The chief problem with grasses for the novice is knowing what he or she is looking at. Once one becomes familiar with the specialized parts of the grass inflorescence, these plants are no more difficult than others to identify. The overall inflorescence of a grass is described in the usual terms, such as “panicle” or “spike,” although each unit whether sessile (in a spike) or pediceled is not a single flower alone. The basic unit of the grass inflorescence is a spikelet containing one or more flowers, each called a floret. Except in the rare instances when one or both are absent, there is a pair of scales or bracts called glumes at the base of each spikelet. In addition, at the base of each floret are two often similar scales or bracts enclosing the reproductive parts. The lower (outer) scale is the lemma and the upper (inner) one the palea. The palea is generally two-nerved or two-ribbed and is often smaller and less firm in texture than the lemma and sometimes difficult to distinguish without a careful dissection. Most grass flowers—within the lemma and palea—consist of 3 stamens and a single pistil. Some flowers are sterile (lacking a pistil or all reproductive parts), some have 1, 2, or 6 stamens, and some are unisexual. The axis of a spikelet, to which the florets are attached, is the rachilla. The hard often enlarged area at the base of a floret, sometimes bearing hairs, is the callus. Tiny swollen scale-like structures at the base of the stamens are lodicules and may represent a very much reduced perianth; they are not mentioned in any of the keys here. A pulvinus is a swelling at the base of a branch of the inflorescence. The fruit produced by a grass flower is almost always a seedlike grain or caryopsis, a dry indehiscent one-seeded fruit with the pericarp adnate to the seed (in an achene the pericarp is free).
The leaf of a grass consists of a blade, generally narrow and elongate, and a sheath which surrounds the stem like a sleeve. The sheath is usually open, i.e., with the margins separate, but in some genera (including the common Bromus and Glyceria) it is closed, i.e., with the margins fused. At the inside summit of the sheath is almost always a small appendage, the ligule, which may be either a fringe of hairs or a membranous collar (or, rarely, a combination). At the junction of the blade and sheath in some species is a pair of prolongations called auricles; these are lateral and often tend to clasp the stem. When checking leaf surfaces, many grasses rotate their leaves 180 degrees soon after diverging from the sheath, such that what appears to be the lower surface is actually the upper (i.e., adaxial).
The following notes may help in interpreting statements in the keys and other comments. Remember that the florets in a spikelet are basically alternate in arrangement.
In 1-flowered spikelets, the first (or lowest) glume is below the lemma and the second glume is below the palea. In several-flowered spikelets, the first (or lowest) glume is below the lowest lemma and the second glume is below the second lowest lemma. However, in order to avoid the necessity of specifying “first glume” or “second glume,” the keys often refer merely to the “larger” or “smaller” glume, because the relative size is more readily seen for quick identification; such references to size carry no implication as to whether the larger glume is the first or second one.
When it is necessary to determine the place of articulation (whether above or below the glumes at the summit of the pedicel) or whether there is an articulation at all, it can often be determined in dried specimens, even those not yet mature enough to be disarticulating naturally, by attempting a break with a dissecting needle or fine forceps. The break will usually occur at the place of articulation. Ligule measurements are based on ligules from the upper and middle portion of a plant, not necessarily the lower. For ease in keying, the genera are here arranged in artificial groups that have their origins in older classifications dividing the family into Tribes based on spikelet morphology and arrangement. We do note instances where a group does correspond largely or in part to a natural entity. When a single genus is keyed out in the key to groups, it is nevertheless also included in the numbered generic key for completeness.
KEY TO GROUPS
1. Culms woody, perennial (bamboo), intricately branched from the upper nodes; leaf blades narrowed to a distinct petiole-like base.
1. Culms herbaceous, normally annual, branchless or few branched from the upper nodes; leaf blades lacking a petiole-like base.
2. Apex of plant with a large “tassel” or spike-like raceme bearing staminate florets in pairs, the pistillate florets either sunken in hardened joints of rachis below the staminate portion or in separate “ears” lower on the plant.
2. Apex of plant not as above: upper portion of inflorescence bearing pistillate or bisexual florets or rarely staminate florets in short one-sided spikes.
3. Spikelets concealed within ± globular, hard, bur-like structures.
4. Bur-like clusters intensely spiny, the spines radiating out in all directions; plants tufted fibrous rooted annuals.
GROUP 8 (Cenchrus)
4. Bur-like clusters not spiny, the projections all pointing upward; plants perennial and colonial from stolons.
GROUP 5 (Bouteloua dactyloides, female plants)
3. Spikelets exposed, not concealed within ± globular, bur-like structures.
5. Spikelets all unisexual, segregated into different and dissimilar appearing parts of the inflorescence or on different plants.
6. Staminate and pistillate spikelets on different plants; staminate plants with 1–4 sessile or short peduncled one sided racemes; stamens 3.
GROUP 5 (Bouteloua dactyloides, male plants)
6. Staminate and pistillate spikelets on the same plant, upper panicle branches bearing awned pistillate spikelets, lower branches bearing staminate spikelets; stamens 6.
