POSITIVE HELIOTROPISM 585 



multiple responses become more frequent by the increased 

 absorption of energy, and their individuality is lost. 



The extreme sensitiveness of some plant-organs to 

 light. — I shall now say a few words about the extraordinary 

 sensitiveness of some plants to the stimulus of light. Darwin 

 gives a striking example of this in a case where the cotyledons 

 of Phalaris canadensis, after three hours of continuous exposure 

 to a small lamp at a distance of twelve feet, became doubtfully 

 curved towards the light, and after seven hours and forty 

 minutes from the first exposure were plainly, though slightly, 

 curved towards the lamp. The candle-power of the lamp is not 

 given, but it may be taken to be about four candles. Reducing 

 this to the standard distance of one metre, we find four candles 

 at a distance of twelve feet (four metres approximately) to 

 be equal to a quarter candle at a distance of one metre. If 

 three hours' exposure induced a doubtful curvature, then the 

 smallest amount of light to be effective must have been 

 I or 75 candle-hour ; the candle-hour giving an indication 

 of the quantity of light that had to be absorbed by the plant 

 in order to induce a movement that was just perceptible. 



Now, with the terminal leaflet of Desmodium, exposure to 

 the light of a candle at a distance of 20 cm. for ten seconds 

 was sufficient to initiate responsive movement. This when 

 reduced to standard conditions is equivalent to 'OJ candle- 

 hour. In other words, we find, as far as these two experi- 

 ments can determine the point, that the terminal leaflet of 

 Desmodium in this experiment was at least ten times more 

 sensitive than Darwin's cotyledons of Phalaris canariensis. 



Darwin, however, mentions another instance which is more 

 like the sensitiveness of which I have just given an example. 

 He has been using a small wax taper, in order to observe the 

 cotyledons of Phalaris ; he used this light for one or two 

 minutes at each observation, and observed the seedlings 

 seventeen times in the course of the day, in consequence of 

 which he found that zigzag responsive movements had been 

 induced. 



I must again point out here that the specimen of 



