CHAP. IX. SENSITIVENESS TO LIGHT. 457 



the room. That light passing through a hole only 

 004 inch in breadth by '016 in length, should induce 

 curvature, seems to us a surprising fact. 



Before we knew how extremely sensitive the coty- 

 ledons of Phalaris were to light, we endeavoured to 

 trace their circumnutation in darkness by the aid of 

 a small wax taper, held for a minute or two at each 

 observation in nearly the same position, a little on the 

 left side in front of the vertical glass on which the 

 tracing was made. The seedlings were thus observed 

 seventeen times in the course of the day, at intervals of 

 from half to three-quarters of an hour ; and late in the 

 evening we were surprised to find that all the 29 coty- 

 ledons were greatly curved and pointed towards the 

 vertical glass, a little to the left where the taper had 

 been held. The tracings showed that they had tra- 

 velled in zigzag lines. Thus, an exposure to a feeble 

 light for a very short time at the above specified 

 intervals, sufficed to induce well-marked heliotropism. 

 An analogous case was observed with the hypocotyls 

 of Soldnum lycopersicum. We at first attributed this 

 result to the after-effects of the light on each occasion ; 

 but since reading Wiesner's observations,* which will 

 be referred to in the last chapter, we cannot doubt that 

 an intermittent light is more efficacious than a con- 

 tinuous one, as plants are especially sensitive to any 

 contrast in its amount. 



The cotyledons of Phalaris bend much more slowly 

 towards a very obscure light than towards a bright 

 one. Thus, in the experiments with seedlings placed 

 in a dark room at 12 feet from a very small lamp, they 

 were just perceptibly and doubtfully curved towards it 

 after 3 h., and only slightly, yet certainly, after 4 h. 



* ' Sitz. der k. Akad. dcr WissenscL.' (Vienna), Jan. 1880, p. 12. 



