1893 EXPEEIMENTS ON HELIOTROPISM 355 



room and exposed to flashes of light in the form of 

 electric sparks passed at regular intervals, or they 

 were put in a camera obscura, before which was 

 placed a Swan burner or arc lamp, the light from 

 which was rendered intermittent by the regular 

 opening and shutting of the photographic shutter. 

 The heliotropic effect on the seedlings was found in 

 all cases to be very marked, the most vigorous ones 

 beginning to bend towards the light ten minutes after 

 the flashing began, bending through 45 in as many 

 minutes, and often through another 45 in as many 

 minutes more. By protecting half of the seedlings 

 from the interrupted light, by means of a cardboard 

 cap, then after the experiment uncovering them and 

 exposing that half for the same duration of time to 

 constant sunlight, Mr. Romanes found that the bend- 

 ing was less in this latter case ; that is, when the light 

 was continuous. This result was confirmed by 

 placing two sets of plants under exactly similar con- 

 ditions before a Swan burner, the light from which 

 was constant for one set of seedlings, and rendered 

 intermittent for the other set by working the flash 

 shutter ; in all cases the interrupted light caused the 

 plants to start bending more quickly, and through a 

 greater angle in a given time. 



As regards the rate the flashes must succeed 

 one another to produce this heliotropic effect, Mr. 

 Romanes found that sparks passed at the rate of fifty 

 in an hour would cause considerable bending in half 

 an hour. It is of interest to note that in no single 

 case was there any green colouring matter produced, 

 the seedlings remaining colourless even when the 

 sparks were passed at the rate of 100 per second 

 continuously during forty- eight hours. 



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