27-- 



is dependent on the duration of t;:e exposure. At the -^nd of 

 only three minutes, the organisms of the gren, blue and violet were 

 all over to the brighter side of the dish. At the end of 15 mi-iutes 

 the red, orange and yellow had still oroduced no visible effect. But, 

 after 20 minutes, all but the longest waves, 6600 to 7000 had caused 

 a definite ag regation;and after thirty uinutes,' the effect was se^n 

 benind this r^d screen. 



As striding a? tnis relation of the time factor to the power 

 of light stiumli to produce the phc totropisrn was the difference in the 

 response behind the 'four filters of the green, blue, and violet. After 

 only 3 minutes of exposure the aggregation effect was most marked in 

 the violet, less in the blue and still less in the green. The 

 difference in the appearance of the four disnes was due to the 

 difference in the number of individuals that had responded positively 

 and the completeness of their response, i.e. in 4-0 o-j i+j o 

 practically every colony -va igainst the glass on the outer side of 

 the dish; in A =. *f 9o - frffctf ^- A there were more scattered about in 

 the water. In 5200 - 560^tf still more and in 560 - 590^ 



there wre just barely enough affected to make it evident that some 

 were positively [z?o%4'(l; After 10 minutes this gradiant was still 

 visible. After about 15 minutes, the dishes all looked alike. Tnis 

 again shows the importance of the time factor, since the less sensitive 

 organisms which respond less rapidly are after a longer time afteoted 

 to the sane decree. 



In all the succeeding exposures with sunlight this rule held 

 good, that the organisms behind 76snd75 respond most rapidly and most 



