98 OUTLINES OF PLANT LIFE. „ 



121. 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 (i) the 

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

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

 cessive 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 ; or the sixth over the first, and so on. A transverse sec- 

 tion of an opening 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. 



122. 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 abun- 

 dant form of secondary leaves is foliage leaves. These may 

 be very simple, as the ' ' needles ' ' of the pines, or differen- 

 tiated 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 pro- 

 duced each season, as in many ferns, the mature form may 

 not appear for several years. 



123. Foliage leaves. — ^A well-developed foliage leaf has 

 three parts, the base, the stalk, the blade (fig. 87). The 