GROUP 7 (Zizania)
5. Spikelets perfect, or if unisexual then scattered among bisexual ones.
7. Spikelets forming a simple spike or spikes, directly sessile or subsessile on main axis of inflorescence or at most on secondary branches.
8. Spike solitary, terminal (its rachis a continuation of the culm), the spikelets on opposite sides of rachis (one-sided only in Nardus, which lacks glumes).
8. Spikes several, one-sided (spikelets in two rows on one side of rachis).
9. Glumes keeled and ± equal (or the smaller half or more as long as the larger).
9. Glumes rounded on the back (not keeled), very unequal.
7. Spikelets not forming simple spikes as above: pediceled and/or on tertiary or further branches of the inflorescence; in some species congested and hence spike-like, but not directly sessile or subsessile (reduced panicle branches usually visible on close examination and removal of some spikelets).
10. Spikelets containing only 1–2 florets.
11. Spikelets with an involucre consisting of long subtending bristles.
11. Spikelets without an involucre of bristles (although glumes or lemmas may be awned).
12. Glumes or lemmas (or both) ± laterally compressed or keeled (lateral nerves, if present, less prominent than midnerve).
go to couplet 17
12. Glumes and lemmas rounded on back, not keeled (nerves, if present, about equally prominent).
13. Glumes very unequal in length, one of them minute, or absent, or at most about half as long as the spikelet. [Note: In the species of group 8, a sterile lemma is present that closely resembles the large second glume opposite it and might easily be misinterpreted as a glume. Look for the true first glume as a small, sometimes minute and membranous, even deciduous, scale at the very base of the spikelet; a reduced palea, often associated with the sterile lemma, will also help to identify the latter as part of a sterile floret and not a glume].
14. Spikelets disarticulating below the glumes (except in Setaria italica), ± elliptic (less than 3 times as long as wide); a sterile lemma resembling the larger glume present.
14. Spikelets disarticulating above the glumes, ± lanceolate (3–10 times as long as wide); no sterile lemma present.
13. Glumes ± equal in length, neither of them much reduced nor absent.
15. Spikelets paniculate, all (or mostly) 1-flowered, bisexual, the florets all alike (no sterile lemmas or separate sterile pedicels present); spikelets less than 4 mm long, except in Hesperostipa and Piptochaetium with awns over 5 cm long.
15. Spikelets paniculate or racemose, basically 2-flowered (the lower floret staminate or sterile with often suppressed palea); spikelets 3 mm or more long.
16. Spikelets all alike, not paired with a pedicel bearing a rudimentary, staminate, or no floret.
GROUP 8 (Panicum virgatum)
16. Spikelets in pairs, usually of two kinds (except in Miscanthus): one sessile and with a bisexual floret, the other a hairy pedicel with or without a staminate or rudimentary floret (rarely 2 stalked florets with 1 sessile one).
10. Spikelets containing 3 or more florets, including any sterile ones [Note: couplet 17 is also reached from couplet 12, if the spikelets are 1- or 2-flowered].
17. Spikelets all or mostly containing 1 bisexual floret and no sterile or vestigial ones below it.
18. Glumes both completely absent; spikelets strongly flattened, appressed and ± overlapping, the lemmas scabrous or hispid-ciliate.
GROUP 7 (Leersia)
18. Glumes (one or both) usually present; spikelets various but not as above.
17. Spikelets all or mostly containing 2–several florets, the lower ones sometimes staminate or rudimentary (scale-like or reduced to tiny hairy appendages; look carefully).
19. Glumes shorter than the lowest floret (excluding awns if present); awn of lemma none, terminal, arising from between terminal teeth and not twisted, or at most subterminal.
19. Glumes (at least one of them, not necessarily the first glume) longer than lowest floret; awn of lemma none or arising from between terminal teeth and strongly twisted below, or (the usual condition) inserted on the middle or lower part of the lemma.
20. Spikelets containing one bisexual awnless floret (the lemma sometimes membranaceous) with two additional, often dissimilar (sometimes awned) staminate, sterile, or vestigial lemmas below it.
20. Spikelets usually containing 2 or more bisexual florets (staminate or sterile florets, if present, above the fertile one and/or fertile lemma awned) [Note: Some species of group 3 will also run in this key to group 1 and are also included in the Key to Genera for that group].
KEY TO GENERA OF GROUP 1
1. Rachilla (above the lowest floret) with silky beard about equaling or exceeding the lemmas; plants tall and stout (usually over 1.5 m tall) with larger leaf blades 1–3.5 cm wide; spikelets ca. 11–17 mm long; ligule a densely ciliate brown band.
1. Rachilla with beard shorter or absent; plants generally less than 1.5 m tall with narrow leaves less than 1 cm wide; spikelets and ligule various.
2. Spikelets sessile or at most very short-pediceled, crowded into dense clusters, these either at the ends of elongate panicle branches or in a single congested, rather spike-like inflorescence.
