i52 SENSITIVENESS TO LIGHT. Chap. IX 



taken by each semicircle, within a limit of error of at 

 most 5 minutes. Although the rate of movement in 

 different parts of the same revolution varied greatly, 

 yet 22 semicircles to the light were completed, each 

 on an average in 73-95 minutes ; and 22 semicircles 

 from the light each in 73-5 minutes. It may, there- 

 fore, be said that they travelled to and from the light 

 at exactly the same average rate ; though probably 

 the accui-acy of the result was in part accidental. In 

 the evening the stems were not in the least deflected 

 towards the window. Nevertheless, there appears to 

 exist a vestige of heliotropism, for with 6 out of the 

 7 plants, the first semicircle from the light, described 

 in the early morning after they had been subjected to 

 darkness during the night and thus probably rendered 

 more sensitive, required rather more time, and the first 

 semicircle to the light considerably less time, than the 

 average. Thus with all 7 plants, taken together, the 

 mean time of the first semicircle in the morning from 

 the light, was 76"8 minutes, instead of 73'5 minutes, 

 which is the mean of all the semicircles during the 

 day from the light ; and the mean of the first semi- 

 circle to the light was only 63-1, instead of 73-95 

 minutes, which was the mean of all the semicircles 

 during the day to the light. 



Similar observations were made on Wistaria Sinensis, 

 and the mean of 9 semicircles from the light was 

 117 minutes, and of 7 semicircles to the light 122 

 minutes, and this difference does not exceed the pro- 

 bable limit of error. During the three days of expo- 

 sure, the shoot did not become at all bent towards the 

 window before which it stood. In this case the first 

 semicircle from the light in the early morning of each 

 day, required rather less time for its performance thar' 

 did the first semicircle to the light; and this result, 



