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If a leaf be turned, so as to present its lower sur- 

 face to the sun, its fibres will twist so as to bring it as 

 much as possible into its original position ; and all 

 leaves elevate themselves on the foot-stalk during their 

 exposure to the solar light, and as it were moved to- 

 wards the sun. 



This effect seems in a great measure dependent 

 upon the mechanical and chemical agency of light and 

 heat. Bonnet made artificial leaves, which, when a 

 moist sponge was held under the lower surface, and 

 a heated iron above the upper surface, turned exactly 

 in the same manner as the natural leaves. This how- 

 ever can be considered only as a very rude imitation 

 of the natural process. 



What Linnaeus has called the sleep of the leaves, 

 appears to depend wholly upon the defect of the ac- 

 tion of light and heat, and the excess of the operation 

 of moisture. 



This singular but constant phenomenon had 

 never been scientifically observed, till the attention of 

 the botanist of Upsal was fortunately directed to it. 

 He was examining particularly a species of lotus, in 

 which four flowers had appeared during the day, and 

 he missed two in the evening ; by accurate inspection, 

 he soon discovered that these two were hidden by the 

 leaves which had closed round them. Such a circum- 

 stance could not be lost upon so acute an observer. 

 He immediately took a lantern, went into his garden, 

 and witnessed a series of curious facts before un- 

 known. All the simple leaves of the plants he exam- 

 ined, had an arrangement totally different from their 