3. Lemma with a prominent, somewhat twisted or spreading dorsal awn; rachilla villous.
GROUP 3 (Trisetum)
3. Lemma with awn absent or short and strictly terminal; rachilla not villous.
4. Clusters of spikelets at the ends of elongate naked branches of the panicle; sheaths closed much of their length.
Dactylis (in part)
4. Clusters of spikelets all crowded into a congested, rather spike-like inflorescence; sheaths open their entire length.
5. Spikelets of two kinds in a cluster: normal fertile and special sterile fan-like ones; fertile lemmas mostly short-or long-awned.
5. Spikelets all similar, fertile; lemmas not awned.
6. Foliage glabrous; flowering stems 3–20 cm, inflorescence 1–3.5 cm long; ± prostrate annuals.
GROUP 2 (Sclerochloa)
6. Foliage (at least the lowermost sheaths) pubescent or puberulent; flowering stems 20–100 cm tall, inflorescence (3–) 5–25 cm long; ± erect, perennials.
2. Spikelets short- to long-pediceled in a ± open panicle.
7. Callus at base of floret with dense beard of straight hairs 0.5 mm or more long.
8. Awn of lemma arising near base.
GROUP 3 (couplet 10)
8. Awn of lemma absent, terminal, subterminal, or arising between terminal teeth.
9. Lemmas awnless, weakly 5-nerved; sheaths open.
GROUP 3 (Graphephorum)
9. Lemmas with short or long awn, either 3-nerved or plants with closed sheaths.
10. Sheaths open; lemmas 3-nerved, ± truncate (ragged or lobed) at apex, the nerves hairy.
11. Panicles terminal and axillary, small, the former often partly and the latter entirely included in the swollen sheaths; palea villous on apical half; nodes bearded; plants annual, with the leaves, sheaths, and culms toward base upwardly scabrous.
11. Panicles terminal, large, exserted; palea not villous on apical half; nodes glabrous; plant a stout perennial smooth toward base.
10. Sheaths closed; lemmas 5–7-nerved, tapering to an apparently 2-lobed or sharply bifid apex, glabrous or hairy.
12. Callus with distinct beard, the lemma glabrous or nearly so; grain glabrous.
12. Callus lacking a distinct beard, the pubescence like that of the lemma; grain pubescent at the summit.
Bromus (in part)
7. Callus glabrous, minutely puberulent, or cobwebby (not densely bearded with straight hairs more than 0.5 mm long).
13. Glumes (at least one of them) and usually also lemmas strongly keeled (lemmas in a few species rounded on the back); awns absent or not over 2 mm.
14. Larger glumes ca. 4.5–7 mm long.
15. Spikelets nearly sessile, numerous, ascending in a ± crowded panicle; ligules ca. 2–8 mm long; sheaths closed much of their length.
Dactylis (in part)
15. Spikelets on long pedicels, ascending to drooping in a much expanded raceme or panicle; ligules less than 1.5 mm long; sheaths open [if confronted at this point with a plant bearing an open panicle and closed sheaths, try Bromus].
16. Lemmas smooth except on keel; spikelets very strongly flattened and keeled; lowest lemma sterile; leaf blades mostly more than 1 cm wide, flat.
16. Lemmas scabrous over the back; spikelets not strongly flattened, the glumes and lemmas ± obscurely keeled; lowest lemma fertile; leaf blades less than 1 cm wide, the lower ones ± involute.
14. Larger glumes not over 4.4 mm long (to 5 mm in Leptochloa, with ± one-sided inflorescence branches).
17. Larger glumes ± obovate, broadest above the middle.
17. Larger glumes broadest at or below the middle.
18. Ligule a fringe of hairs; lemmas with 3 prominent nerves, glabrous; spikelets 2–30-flowered.
18. Ligule a membranous scale, the cilia, if any, shorter than the scale; lemmas with 3–5 nerves, glabrous, hairy, and/or cobwebby at base; spikelets various [if specimens will not fit here, try couplet 20].
19. Glumes very unequal, the smaller (first) usually only slightly more than half the length of the larger; lemmas obscurely 2-toothed, with minute awn between teeth, the nerves silky-pubescent basally; callus glabrous.
GROUP 5 (Diplachne fusca)
19. Glumes slightly unequal; lemmas rounded or pointed, but neither toothed nor awned, the nerves pubescent or glabrous; callus often with a tuft of cobwebby hairs.
13. Glumes and lemmas ± rounded on the back, not keeled (or becomeing somewhat keeled only toward apex); awns absent or present.
20. Lemmas distinctly 3-nerved, leathery in texture.
21. Ligules membranous, finely ciliate at apex; plants rhizomatous colony formers of floodplain forests.
21. Ligules a fringe of hairs; plants densely tufted in open habitats.
20. Lemmas 5- (or many-) nerved (the nerves sometimes very indistinct), in most species thin textured.
22. Lemmas at least as broad as long; mature glumes and florets spreading nearly at right angles to rachilla, the spikelets nearly or quite as broad as long.
