82 THE LEAF. 



CHAPTEU XII. 



THE LEAF. 



213. The 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 tlie 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 uir 

 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, wliicli 

 of all colors is the most agreeable to the eye ; but its intensity 

 varies by infinite shades, and is often finely contrasted vi-ith 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 gi-ains, called chlorophyll 

 {;^reen leaf), adhering to the insides of the cells, just beneath the cuticle, aiid 

 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 liy day, 

 an excess is gradually added to the chlorophyll, which changes the green iirst to 

 vellow, 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. 



