63 
experiment 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 an- 
nually 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 leaved pre- 
serve their functions in common 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 developed, 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, yellow ; 
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 ap- 
pears that the force of the sap is much less in 
plants of this species, and probably there is a cer- 
tain degree of motion in it, in warm days, even in 
winter ; their juices are less watery than those of 
other plants, and probably less liable to be con- 