22. Lemmas longer than broad; glumes and florets not so widely spreading, the spikelets in most species much longer than broad.
23. Lemmas usually 2-toothed or minutely 2-lobed at the apex and usually with at least a short awn arising from just below or between the teeth (if teeth apparently united, as in some species of Bromus, the awn thus subterminal); sheaths closed nearly to their summit.
24. Spikelets narrowly linear-lanceolate on much shorter, densely hispid pedicels; lemmas awned, minutely strigose or scabrous, at least on the nerves; sheaths retrorsely scabrous; ligules 3–6 (–7) mm long; grain glabrous.
24. Spikelets broadly linear to oblong, usually on ± elongate pedicels; lemmas various; sheaths glabrous or pubescent but not scabrous; ligules less than 2.5 (–4) mm long; grain pubescent at the summit.
Bromus (in part)
23. Lemmas not 2-lobed or 2-toothed at apex, awnless or with strictly terminal awn; sheaths open (or closed in Glyceria, with prominently nerved and awnless lemmas, and in the youngest shoots of Festuca rubra).
25. Lemmas acute at the tip, awned.
26. Blades of leaves flat (or merely once-folded), at least the larger ones (2.5–) 3–8 mm broad.
Lolium (in part)
26. Blades of leaves ± strongly involute, less (usually much less) than 3 mm broad.
Festuca (in part)
25. Lemmas acutish or obtuse, awnless.
27. Nerves of lemma prominent, straight and becoming parallel at the tip; sheaths closed or open.
28. Sheaths closed much of their length (but easily splitting); second (larger) glume with 1 distinct nerve; plants rhizomatous.
28. Sheaths completely open; second glume with 3 (–5) nerves distinct at its base; plants without rhizomes (though culms may be decumbent or prostrate).
27. Nerves of lemma very weak (or if visible, then converging, not parallel, at the tip); sheaths open.
29. Lemmas ca. 2 mm long.
29. Lemmas 2.5–8 mm long.
30. Blades of leaves ± strongly involute, less (usually much less) than 3 mm broad.
Festuca (in part)
30. Blades of leaves flat (or merely once-folded), at least the larger ones (2.5–) 3–8 mm broad.
31. Larger lemmas 2.5–4.5 mm long; anthers 0.8–1.4 mm long; spikelets mostly containing 2–4 (–5) florets and borne beyond the middle of the primary panicle branches.
31. Larger lemmas 5.5–8 mm long; anthers 2.2–3.5 (–3.8) mm long; spikelets often containing 5 or more florets, borne below as well as above the middle of the primary panicle branches.
Lolium (in part)
KEY TO GENERA OF GROUP 2
The core of this group is the natural tribe Triticeae, containing wheat, barley, and rye. It is one of the most important groups of economic plants in the world. The spikelets and their parts in some species are oriented asymmetrically, leading to easy confusion in their interpretation: glumes may be missing, reduced to mere awns, or apparently beside each other on one side of the spikelet; keels may not be centered, the inward-turned portion of glumes or lemmas then narrower and sometimes much more membranous than the outward portion. The key below is based on the superficial appearance of our species and avoids as far as possible the morphological problems of the spikelets. Intergeneric hybrids have frequently been reported, and delimitation of genera is still somewhat unsettled.
1. Spikelets fitting tightly into cavities in the rachis, their shape modified to form the inflorescence into a narrow cylinder.
2. Spikelets awned, those uppermost in the inflorescence with awns much longer than the spikelet.
2. Spikelets awnless.
1. Spikelets standing out from the rachis and thus prominently visible in the inflorescence, the inflorescence not a narrow cylinder.
3. Lemmas smooth and glabrous, except for a spiny-ciliate keel and exposed margin, tapering into a long awn.
3. Lemmas smooth to scabrous or pubescent, but not simply with spiny-ciliate keel and margin, awned or awnless.
4. Larger glumes 3.3–6.5 mm broad with at least 3 prominent nerves, the keel or midnerve not centered.
5. Glumes glabrous, or pubescent toward the base on nerves and margins (rarely pubescent throughout), the larger ones (3.7–) 5–6.5 mm wide, less than 3 times as long (excluding awns if present); lemmas awned or awnless.
5. Glumes softly hairy or glabrous throughout, 3.3–4.2 mm wide, ca. 6–10 times as long; lemmas awnless.
4. Larger glumes less than 2.5 mm broad, variously nerved (or glumes absent).
6. Spikelets mostly 2–3 at each node of the rachis; glumes usually 4–6 (the spikelet arrangement may be obscured by reduction or asymmetric positions of some spikelets, but the basic structure is revealed by the presence of usually 4–6 glumes subtending the entire group of spikelets); glumes usually vestigial or absent in Elymus hystrix with mostly 2 easily recognized narrow spikelets at each node.
7. Spikelets 2 at each node of the rachis (or at some nodes, only 1, rarely 3, but total number of glumes (awn-like or broader) developed at a node not more than 4).
