Digital Key and Grass Images
The grass family, Gramineae (Poaceae), is a diverse, widely-distributed group of plants. Its wide distribution and large number of economically-important food crops (corn, wheat, rice, sorghum) make the grass family one of the most globally important plant families. Rangelands, principally dominated by grasses, represent nearly one-half of the earth's surface. As rangeland species, they provide tremendous grazing resources, shaped by evolution to withstand grazing, fire, disease, and periodic droughts. They represent the best use for lands that are typically unsuited for tillage agriculture, since most rangelands are too arid, rocky, or rough to be cultivated. Domesticated grasses on the other hand provide the bulk of the basic food resources, and world politics are influenced by production and distribution of corn, wheat, or rice.
The grass family ranks third in number of genera and fifth in number of species among higher plant families with some 600 genera and about 7500 species. Kansas grasses include some 65 genera and 216 species, of which we have included 195 species.
Identification of Grasses
This manual is not intended as a taxonomic treatise, rather it is designed as an identification tool. Consequently, the keys are strictly mechanical, and the species are presented alphabetically. In order to use this manual, a working knowledge of certain morphologic characters which aid in differentiation among the many grass species is necessary and are described below Fig. 1).
Figure 1. The grass plant.
Roots. Grasses typically have a fibrous root system. The primary or seminal root is usually short-lived, and the secondary or adventitious roots arising from the lower portion of the culm (grass stem) form the bulk of the root system. Grass roots are relatively short-lived with the entire root system being replaced over a 3- to 4-year period. Some roots live for only a matter of hours. Particularly in prairies, the majority of the root mass is in the upper 30-50 cm of soil. Root characteristics are generally of little benefit in identification, but may be of taxonomic importance (Pohl, 1966).
Stems. Terminology of stem types depends on whether the stem is erect or repent. Erect stems are termed culms. The stem may not elongate until late in seasonal development or not at all. Usually, the term, culm, is reserved for elongated, erect stems. Leafy, non flowering basal shoots are called innovations. The culm is jointed with elongated sections, termed internodes, which may be hollow, solid, or intermediate. Culms can have three basic shapes; flat, elliptical, or round (Fig. 2). Where these sections join, the solid and swollen juncture is called a node . Depending upon mode of asexual reproduction, erect stems may be tightly grouped and referred to as bunchgrasses (typical of innovatively-reproduced plants). Those species which reproduce by elongated, repent aerial or subterranean stems are called sod-formers. Those repent stems that are below the soil surface are called rhizomes and the aerial ones, stolons. (Fig. 1). Both these stem types usually root at the nodes. A few species have thickened stem bases modified for food storage, typically called corms, but sometimes erroneously referred to as bulbs. Often a stem may expand horizontally for a short distance from the central stem base before turning upward. Harrington (1977), for want of a better term, has named them false rhizomes. A good example of this phenomenon is eastern gamagrass (Tripsacum dactyloides), often incorrectly referred to as rhizomatous.
Figure 2. Cross-section of grass culms (stems.
Stem growth in grasses begins at bud break or germination. For most species, particularly perennials, there is little stem elongation and internodes are short, which serves to keep the apical meristem at/or below the soil surface. That condition persists until the stem becomes reproductive. At that time, internode elongation of newly-derived stem tissue pushes the apical meristem upward and floral structures are differentiated. Many species have only a small percentage of their tillers which become reproductive. That adaptation allows for leaf replacement to occur from a protected apical meristem.
Basically, grasses are herbaceous with annual culms. A few species, like bamboo, have perennial culms. If a grass plant has modified food storage organs such as rhizomes, corms, enlarged stem bases, and stolons, they will exhibit perenniality. Many identification decisions are based on whether a plant is a perennial or an annual. Presence or absence of modified food storage organs and previous year's stem bases are good clues as to perenniality.
Leaves. The grass leaf is made up of two basic parts, the blade and the sheath (Fig. 1). They are 2-ranked, arising alternately on the culm. The sheath encloses the culm and provides support for leaf attachment (Fig. 8). Leaves arising during early growth from an unelongated stem have sheaths that are very short and nonfunctional. The intercalary meristem at the sheath base differentiates sheath cells, but only with stem elongation do sheath cells elongate. That adaptation keeps the blade intercalary meristem at/or below the soil surface, thereby protecting it from grazing or other removal. The leaf blade is the expanded, dependent portion which extends away from the culm. Leaf blades, though basically narrowly linear, vary in shape (Fig. 3). The collar region is the juncture of the sheath and the blade. Auricles are ear-like projections that may extend from the lower edge of the leaf blade (Fig. 4). Since they are uncommon, auricles are an excellent identification tool. The ligule is a collar-like projection of the sheath at the base of the blade (Fig. 7). Ligules vary from membranous to rigid or may be made up of hairs. Their varied shape and consistency are excellent identification tools. Leaf surfaces also offer opportunity for differentiation among species. Presence or absence of hairs and their characteristics along with surface topography (strongly-veined vs. smooth) are important key characters (Fig. 5). Leaf margins may have hairs, pustules, serrations, and be wavy or smooth (Fig. 4).
