296 BOTANY. 



shrubs and trees the roots are of course perennial. Many rootlets, 

 however, even in trees and shrubs, die off in the autumn, and new 

 ones are produced in the spring. 



Branching — The branching of roots is usually very irregular. 

 Where roots are branched, the main root is called the primary root, 

 while its branches are secondary roots. In examining the branches of 

 roots, notice that they spring from beneath the surface of the main 

 root. In this they differ from the branches of stems. In stems the 

 surface of the main stem is continuous with that of its branches, but 

 in roots the surface is broken at the points where branches emerge. 



§ 3. The Leap. 



Position on the Stem — Leaves grow upon the stem in several 

 ways. In some cases they are scattered (or alternate Fig. 184); in 

 others they are opposite (Fig. 185) ; in others again they are whorled 

 (i.e., several occupy a circle around the stem). 



Parts — Many leaves have three well-defined parts : 1. A broad 

 or flattened part, t)ieMade; 2. A leaf -stalk, upon which the blade is 

 supported, the petiole ; 3. Two little appendages or lobes at or near 

 the base of the petiole, the stipules. (Fig. 186.) 



Blade — The blade is always one piece when the leaf is very 

 young (i.e., very early in its growth in the bud). In many cases it 

 remains so in all its subsequent growth, and is said to be simple. 

 Very commonly, however, even in simple leaves the blade has 

 branched more or less in its growth, giving 

 rise to lobes of various sizes and forms (the 

 lobed leaf). The indentation between two 

 lobes is termed a sinus (Fig. 187). When 

 the branching is so profound that the lobes 

 have become separable leaflets, the blade is 

 said to be compound. 



The branches of the blade may radiate 

 from a common central point [radiately 

 lobed, radiately compound, or, more com- 

 monly, palmately lobed. Fig. 188, palmate- 

 patoVelyTobtdtaf.'' *"■ '^ compound. Fig. 189) ; or they may grow 

 out on opposite sides of an axial portion 

 (pinnately lobed, Fig. 187, pinnately compound, Fig. 190). Leaf- 

 branches may branch again ; thus we may have twice palmately lobed 

 and tioice palmately compound leaves, and likewise twiee pinnately 

 lobed, twice pinnately compound leaves, etc. , etc. 



Forms of Blade. — The forms of the blade may be concisely ar- 

 ranged as follows (Fig. 191) : 