Elymus (in part)
7. Spikelets basically 3 at each node of the rachis (the lateral 2 in commonest species reduced to bristles), this arrangement most easily recognized by the presence of 6 awn-like or narrowly lanceolate and awn-tipped glumes at a node.
8. Body of larger lemmas ca. 3.5–6 mm long; rachis of spike readily disintegrating at maturity.
Hordeum (in part)
8. Body of larger lemmas ca. 8–12 mm long; rachis not disintegrating.
9. Awn of lemmas much stouter than awn of glumes, ± straight.
9. Awn of lemmas as slender as awn of glumes, spreading to recurved at maturity.
Elymus (in part)
6. Spikelets clearly 1 at each node (or most nodes) of the rachis; glumes variously arranged (or absent), but not more than 2.
10. Glumes absent; spikelets 1-flowered, in two rows appearing as one, all on one side of rachis.
10. Glumes (1 or 2) present; spikelets 1–many-flowered, on opposite sides of the rachis.
11. Glumes 1 (except terminal spikelet with 2), the narrow edge of the spikelet against the rachis and lacking a glume.
Lolium (in part)
11. Glumes 2 on all spikelets.
12. Flowering stems 3–20 cm; inflorescences 1–3.5 cm long; ± prostrate annual.
12. Flowering stems 20–150 cm tall; inflorescences (3–) 5–20 cm long; plants erect, perennial (except Agropyron).
13. Inflorescence lax and arching; narrow edge of the spikelets against the rachis; nodes retrorsely pilose.
13. Inflorescence stiff, usually straight and erect; broad side of the spikelet against the rachis; nodes glabrous.
14. Body of the glumes 2.7–4 mm long, the midrib asymmetrically positioned and formed into a prominent keel; spike densely crowded and strongly 2-ranked with middle internodes ca. 1–2 mm long.
14. Body of the glumes more than 4 mm long; midrib symmetrically positioned and not raised into a prominent keel; spikes with the middle internodes ca. 3–15 mm long.
15. Lemmas with awns strongly divergent or recurved at maturity.
16. Anthers ca. 1–2.2 mm long; glumes with body mostly longer than the internodes of the spike; broadest leaf blades ca. 4–7.5 (–8.5) mm wide, ± flat.
16. Anthers ca. 4–5 mm long; glumes with body mostly shorter than the internodes of the spike; broadest leaf blades not over 3 mm wide, ± involute.
15. Lemmas with awns ± straight or absent.
17. Lemmas densely hairy.
17. Lemmas glabrous (rarely slightly pubescent), smooth or scabrous.
18. Leaf blades mostly broad and flat, slightly or not at all involute when dry, no more deeply grooved above than below between the numerous fine nerves (not strongly scabrous, usually with scattered long hairs above); glumes completely lacking cilia toward the base; cartilaginous belt at upper nodes nearly or fully as long as its diameter.
Elymus (in part)
18. Leaf blades ± strongly involute when dry, deeply grooved above between the prominent raised nerves (and usually strongly scabrous above); glumes mostly with margin minutely ciliate toward base; cartilaginous belt (sharply defined, usually darker, non-green zone) at upper nodes of culm usually less than half as long as its diameter.
KEY TO GENERA OF GROUP 3
The awns on the lemmas of many species in this group are distinctive in being tightly twisted or coiled, and often darker in color, on the lower portion. The genera Graphephorum, Koeleria, Sphenopholis, and Trisetum are also included in the key to genera of group 1 because of the tendency to short glumes.
1. Lemmas all awnless; larger glumes ± obovate (broadest above the middle), generally shorter than the lowest floret.
2. Rachilla and callus prominently bearded with long straight hairs.
2. Rachilla and callus glabrous or at most with short hairs (under 0.5 mm long).
3. Axis and branches of inflorescence glabrous, at most scabrous; larger glumes not over 3 (–3.2) mm long.
3. Axis and branches of inflorescence densely short-pubescent; larger glumes 3–4.2 (–4.7) mm long.
1. Lemmas with distinct twisted, jointed, or curved awn (sometimes largely hidden by the glumes or absent on some florets of a spikelet); glumes mostly ovate to lanceolate, at least one of them longer than the lowest floret.
4. Larger glumes 6–27 mm long.
5. Ligule a fringe of short hairs with a long tuft at each side; lemma with awn arising between terminal teeth.
5. Ligule membranous, hairless; lemma with awn arising dorsally.
6. Spikelets less than 10 mm long (excluding awns), the lower floret staminate with strong awn and the upper floret bisexual with (usually) weak awn.
6. Spikelets ca. 20–27 mm long, the florets all bisexual or the upper rudimentary; awns various.
4. Larger glume less than 6 mm long.
7. Bisexual (lowermost) floret awnless; awn subterminal on a reduced staminate floret; nodes pubescent.
7. Bisexual florets awned; awn twisted or spreading; nodes glabrous or (sometimes in Trisetum) pubescent.
8. Awn jointed near the middle, the lower portion brown, the upper whitish and thinner, narrowly club shaped, the joint with a ring of short hairs; blades densely scabrous.
