| ||Vegetative parts | Other grasslike plants | Glossary
It is strange that grass, the most common plant in most places in the world, is intimately known to rather few people. The structure of a grass is, however, just as easy to understand as that of any other plant. A typical grass is shown in Figure 1.
The roots of grasses, like those of other land plants, serve to anchor the plant in the soil and to conduct water and nutrients in solution from the soil to the plant parts. The main body of a grass comprises the culm, or stalk, and the leaves. In most grasses the culm is hollow and is composed of several tubes closed at the joints by solid tissue. The joints are nodes; the portions between them are internodes. One or more buds, which are undeveloped leaf or flower branches, may form at every node.
The leaves are arranged in two ranks, that is, alternately on opposite "sides" of the culm. The base of each leaf is at a node. The expanded upper part of the leaf is the blade; the lower part, which is usually split on the "side" opposite the blade, surrounds the culm, and is called the sheath.
At the junction of the sheath and blade, and facing the culm, there is almost always a small appendage termed the ligule. At the same place, but on the outer or lower "side", there is a band called the collar. Usually, on either side of the split in the sheath the collar bears projections termed auricles.
An important feature in the identification of grasses is the emerging leaf, that is, the new growth as it emerges from the sheath. The emerging leaf is described on page 13.
A description of the flowers is not needed here, because in this book grasses are identified by vegetative characters alone.
A fuller account of essential vegetative characters follows.
In grasses there are mainly three types of roots (Figure 2). A fibrous root branches in all downward directions from the crown, as in crested wheat grass (Agropyron cristatum) or rough fescue (Festuca campestris). Rhizomatous roots, or rootstalks, are actually underground stems from which stalks and roots develop at intervals, as in western wheat grass (Agropyron smithii) , smooth brome (Bromus inermis), and some other grasses. Stolons are horizontal stalks that root at the nodes. The roots of creeping bent (Agrostis stolonifera var. major) are stoloniferous.
The sheath may be round or compressed. Occasionally it is keeled at the midrib. It is usually of a paler shade of green than the blade, and is often tinged with pink or purple at its base. These tints are not consistent enough to be of much value in identification, except in a few grasses such as meadow foxtail (Alopecurus pratensis). A sheath may be split completely, split with one margin overlapping the other, or without a split (Figure 5). A ruptured sheath must be distinguished from one that is naturally split.
The ligule may be a membrane or a fringe of hairs. In some species the ligule is not present. In texture and color it varies from thin and white, as in Canby blue grass (Poa canbyi), to thick and opaque, as in fringed brome (Bromus ciliatus). The length of a ligule may vary within a species, but its shape and color and the character of its margins are usually quite constant. The outer surface of the ligule is usually smooth, but it may be pubescent, as in reed canary grass (Phalaris arundinacea) and some species of Agrostis. The types, shapes, and margins of ligules are shown in Figure 6. Large ligules and those in old or dry material may be so frayed and broken that they are useless for identification purposes.
Figure 7. Types of collars
The band that forms the collar (Figure 7) may be vertically broad or narrow, continuous, or divided by a midrib. If it extends diagonally it is called oblique. The collar is usually smooth, but it may be hairy (pubescent) over the whole surface, as in quack grass (Agropyron repens), or it may have minute hairs (cilia) on the inner and lowermost portion of the margin. It is usually pale green or yellowish green, but it may be temporarily tinged with red, as in yellow foxtail (Setaria glauca).
Figure 8. Shapes of auricles and (right) a collar without auricles.
Auricles (Figure 8) are appendages extending from the collar. They may be horizontally clawed, fully or slightly rounded, or rudimentary. Sometimes they are not present. When old or dry specimens are being examined care must be taken to avoid breaking off the auricles.
