120 GEOEGE JOHN EOMANES 1881- 



I observed an analogous case described in my book. 

 Wiesner and Tieghem seem to think that this is 

 explained by calling the whole process ' induction,' 

 borrowing a term used by some physico-chemists (of 

 whom I believe Roscoe is one), and implying an 

 agency which does not produce any effect for some 

 time, and continues its effect for some time after the 

 cause has ceased. I believe (?) that photographic 

 paper is an instance. I must ask Leonard whether 

 an interrupted light acts on it in the same manner 

 as on a plant. At present I must still believe in my 

 explanation that it is the contrast between light and 

 darkness which excites a plant. 



I have forgotten my main object in writing, viz. 

 to say that I believe (and have so stated) that seedlings 

 vary much in their sensitiveness to light ; but I did 

 not prove this, for there are many difficulties, whether 

 time of incipient curvature or amount of curvature 

 is taken as the criterion. Moreover, they vary 

 according to age and perhaps from vigour of growth ; 

 and there seems inherent variability, as Strasburger 

 (whom I quote) found with spores. If the curious 

 anomaly observed by you is due to varying sensitive- 

 ness, ought not all the seedlings to bend if the flashes 

 were at longer intervals of time ? According to my 

 notion of contrast between light and darkness being 

 the stimulus, I should expect that if flashes were 

 made sufficiently slow it would be a powerful stimulus, 

 and that you would suddenly arrive at a period when 

 the result would suddenly become great. On the 

 other hand, as far as my experience goes, what one 

 expects rarely happens. 



