[ 60 j 



arrangement in the day : and the greater number of 

 them were seen closed or folded together. 



The sleep of leaves is, in some cases, capable of 

 being produced artificially. Decandolle made this ex- 

 periment on the sensitive plant. By confining it in 

 a dark place in the day time, the leaves soon closed ; 

 but on illuminating the chamber with many lamps, 

 they again expanded. So sensible were they to the 

 effects of light and radiant heat. 



In the greater number of plants the leaves annu- 

 ally decay, and are reproduced ; their decay takes place 

 either at the conclusion of the summer, as in very hot 

 climates, when they are no longer supplied with sap, 

 in consequence of the dryness of the soil, and the 

 evaporating powers of heat ; or in the autumn, as in 

 the northern climates at the commencement of the 

 frosts. The leaves preserve their functions in com- 

 mon cases no longer than there is a circulation of 

 fluids through them. In the decay of the leaf, the 

 colour assumed seems to depend upon the nature of 

 the chemical change, and as acids are generally devel- 

 oped, it is usually either reddish brown or yellow ; 

 yet there are great varieties. Thus in the oak, it is 

 bright brown ; in the beech, orange ; in the elm, yel- 

 low ; in the vine, red ; in the sycamore, dark brown ; 

 in the cornel tree, purple ; and in the woodbine, blue, 



The cause of the preservation of the leaves of 

 evergreens through the winter is not accurately 

 known. From the experiments of Hales, it appears 

 that the force of the sap is much less in plants of this 

 species, and probably there is a certain degree of circu- 