8. Awn lacking a joint, uniform throughout its length; blades glabrous or pilose.
9. Awn arising above middle of lemma; panicle ± crowded and spike-like.
9. Awn arising well below middle of lemma; panicle at maturity very open and diffuse.
10. Awn conspicuously exserted, becoming bent at middle; lemma scabrous or minutely hispidulous; leaf blades involute-filiform; ligule 0.5–3 (–5) mm long.
10. Awn scarcely if at all exserted beyond tip of glumes, ± straight; lemmas smooth; leaf blades involute or often flat (1.5–5 mm wide); ligule 3–10 (–17) mm long.
KEY TO GENERA OF GROUP 4
1. Lemma with awn (or awns) strictly terminal.
2. Awns of lemma 3 (lateral ones sometimes very short).
2. Awn of lemma solitary.
3. Body of lemma 8–23 mm long.
4. Glumes 9.5–45 mm long.
5. Glumes 9.5–12 (–14) mm long; awn of lemma ca. 5–6.5 cm long; body of lemma 8–10 mm long.
5. Glumes 17–45 mm long; awn of lemma ca. 9–20 cm long; body of lemma 8–23 mm long.
4. Glumes rudimentary or one of them up to 5 mm long.
3. Body of lemma less than 7 mm long.
6. Glumes acute to obtuse, more than 1 mm wide, scarcely if at all keeled, the spikelets nearly terete; lemma rounded on the back, ± firm and hardened.
7. Principal leaf blades basically flat (just the margins involute), basal or nearly so, densely and very finely rough-puberulent, with strong closely spaced veins.
7. Principal leaf blades involute or, if flat, all cauline and glabrous.
8. Leaf blades flat, the larger ones (4–) 5–18 mm wide; body of lemma 5.5–7 mm long; ligules virtually absent or up to ca. 0.5 mm long.
8. Leaf blades involute, less than 2 (–2.3) mm wide; body of lemma 2.5–4 mm long; ligules of upper leaves ca. (1–) 1.5–3 mm long.
Piptatheropsis (in part)
6. Glumes acuminate, not over 1 mm wide, keeled, the spikelets somewhat compressed (or glumes vestigial in M. schreberi); lemma ± keeled, membranous or thin.
9. Inflorescence various (if spike-like, the plants with scaly rhizomes); ligules up to 2 mm long; glumes gradually tapered into awn or awnless.
Muhlenbergia (in part)
9. Inflorescence a dense thick spike-like panicle, the plants annual, without rhizome; ligules at least 3 mm long; glumes ± 2-lobed or rounded at apex, not tapered into the very slender awn (ca. 5–8 mm long).
Polypogon (in part)
1. Lemma with awn absent or dorsal or subterminal.
10. Spikelets 10–15 mm long; anthers (4–) 5–8 mm long; panicle crowded and ± spike-like, (10–) 12–20 (–28) mm across at the middle.
10. Spikelets less than 8 mm long (excluding awns); anthers up to 4.5 mm long (usually much shorter); panicle various.
11. Spikelets sessile or nearly so, crowded in a very dense spike-like panicle (branches of panicle suppressed, scarcely if at all visible without dissection of panicle).
12. Glumes awned.
13. Plants with scaly rhizomes; glumes gradually tapered into awn; ligule up to ca. 1 mm long.
Muhlenbergia (in part)
13. Plants without scaly rhizomes; glumes abruptly rounded or truncate, the awn distinct; ligule over 1 mm long.
14. Awn of glume rather stout and stiff, not over ca. 3 mm long; anthers ca. 1–2 mm long; spikelets articulated above the glumes; glumes prominently pectinate-ciliate on keel basally, otherwise glabrous or variously (but not so prominently) pubescent or ciliate; lemmas awnless.
14. Awn of glume very slender, ca. 5–8 mm long; anthers less than 1 mm long; spikelets articulated below the glumes; glumes ± evenly hispidulous basally; lemmas often with delicate awn.
Polypogon (in part)
12. Glumes awnless.
15. Panicle ± ovoid, ca. 2–3.5 times as long as wide; spikelets articulated above the glumes; ligule a fringe of hairs; lemmas awnless [if ligule membranous, try Phalaris—see text].
15. Panicle cylindrical (in common species slender and pencil-like), 3–15 times as long as wide; spikelets articulated below the glumes; ligule membranous; lemma with slender awn attached near the middle or base of keel (in common species, often very inconspicuous, shorter than the glumes; [if spikelets articulated above the glumes and lemmas stoutly awned, try Anthoxanthum—see text].
11. Spikelets in ± open or contracted (but not densely spike-like) inflorescences, with evident pedicels and/or panicle branches.
16. Spikelets rounded on back, not keeled (neither glumes nor lemma with a midvein more prominent than other nerves), at least 2.5 mm long; lemma ± shiny, distinctly firmer in texture than the glumes.
