LEAVES. 
ons trees ; these change their leaves annually^ but the young 
leaves appearing before the old ones decay, the plant is always 
green. In our climate the leaves are mostly deciduous, return- 
ing in autumn to their original dust, and enriching the soil 
from which they had derived their nourishment, tn the re- 
gions of the torrid zone, the leaves are mostly persistent and 
evergreen ; they seldom fade or decay in less than six years ; 
but the same trees, removed to our climate, sometimes become 
annual plants, losing their foliage every year. The passion- 
flower is an evergreen in a more southern climate. 
59. The GREEN coLOK of leaves is owing to a coloring matter 
called C/iloropIiyl (from chloros, green, smd pJiyllon^ leaf), which 
floats in minute globules in the fluid of cells accompanied by 
starch grains. The green color becomes lighter or deeper ac- 
cording to the quantity of chlorophyl and the aggregation of 
cells. Leaves have not that brilliancy of color which is seen 
in the corolla or blossom ; but the beauty of the corolla has 
only a transient existence ; while the less showy leaf remains 
fresh and verdant after the flower has withered away. The 
substance of most leaves is so constituted as to absorb all the 
rays of light except green ; this color is of all others best 
adapted to the extreme sensibility of our organs of sight. Thus, 
in evident accommodation to our sense of vision, the ordinary 
dress of nature is of the only color upon which our eyes, for 
any length of time, can rest without pain. But although green 
is almost the only color which leaves reflect, the variety of its 
shades is almost innumerable. 
" N'o tree in all the grove but has its charms, 
Though each its hue peculiar ; paler some, 
And of a wannish-gray ; the willow such, 
And poplar, that with silver lines his leaf ; 
And ash far stretching his umbrageous arm ; 
Of deeper green the elm ; and deeper still, 
Lord of the woods, the long-surviving oak."* 
The contrast between their shades, in forests, where different 
families of trees are grouped together, has a fine eflect, when 
observed at such a distance as to give a view of the whole as 
forming one mass. A small quantity of carbon, united to 
oxygen in the vegetable substance, and acted upon by light, is 
said to give rise to the various colors of plants. f If this theory 
be correct, the different shades of color in plants must be 
owing to the different proportion in which the carbon and 
* Cowper. 
t This idea coincides with the supposition, that the ffreen color of leaves is changed to brown by tha 
1*83 of an acid principle ; that the petals of flowers cliange from purple to red by an increase of acid. 
The base of this acid is oxygen. 
59. Chlorophvl. 
