THE WONDERS OF VEGETATION. 265 



The positions assumed by leaves during the night 

 depends much upon whether these leaves are simple or 

 compound ; in the latter, the difference is far more 

 distinctly marked. In the oxalis, with compound 

 leaves, the leaflets bend toward the common stalk, 

 and lean against it with their under surface, leaving 

 only their upper surface visible. Sweet peas and 

 common beans fold their leaves up, till one supports 

 the other ; while other plants roll theirs together, in 

 the shape of an ear trumpet. 



The common chickweed (Stellaria medico) fur- 

 nishes a beautiful instance of the sleep of plants. 

 Every night the leaves approach each other in pairs, 

 so as to include between their upper surfaces the ten- 

 der rudiments of young shoots. But they are not 

 alone. If one were to walk in a botanic garden 

 after the setting of the sun, a great number of plants 

 would be found which present a different aspect dur- 

 ing the night from that which they present during 

 the day. In some the leaves are erect and cover the 

 stem, in others they hang down and cover the leaflets 

 with the under side ; while, in still others, they ap- 

 proach each other in such a manner as to form tiny 

 boats. In the mallows, with simple round leaves, the 

 form of the latter is convex or concave, according to 

 the hour of the day. 



To what cause are these general phenomena due ? 

 They seem to be independent of the thermometric or 

 hydrometric condition of the air. De Candolle, fol- 

 lowing the example of Linnaeus, ascertained that 

 light was the most direct cause. He exposed plants 



