TEE MOYEMEMTS AND HABITS OF PLANTS. 27 



any important manner objects so thin as are many cotyle- 

 dons and leaves, and more especially affect differently 

 their upper and lower surfaces ; for, although the tem- 

 perature of their upper surfaces would undoubtedly fall 

 when freely exposed to a clear sky, yet we thought that 

 they would so quickly acquire by conduction the temper- 

 ature of the surrounding air, that it could hardly make 

 any sensible difference to them whether they stood hori- 

 zontally, and radiated into the open sky, or vertically, 

 and radiated chiefly in a lateral direction toward neigh- 

 boring plants and other objects. "We endeavored, there- 

 fore, to ascertain something on this head, by preventing 

 the leaves of several plants from going to sleep, and by 

 exposing to a clear sky, when the temperature was be- 

 neath the freezing-point, these as well as the other leaves 

 on the same plants, which had already assumed their 

 nocturnal vertical position. Our experiments show that 

 leaves thus compelled to remain horizontal at night suf- 

 fered much more injury from frost than those which were 

 allowed to assume their normal vertical position. It may, 

 however, be said that conclusions drawn from such ob- 

 servations are not applicable to sleeping plants, the inhab- 

 itants of countries where frosts do not occur. But in 

 every country, and at all seasons, leaves must be exposed 

 to nocturnal chills through radiation, which might be in 

 some degree injurious to them, and which they would es- 

 cape by assuming a vertical position. 



The Power Any one who had never observed continu- 



in Plants^ 11 °usly a sleeping plant would naturally suppose 

 page 403. that the leaves moved only in the evening 

 when going to sleep, and in the morning when awaking ; 

 but he would be quite mistaken, for we have found no 

 exception to the rule that leaves which sleep continue to 



