46 LEAVES. 



the upper smooth, green, and shining, the under surface gene- 

 rally with the ribs prominent, of a lighter green, often hairy, 

 and abounding in stomata or pores. The particular position 

 which a leaf assumes, is necessary to its properly performing its 

 functions, and even to its existence ; for if a leaf be inverted, it 

 withers and dies. The deeper tint of the upper surfece of leaves 

 is owing to the greater compactness of the parenchyma. 



82. The first organs that appear in dicotyledons after germi- 

 nation, are the cotyledons themselves, or the lobes of the seed, 

 which supply the young plant with food, till it becomes fur- 

 nished with organs for obtaining it from other sources. Before 

 the cotyledons become exhausted, leaves are produced, which 

 are called primordial leaves, which seem to possess an organiza- 

 tion a degree more elevated. These leaves often difier mate- 

 rially from the succeeding true leaves of the plant, and seem to 

 form a kind of medium between the cotyledon and true leaves. 

 A similar course is followed by monocotyledons ; but as the co- 

 tyledon does not rise above the earth, this organ is not ob- 

 served. When leaves have their origin at, or below the surface 

 of the earth, seeming to come from the root, they are called 

 radical leaves, although it is not strictly a correct term ; as it 

 seems to imply that the leaves originate from the root, which 

 is not the case; the root, under ordinary circumstances, not 

 producing leaves. Leaves having their origin on the main 

 stem, are called cauline ; those arising from branches are dis- 

 tinguished by the term rarneal : when leaves are found among 

 the flowers or on fiower-bearing branches, they are called y?om/ 

 leaves. 



83. The structure of the leaf demands the most careful at- 

 tention, both from the singularly curious arrangement of its 

 parts, and the manifest design exhibited in fitting it for the 

 various functions it is found to perform. Although to the com- 

 mon observer the leaf appears a veiy simple organ, composed 

 only of fibrous veins, and cellular substance ; yet, by the aid 

 of the microscope it is found to be one of the most complicated 

 of the vegetable organs. The leaf is covered, like the other 

 parts of the plant, by an epidermis, except such as are sub- 

 merged in water, and is furnished with pores or stomata. These 

 pores, in most plants, are more numerous on the under than on 

 the upper surface. In leaves which grow nearly perpendicularly 



Is the particular position of a leaf necessary ? — 82, What are the first 

 origans that a{)pear ? What are primordial leaves ? What are radical leaves 1 

 What cauline ? What floral ?— 83. With what is the leaf covered ? V/hat 

 are stomata ? On which surface most numerous ? How with perpendicu- 

 lar loaves ? 