A developing leaf is closely surrounded by the sheath of the previously developed leaf (Figure 9). In the developing state, a blade may be conduplicate, that is, folded at the midvein with the margins meeting, or it may be rolled lengthwise with either one margin overlapping the other (convolute) or both margins turned inward toward the midrib (involute). Conduplicate leaves usually form a laterally compressed shoot, rolled leaves usually a cylindrical one. There are exceptions. In yellow foxtail (Setaria glauca) and barnyard grass (Echinochloa crusgalli) the leaves are rolled in flattened shoots, whereas in poverty oat grass (Danthonia spicata), June grass (Koeleria gracilis), and certain fescues, the folded leaves are in round shoots. In plains reed grass (Calamagrostis mortanensis) and Nuttall's alkali grass (Puccinellia nuttalliana) the leaves are involute in somewhat rounded shoots.
The shape of the emerging leaves can best be seen when the stalk is cut just below the ligule and examined with a hand lens.
Figure 1. A grass plant, showing the vegetative parts.
The blade (Figure 3) is usually long and narrow. Though ordinarily rather flat, it may be V-shaped, folded, or rounded. When it is so tightly folded that it seems to be solid and cylindrical, it is termed bristle-like. A rounded leaf may be U-shaped, cylindrical with one margin overlapping the other (convolute), or cylindrical with margins turned inward toward the midrib (involute). A blade may be constricted at the base, or sometimes twisted.
Figure 2. Types of grass roots.
Its tip may be sharp-pointed or boat-shaped, and the general shape may be tapered or mostly with parallel sides (Figure 4). The surface may be smooth, rough, or hairy, and the margins smooth, without hairs (glabrous), or with a fringe of hairs (ciliate). Veins may be so prominent on the upper surface that they form ridges. On the lower surface the midvein or midrib may be prominent enough to form a keel.
Figure 3. Shapes of blades, in cross section.
Figure 4. Shapes of blades and tips.
Figure 5. Split and unsplit sheaths, viewed directly and in cross section.
Figure 6. Ligules, showing (upper row) types (and, right a leaf without a ligule), (middle row) shapes, and (lower row) margins.
Figure 9. Shapes of emerging leaves, with, at right of each leaf, a cross-section view at the juncture of leaf blade and sheath.
Other Grasslike Plants
Some common plants, mainly the sedges and rushes, are in certain ways similar to grasses. Table 1 presents comparative information.
Table 1. A comparison of plant parts in the grasses, sedges, and rushes
|Culm||Usually hollow; cylindircal or flattened|
|Filled with pith, rarely hollow, usually three-sided|
|Filled with spongelike pith, cylindrycal|
|Leaf blade||Usually flat; often folded, involute, or bristle-like; glabrous or pubescent||Flat, folded, or bristle-like; rarely pubescent||Channeled or round, usually glabrous|
|Leaf margins||Smooth, scaborous, ciliate||Usually scabrous||Smooth|
|Leaf sheath||Usually split, |
|Usually closed||Open or closed|
|Ligule||Usually absent||Absent of weakly developed||Absent of weakly developed|
|Auricles||Present or absent||absent||abesent|
Definitions are limited to the sense of the word as it applies to vegetative parts of grasses in this publication.