17. Lemmas 1.3–2.5 mm long, scabrous except for a basal tuft of short hairs, with a dorsal awn; delicate tufted annuals.
Aira (in part)
17. Lemmas 2.3–4.3 mm long, glabrous or with appressed pubescence over the body, awnless; hard-based or rhizomatous perennials.
18. Lemmas with appressed pubescence; leaves with blades usually involute; upper ligules not over 3 mm long.
18. Lemmas glabrous; leaves with blades broad and flat; upper ligules mostly 4–6 (–8) mm long.
16. Spikelets keeled (glumes and/or lemmas with midvein more prominent than other nerves) or less than 2.5 mm long; lemma no firmer in texture than the glumes.
19. Ligule a fringe of short hairs; lemmas awnless.
20. Lemma (4.7–) 5–6.7 mm long, surrounded with a tuft of long hairs (more than half its length) at its base.
20. Lemma 1.5–5.5 mm long, without long hairs at its base.
19. Ligule membranous (at most minutely ciliate at summit of membrane); lemmas awned or awnless.
21. Lemma with long hairs at base (on or near callus).
22. Long hairs at least in part arising from lower portion of lemma; glumes (excluding awn-tips if present) shorter than lemma.
Muhlenbergia (in part)
22. Long hairs restricted to callus at base of floret; glumes slightly exceeding lemma.
21. Lemma without long hairs at base (at most with hairs on callus less than 0.5 mm long).
23. Glumes both distinctly shorter than lemma; lemma awnless.
Muhlenbergia (in part)
23. Glumes (one or both of them) equaling or exceeding the lemma and/or the lemma awned.
24. Floret raised above base of glumes on a minute stalk; spikelet articulated below the glumes; lemma with a small subterminal awn; stamen 1.
24. Floret not stalked; spikelets articulated above the glumes; lemma awnless or with long subterminal awn or with dorsal awn; stamens 3.
25. Lemma with a long subterminal awn, much exceeding the body in length; rachilla prolonged (scarcely 0.5 mm) behind the palea.
25. Lemma awnless or with mid-dorsal awn; rachilla not prolonged.
26. Plants tufted to rhizomatous or stoloniferous perennials; spikelets 1-flowered; lemmas usually awnless.
26. Plants delicate tufted annuals; spikelets mostly 2-flowered; lemmas awned.
Aira (in part)
KEY TO GENERA OF GROUP 5
1. Spikelets concealed within ± globular, hard, bur-like structures.
(Bouteloua dactyloides, female plants)
1. Spikelets exposed, not concealed within ± globular, bur-like structures.
2. Spikes radiating from summit of culm (i.e., umbellate or nearly so) or at least the lower ones whorled (solitary in depauperate individuals); anthers not over 1.5 mm long.
3. Lemmas conspicuously awned; glumes short-awned to narrowly acuminate; ligule white, membranous with ciliate edge.
3. Lemmas and glumes awnless; ligule various.
4. Glumes ca. 2.5–4.5 mm long; spikelets at least 2–3-flowered; larger spikes 4–6 mm broad; plant a tufted annual; ligule membranous.
4. Glumes less than 2.5 mm long; spikelets 1-flowered; larger spikes 1.5–3 mm broad; plant a creeping perennial; ligule a fringe of white hairs.
2. Spikes all racemose or panicled; anthers various.
5. Glumes equal, ca. 2–3 mm long, deeply pouch-like and largely covering the floret, the spikelet strongly flattened and about as wide as long; ligule membranous, not ciliate; anthers ca. 0.5–1 mm long.
5. Glumes unequal, the longer ones ca. 2.5–11 mm long, not pouch-like, equaling or shorter than florets, the spikelet not strongly flattened, much narrower than long; ligules and anthers various.
6. Ligule membranous, not ciliate, ca. (2.5–) 4–6 mm long, becoming shredded; anthers up to ca. 0.5 mm long; end of rachis of spike spikelet-bearing, not prolonged; spikelets short-pediceled.
6. Ligule ciliate (entirely of hairs, or hairs longer than any membranous portion), up to 3.5 mm long (a few hairs rarely to 4.5 mm); anthers 1.8–6 mm long; end of rachis of spike prolonged into a naked projection 0.5–14 mm beyond the last spikelet (except in Bouteloua gracilis); spikelets sessile or subsessile.
7. Spikes short peduncled, peduncles glabrous to hispid, 0.5–20 mm long.
7. Spikes ± sessile or on finely hispidulous or short-pubescent peduncles up to 3 mm long.
KEY TO GENERA OF GROUP 6
The spikelets in this tribe basically have 3 florets, but the lower pair may be only staminate (Anthoxanthum) or reduced to tiny scales (Phalaris), and hence easily overlooked or misinterpreted. Only the upper (terminal) floret is bisexual.
1. Lower florets staminate, at least as large as bisexual floret, awnless or awned, but evident in the spikelet.
1. Lower florets sterile, small and inconspicuous, awnless, only the awnless bisexual floret evident in the spikelet
KEY TO GENERA OF GROUP 7
This includes all our members of the Tribe Oryzeae, which includes rice (Oryza sativa L.), perhaps the oldest of food crops and eaten by more people than any other grain.
