82 THE LEAP. 
CHAPTER XII. 
THE LEAF. 
213. Tue leaf constitutes the verdure of plants, and is by far 
the most conspicuous and beautiful object in the scenery of 
nature. It is also of the highest importance in the vegetable 
economy, being the organ of digestion and respiration. 
214. The leaf is characterized by a thin and expanded form, 
presenting the largest possible surface to the action of the air 
and the light, which agents are indispensable to the life and in- 
crease of the plant. 
215. The color of the leaf is almost universally green, which 
of all colors is the most agreeable to the eye; but its intensity 
varies by infinite shades, and is often finely contrasted with the 
more delicate tints of the flower. Towards maturity its verdure 
is changed, often to the most brilliant hues, as red, crimson, 
orange, yellow, giving our autumnal forest scenery a gaiety, 
variety, and splendor of coloring, which the wildest fancy could 
scarcely surpass. 
a. The color of the leaf is due to minute globules, or grains, called chlorophyll 
(green leaf ), adhering to the insides of the cells, just beneath the cuticle, and 
composed of carbon and hydrogen, with a small proportion of oxygen. Their 
change of color in autumn, is stated by Macaire to depend upon their oxydation. 
As the leaves in autumn absorb more oxygen by night than they evolve by day, 
an excess is gradually added to the chlorophyll, which changes the green first to 
yellow, then to orange, red, and crimson successively, according to the quantity 
absorbed. ‘The same effect may be produced by acids. 
b. As flowers are modifications of leaves, it is probable that their various and 
splendid coloring is due to the same source, namely, the modifications of the 
chlorophyll by various degrees of oxydation, or by the presence of acids or alka- 
lies in the cells. 
§1. VERNATION. 
. 216. A leaf-bud contains a collection of undeveloped leaves, 
folded together in such a manner as to occupy the least possible 
space. The particular manner in which the young leaves are 
folded in the bud, varies in different species, and is called vEr- 
NATION. 
