GRAMINE^ (POACE^), GRASS FAMILY 77 



sod that would be as enduring under conditions affecting the 

 sod surface, such as heavy trampling or close grazing. 



Leaves. — General Characteristics. — In grasses, a single leaf 

 arises at each node. Leaves disposed in this fashion along a 

 stem are said to be alternate. If one starts with a certain 

 leaf, the leaf next above or next below, is on the opposite side 

 of the stem, i8o° around the circumference. This arrange- 

 ment gives two vertical rows of leaves opposite each other 

 on the stem. Such an arrangement is said to be two-ranked, 

 distichous, or one-half spiral. We shall have occasion further 

 on to discuss other leaf arrangements, and to emphasize 

 the fact that leaves are developed on a stem in a definite 

 order. 



The grass leaf in general appearance is unlike that of such 

 common plants as apple, cottonwood, maple and beet. In 

 these the leaf has a definite, narrow stalk or petiole and an 

 expanded blade (Fig. 159). The grass leaf is divided into 

 two distinct parts, sheath and hlade (Fig. 26). The sheath 

 represents the leaf base, and forms a tube around the culm. 

 At the base of the leaf sheath, there is a distinct swelling. 

 The more or less flattened part of the leaf which spreads 

 away from the culm is the hlade (lamina). The blades 

 are parallel- veined, that is, have many veins, about equal 

 in size, running parallel, and joined by inconspicuous vein- 

 lets. Parallel venation is characteristic of the leaves of 

 grasses, sedges, rushes, lilies and most all other monocoty- 

 ledonous plants. 



Growth of Leaves. — In the early life of the grass plant, 

 leaves grow faster than internodes. This results in a tuft 

 of leaves. Some leaves elongate indefinitely. The tip of 

 the leaf blade is the oldest portion. The growing point is 

 at the base of the blade. This growing zone, as a rule, is 

 marked by a whitish or light green semicircle (Fig. 26). 