Figure 3. Cross-section of grass leaf blades
Figure 4. Margin of grass leaf blades.
Figure 5. Grass leaf blade hairs.
Figure 6. The collar region of grass leaf blades.
Figure 7. Grass leaf blade ligules.
Figure 8. Sheath characteristics of grass leaf blades.
Inflorescence. That portion of a flowering culm upward from the node at the base of the uppermost leaf is the inflorescence. Since true flower parts in grasses are inconsequential in identification, spikelet arrangement will be used for description of the inflorescence type. The inflorescence of grasses has been classified variously. Nonetheless, there are three basic types: the panicle, the raceme, and the spike (Fig. 9). Spike - the spikelets are attached directly (sessile) to the central axis of the inflorescence (rachis), without a subtending stalk. Raceme - the spikelets are attached directly to the rachis by a single stalk (pedicel). Panicle - the spikelets are born on branches from the central axis of the inflorescence.
Figure 9. Inflorescence types.
Certain problems arise when using only these three inflorescence types to describe a number of genera. Allred (1982) suggested that descriptive adjectives be used to qualify the pattern variation (Fig. 9). For example, green spangletop (Leptochloa dubia) would be a panicle of racemose primary branches, blue grama (Bouteloua gracilis) is described as a panicle of spicate primary branches, and windmillgrass (Chloris verticellata) a panicle of digitate spicate branches. The genera, Andropogon and Hordeum, usually have both sessile and pedicellate spikelets on the main axis and are spicate racemes. If the main axis is branched, with the branches having pedicellate spikelets borne singly along the branch, it is a panicle of racemose branches. Pohl (1968) termed the inflorescence of Andropogon a rame, which is an inflorescence branch with repeating pairs of sessile and pedicellate spikelets. Therefore, the genus Andropogon, with certain exceptions is a ramose panicle. Some genera such as Phleum have very short pedicellate branches, and are called spikelike panicles.
Within an inflorescence the term rachis refers to the central axis. For branched inflorescences, the branches from the rachis are called the primary branch. Further branching leads to secondary branches, tertiary branches, and so forth (Fig. 9). Another term often used in inflorescence description is peduncle, defined as the stalk of a cluster of spikelets (Gould, 1978).
Spikelets. The basic unit of the grass inflorescence is the spikelet. The central axis of the spikelet is the rachilla to which spikelet parts are attached. Attached to the lower most portion of the spikelet are two (sometimes one or none) sterile bracts called glumes (Fig. 10 ). Above the glumes on the rachilla may be one or more florets. A floret consists of two bracts, the lemma and the palea, which enclose the grass flower (Fig. 11). The grass flower consists of a vestigial perianth called lodicules and the stamen and pistil. Lodicules increase in size at flowering and force the lemma and palea open such that stigma and anther exertion can occur.
Characteristics of the glumes, lemma, and palea are usually the mechanistic basis for separation of grass species. They, like the leaf from which they were derived, have midribs which vary from obscure to prominent and nerves (veins) which vary in number, location, and prominence. Often the midrib of the lemma is extended into an awn. Awns may be fused at the base as in Aristida, twisted as in Stipa, or bent or geniculate as in Danthonia. Most often the palea fits within the enrolled edges of the lemma and does not often offer distinguishing characteristics.
Figure 10. Illustration of a bluegrass (Poa) spikelet and individual floret.
Figure 11. Illustration of a bristlegrass (Setaria) spikelet.
Disarticulation. Grasses shed their seeds as naked caryopsis, florets, spikelets, or entire inflorescences. Shedding of their seeds is accomplished by disarticulation. Certain points of attachment form abscission layers (points of weakness) which readily break at maturity thus disseminating the seeds or fruits. Members of the genus, Poa and related genera, typically disarticulate above the glumes and within the spikelet, while Panicum and related genera disarticulate below the glumes shedding the entire spikelet. Some species shed naked caryopses such as Sporobolus. Others shed the entire inflorescence such as Schedonnardus.
Compression. Spikelets are seldom round, but usually are flattened or compressed. Lateral compression (Fig. 12 ) occurs when the bracts of the spikelet are flattened from the sides, while dorsal compression occurs by flattening from the back of the bracts. Poa and related genera generally exhibit lateral compression. Dorsal compression is most often found in spikelets of Panicum and related genera where a sterile floret is borne below the perfect one.
Figure 12. Illustration of compression of spikelets.
Figure 13. Illustration of a typical grass flower.
Copyright 2002. Clenton Owensby