116 IRRITABILITY. 



bringing about the phenomena to be mentioned, no 

 doubt is considerable ; and an original tendency to a 

 periodicity of action, the excitement to which has 

 been originally impressed upon the plant, also may 

 prevail ; but that these are accompanied or modified by 

 some powers inscrutable to us, is just as apparent. In 

 what we have to mention, a few only of the many 

 facts known upon this subject, can here be noticed. 



163. At certain hours, most generally when the 

 light comes to be moderated, and the dews of the 

 evening begin to be deposited, plants, whose leaves and 

 flowers have been erect and expanded during the day, 

 begin to droop, the leaflets of the compound leaves 

 incline toward each other, and the general petiole is lax 

 and somewhat recurved, the flower may droop a little, 

 and its petals close one upon the other ; after the ex- 

 piration of a certain time, and towards the morning the 

 leaves and flowers gradually become erect and unfold, 

 and burst out again in all the freshness and beauty which 

 can mark them, at this their most fragrant and lovely 

 period. 



164. This phenomenon, however, in all its relations is 

 subject to innumerable modifications ; in some cases 

 instead of a dormant state being induced by the nega- 

 tion of light, a most active state of vital energy arises, 

 and plants burst out in all their beauty of flower and 

 deliciousness of scent. 



165. The younger the plant, the more evident the 

 nocturnal position of the leaves ; as a general rule 

 their daily position changes with the sinking sun, but 

 more depends upon the action of the solar light, than the 

 actual position of the sun above the horizon, because in 

 dark weather, or when the sun is obscured by a thick 

 cloud, the leaves often shut, or even never open again 

 the same day, when this state continues for some time. It 

 was noticed, that on one of the cold days experienced 

 in Germany in July 1838, the larger leaves of the 

 Hedysarum sank at 3 o'clock p. m. clothing the stem 



