( 233 ) 



I (Exposure). II (Intensity of light). Ill (Candles-seconds). 



100 minutes 

 60 

 30 

 20 

 15 



8 



4 

 40 seconds 

 25 



8 



4 



2 



1 



2/5 



2/25 „ 



1/25 „ 



1/55 „ 



1/100 „ 



1/400 „ 



1/800 „ 

 1/1000 „ 



It follows at once from columns I and II that with a shorter 

 exposure the strength of the light has to be increased, in order to 

 obtain a curvature. The calculated values in column III show, in 

 addition, that the intensity of light is inversely proportional to the 

 length of exposure, or, in other words, that a definite quantity of 

 light, independent of the exposure-time, is required to produce a 

 reaction. It is true that the values in column III are not identical, 

 but they clearly oscillate about a mean. Perfect identity cannot be 

 expected in experiments of this nature, when it is remembered that 

 the limit between curvature and non-curvature cannot always be 

 determined exactly ; moreover the oats seedlings are of course subject 

 to individual variations, which could only be eliminated by making 

 for each determination a long series of experiments; finally external 

 conditions of humidity, temperature, etc. could not be kept perfectly 

 constant in the various experiments. 



There was not much point in choosing exposures of less than 

 0.001 second, nor of more than 13 hours, since the results obtained 

 show clearly that the essential condition for the production of a 

 phototropic curvature is the supply of a definite quantity of radiant 



