[Chap. Ill LEARNING TO NAME PLANTS 19 



Some leaves have no apparent petioles and are described as sessile. 

 The leaves of many grasses, such as those of oats, wheat, and bluegrass, 

 have neither petioles nor stipules; the blades are attached to the stem by 

 a sheath which may be long or short. At the top of the sheath is a collar- 

 like extension called the ligule. The ligule of the bamboo and certain 

 other grasses may consist merely of several long bristles. 



Fig. 14. Diagrams of some parts of leaves. A, blade, petiole, and stipules of 

 apple leaf; B, stem of pea bearing leaves composed of two large stipules, leaflets, 

 and a terminal tendril; C, sessile leaves of zinnia; D, blade, ligule, and sheath of a 

 grass leaf. 



The needle leaves of such trees as pines and spruces appear super- 

 ficially to be quite unlike those of broad-leaved trees. The leaves of 

 pines and larches are in clusters, or fascicles, at the end of short dwarf 

 branches. Spruce, fir, and hemlock have solitary leaves. The leaves of 

 arbor vitae are small scales oppositely arranged on the stem. 



The veins. The most conspicuous structures of an elm leaf are the 

 veins. The large vein near the middle of the blade is the inidrib. In a 

 maple leaf there are several prominent veins which are called the prin- 

 cipal veins. In general, the smaller veins form a network and unite with 

 larger veins, which in turn connect with the midrib or the principal 



