THE VEGETABLE KINGDOM. 431 



speaking of to the difference between the temperature of 

 the day and the temperature of the night ; but when it was 

 seen to take place in greenhouses, where the heat was equal 

 night and day, they were obliged to seek for some other 

 cause. 



De Candolle showed by some interesting experiments 

 that within the empire of Flora sleep is to be attributed 

 to the absence of light. By throwing a very bright light 

 upon sensitive plants during the night, and, conversely, by 

 placing them in profound darkness during the day, the 

 learned botanist succeeded in completely changing their 

 habits. These plants closed up their leaflets and slept the 

 whole day, deceived by the artificial gloom ; and they re- 

 mained awake the whole night, when six lamps projected 

 upon them a brilliance equal to five sixths of that of day- 

 light. 



It is principally among plants which inhabit intertropical 

 countries that the phenomenon in question is seen. It is 

 particularly noticeable in the family of the Leguminosae, 

 and most of all in the sensitive plants. Many of those in 

 our fields show it plainly. 



If at the close of summer we examine a clover-field 

 about six o'clock in the evening, we are struck with the 

 aspect which all the plants present at this moment, the first 

 of their sleep. The two side leaflets of each leaf are laid 

 close against one another, and the middle one covers them 

 like a protecting roof; the whole aspect of the crop has 

 changed. 



