HELIOTROPES, OR SUNFLOWERS. 177 



ways, but with substantially similar results. The 

 movement is, of course, very slow where the light is 

 dim, but more rapid where the light is more intense. 

 Placed before a bright lamp the tips of the shoots 

 were all curved at right angles towards it in two 

 hours and a quarter. From other experiments it was 

 determined that seedlings of this grass commenced 

 travelling in the direction of the lamp within from 

 six to ten minutes after their first exposure. Also 

 that the rate of progression was irregular, sometimes 

 almost stationary for ten minutes, and then onwards 

 again. It may be mentioned in this connexion that 

 a series of observations was made in order to 

 determine how long the influence of light would 

 continue to be exerted after the source of light was 

 obscured. It was found that the young shoots would 

 continue to bend in the same direction as that from 

 which the illumination proceeded for from a quarter 

 to half an hour after the light was extinguished. 1 



Movements in the direction of the light have been 

 so universally observed, that heliotropism has been 

 accepted as a fact, independently of any recent 

 observations. The experience of any one who has 

 been concerned with vegetation will furnish numerous 

 instances of such phenomena. The farmer, the nur- 

 seryman, the gardener, and even the field labourer, 



1 Darwin, " Movements of Plants," pp. 457 to 463. 



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