22 GRASSES OF IOWA. 



The Leaves. 



Orossoharacter. — The leaves of grasse3 have two distinct 

 parts: the sheath and the blade. The sheath, or basal portion, 

 usually closely surrounds the stem, and is split or open upon 

 the side opposite the blade, or is entire, then forming a closed 

 cylindrical sheath (vagina) about the stem. When split, the 

 free edges usually overlap each other. At the top of the leaf- 

 sheath, at the point where the blade originates, there is upon 

 the inside usually a thin and delicate prolongation, often very 

 short, called the ligule. Sometimes the ligule takes the form 

 of a fringe of hairs. The leaf-blade is generally narrow, usu- 

 ally many times longer than broad, with nearly parallel edges. 

 Such leaves are called linear. From this form the leaves may 

 vary to lanceolate or ovate in outline. Sometimes the narrow 

 leaves have their edges rolled inward, when they are said to 

 be involute. Occasionally the leaf-blade is very short, and 

 sometimes it is wanting altogether, the sheath alone remain- 

 ing. As to the surfaces of the leaves and sheaths, they may 

 be smooth or rough, or more or less hairy. The terms used 

 here are those of general application. 



The position of the leaves on the stem is to be noted as 

 affording a ready character for distinguishing grasses from the 

 nearly allied grass-like sedges. Starting with any leaf on the 

 stem of a grass, the next leaf above will be exactly on the 

 opposite side of the stem; while the next or second leaf above 

 will stand directly over the starting point. Such an arrange- 

 ment is called distichous or two-ranked; i. e., in counting two 

 leaves from the first we pass completely around the stem. In 

 sedges the leaf arrangement is three-ranked; it is the third 

 leaf from the first which stands directly above the first. 



The mature blades often show torsion, being either twisted 

 to the left, right, or in both directions; being twisted in one 

 direction above and the other below. Certain grasses are 

 turned 180 degrees at the base so that the upper and lower 

 sides are reversed. In this case the stomata occur on the 

 original upper side. This is true of grasses growing in the 

 w^oods and is brought about through the influence of light. 



Minute anatomy. — The fibro- vascular bundles are separate or 

 they unite to form a strong mid-rib as in corn. The tibro-vas- 

 cular bundles of the leaf consist of the soft bast phloem and 

 woody elements, the xylem. The bundle is protected on each 

 side by sclerenchyma, true at least of the primary veins; this 



