after commencement, were 

 considerably greater in the seedlings exposed to the flashing than to> 

 the constant source stimulation. The same is true if, instead of a 

 Swan burner, the source of light is the sun. 



III. Many experiments were tried, in order to ascertain the smallest 

 number of sparks in a given time which would produce any per- 

 ceptible bending. Of course the results of such experiments varied 

 to some extent with the condition of the seedlings. But in most 

 cases, with vigorous young mustard seedlings and careful observation,. 