|acuminate||of a blade, ligule, stolon, or rhizome: gradually tapering to a sharp point|
|acute||of a blade or ligule: abruptly sharp-pointed|
|annual||of a plant: (noun) a plant that completes its life cycle in one growing season|
(adjective) completing the life cycle in one growing season
|ascending||of a plant part: sloping upward|
|auricle||an appendage of a collar|
|blade||the portion of the leaf above the sheath|
|bulbous||of a stalk: having a bulb-like base|
|chartaceous||of a ligule: having the texture of writing paper|
|ciliate||of a blade, collar, or ligule: fringed with fine hairs|
|ciliolate||of a ligule or blade margin: fringed with minute hairs|
|clasping||of an auricle: having the appendage extending directly into the collar and partly or completely surrounding the blade|
|collar||the band on the outer or lower "side" of the leaf at the junction of the sheath and the blade|
|compressed||of a sheath: flattened laterally|
|conduplicate||of a leaf: folded together lengthwise in the emerging leaf|
|constricted||of a collar: narrowed at the midpoint|
|continuous||of a collar: extending from one margin of the sheath to the other|
|convolute||of a leaf: rolled lengthwise in the emerging leaf, with one margin overlapping the other |
|coriaceous||of a leaf: leathery in texture |
|corrugated||of a ligule: having alternating ridges and grooves|
|crown||of a plant: the region from which both stalks and roots grow|
|decumbent||of a stalk: curved upward from a horizontal or slightly inclined base|
|divided||of a collar: separated at the midrib|
|emarginate||of a ligule: notched at the tip |
|entire||of a blade or ligule: having the margins continuous and not in any way divided|
|fascicle||of leaves: a compact cluster|
|fibrillose||of a root: having fine fibers |
|filiform||of a blade: very slender; threadlike|
|flaccid||of a blade: lax and weak; lacking rigidity|
|flexuous||of a blade: having alternate opposite curvatures|
|glabrous||of a blade, collar, or sheath: without hairs|
|glaucous||of a blade: covered with a waxy layer, which gives the plant part a bluish green color |
|hirsute||of a blade or sheath: having coarse, straight, rather stiff hairs|
|hispid||of a blade or sheath: having stiff hairs or bristles|
|hyaline||of a sheath margin: thin and translucent or transparent|
|hybrid||of a plant: a cross between two species of plants|
|involute||of a blade or an emerging leaf: having margins rolled over the upper surface toward the midrib|
|keel ||a ridge on the back of a sheath or blade usually along the midrib|
|lacerate||of a ligule: having margins deeply and irregularly cut|
|ligule||a thin appendage projecting from the inner surface of a leaf at the junction of the sheath and blade|
|linear||of a blade: long and narrow with parallel sides|
|membranous||of a ligule: thin, rather soft, and more or less translucent and pliable|
|midrib||of a blade: the central vein |
|muriform||of cells: arranged like rows of bricks |
|oblique||of a collar: slanting and of varying width |
|obtuse||of a ligule: blunt or rounded at the tip|
|papillose||of a blade or sheath: having minute nipple-shaped projections |
|perennial||of a plant: (noun) a plant that lives for more than two years. (adjective) living more than two years.|
|pilose||of a plant part: having soft, fairly long, straight hairs |
|procumbent||of a stalk: partly prostrate|
|prostrate||of a stalk: trailing along the ground|
|puberulent||of a sheath: having fine minute hairs|
|pubescent||of a plant part: having soft fine hairs|
|retrorse||of hairs: bent downward or backward|
|rhizome||an underground stalk from which, at intervals, shoots arise above and roots descend below; also called rootstalk|
|ridged||of a blade: having raised veins|
|rudimentary||of a plant part: imperfectly developed|
|scaberulous||of a blade: having minute protuberances that are rough to the touch|
|scabrous||of a blade: having small protuberances that are rough to the touch|
|scabrous||of a sheath or blade: having rather thin, translucent, not green, dry margins|
|serrate||of a ligule or blade margin: having sharp teeth|
|serrulate||of a ligule: having fine teeth|
|sheath||the lower part of the leaf that surrounds the stem or shoot|
|stolon||an aboveground creeping stem that roots at the nodes|
|stoloniferous||of a plant: bearing stolons|
|stoma||(plural: stomata) a minute opening in the surface of a leaf|
|striate||of a sheath: having very narrow longitudinal lines of darker or lighter color than the adjacent tissue|
|truncate||of a ligule: having the tip seemingly cut off at a right angle to the micrib|
|tufted||of plants: having shoots in a loose, compact, or dense cluster arising from a crown|
|undulate||of a Mule: having a wavy summit|
|vein ||of a blade or sheath: one of the parallel structures that serve to support the leaf|
|villose||of a plant part: having long, soft hairs|| |