THE LEAVES. 1 19 



148. Arrangement. — Leaves appear in regular succession 

 upon the stem, the youngest being nearest the apex. Their 

 distribution along the sides of the stem, though extremely 

 various, may be reduced to two main types. Either (1) the 

 leaves are formed singly at the nodes, or (2) more than one 

 leaf occurs at each node. When the leaves are single, succes- 

 sive leaves may stand upon exactly opposite sides of the stem, 

 so that the third leaf, counting from below upwards, stands 

 over the first ; or the fourth leaf may stand over the first j or 

 the sixth over the first, and so on. A transverse section of a 

 bud shows the mode of arrangement, and a study of such 

 sections makes it evident that each leaf appears in the widest 

 space between the two preceding leaves, i.e., where it 

 encounters the least resistance. That this is the determining 

 factor is shown by the fact that the order of arrangement may 

 be artificially altered by pressure or distortion of the bud. 

 When two or more leaves occur at each node, the members 

 of successive circles ordinarily alternate with each other. 

 This alternation is due to the same cause. 



149. Form. — -Leaves show a great variety of form and 

 structure. Even upon the same plant leaves of various forms 

 occur. The primary leaves are usually different from the 

 secondary leaves, both in form and size. The most abundant 

 form of secondary leaves is foliage leaves. These may be 

 very simple, as the " needles " of the pines, or differentiated 

 more completely, as in the deciduous trees. The mature form 

 of the complex foliage leaf is frequently not attained until 

 several nodes above the point at which the primary leaves 

 arise; and, if only one or two leaves are produced each 

 season, as in many ferns, the mature form may not appear for 

 several years. 



150. Foliage leaves. — A well-developed foliage leaf may 

 usually be divided into two equal parts by a plane passing 

 through its base and the axis of the stem to which it is 



