82 THE LEAF. 



CHAPTEE- XII. 



THE LEAT. 



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 the vegetable 

 economy, being the organ of digestion and respiration. 



2 14. The leaf is characterized by a tlain 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, wliich 

 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 ^vildest 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 siiccessively, accordhig 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. 