1. Spikelets bisexual; plant variously scabrous-pubescent, at least the nodes retrorsely bearded; stamens 3.
1. Spikelets all unisexual, segregated into different parts of the inflorescence, upper panicle branches bearing awned pistillate spikelets, lower branches bearing staminate spikelets; plant glabrous; stamens 6.
KEY TO GENERA OF GROUP 8
This group includes all the members of the Tribe Paniceae and is a familiar natural assemblage. The spikelet structure in most genera is distinctive, being basically 2-flowered, with a terminal fertile flower and a sterile flower represented by a sterile lemma, which closely resembles the large second glume opposite it and might easily be misinterpreted as the first glume. Look for the true first glume as a small, sometimes minute and membranous, even deciduous, scale at the very base of the spikelet; a reduced palea, often associated with the sterile lemma, will also help to identify the latter as part of a sterile floret and not a glume. The fertile lemma is usually hard and shiny.
1. Spikelets with an involucre consisting of a spiny bur or of long subtending bristles.
2. Involucre a spiny bur enclosing much or all of the spikelets, the whole readily disarticulating.
Cenchrus (in part)
2. Involucre of long slender bristles subtending but not concealing the spikelet, remaining attached to pedicels when spikelets disarticulate (except in the rare garden escape Cenchrus purpurascens).
3. Bristles at most 1.5 cm long, remaining on the pedicels; annuals lacking a rhizome.
3. Bristles ca. 2–4 cm long, falling with the spikelets; cespitose perennials from hard short rhizomes.
Cenchrus (in part)
1. Spikelets without an involucre (although glumes or lemmas may be awned).
4. Spikelets ± spiny-hispid and usually also awned; ligule none.
4. Spikelets glabrous or pubescent but not coarsely hispid and not awned; ligule present, distinct or nearly absent (of hairs or membranous).
5. Each spikelet subtended by a small cup-like involucre.
5. Spikelets lacking a cup-like involucre.
6. Inflorescence composed of 1-sided spike-like racemes, the rachis of each winged or at least flat on the side opposite the spikelets.
7. Fertile lemma rather leathery in texture, acute, about twice as long as wide (or longer), with thin flat translucent margins; racemes (2–) 3–12 (–15), ± whorled at the top of the stem, rarely some racemes up to 1.5 (–3) cm distant.
Digitaria (in part)
7. Fertile lemma very hard, broadly rounded, less than 1.5 times as long as wide, with thickened, ± inrolled, hardened margins; racemes 1–3 (–4), spaced 2–5 (–6) cm distant, not appearing whorled.
6. Inflorescence an open panicle, not spike-like or distinctly one-sided.
8. Ligule a membranous collar ca. 1–1.5 mm high, without hairs; base of leaf blade and very summit of sheath without special zone of short pubescence or long hairs or cilia; first glume minute (scarcely 0.5 mm) or vestigial; fertile lemma leathery in texture, with thin flat translucent margins.
8. Ligule usually partly or entirely of short or long hairs; (if ligule membranous, plants not otherwise as above: ligule ca. 0.5 mm long or virtually absent; or summit of sheath or basal margin of blade pubescent or ciliate; and/or first glume more than 0.5 mm long); fertile lemma hard and shiny.
9. Spikelets at least sparsely pubescent towards their margins.
Dichanthelium (in part)
9. Spikelets glabrous.
10. Terminal panicle 2.5–8 (–12) cm long; tufted perennials with clear remnants of old, dead leaves from the previous year present at the base, these sometimes formed into a clear overwintering rosette; flowering spring and fruiting in late spring–early summer.
Dichanthelium (in part)
10. Terminal panicle 8–40 cm long, (smaller in occasional depauperate individuals of annual species); annuals or perennials, but without clear remnants of overwintering basal rosette leaves; flowering and fruiting summer-fall.
11. Spikelets all or mostly 3 mm or more in length, strongly nerved.
Panicum (in part)
11. Spikelets less than 3 mm long, strongly nerved or not.
12. Spikelets covered with mealy-looking warts, but nearly nerveless.
12. Spikelets not warty, nerved or not.
13. Sheaths sparsely to heavily pilose on back.
Panicum (in part)
13. Sheaths of middle and upper leaves glabrous on back (may be ciliate on margins).
14. Longer spikelets 2.7–3.4 mm long; pedicel apex lacking ascending hairs; plant usually ± spreading, the stems geniculate at base; annual.
Panicum (in part)
14. Longer spikelets 1.7–2.8 mm long; pedicel apex of at least some spikelets with sparse ascending hairs; plants erect-stemmed perennials with a thickened base.
KEY TO GENERA OF GROUP 9
This group corresponds to the Tribe Andropogoneae, and presents some difficulties in interpreting inflorescence structures. The spikelets fundamentally occur in pairs, one stalked, the other sessile. The stalked spikelet is bisexual in Miscanthus, but usually staminate, rudimentary, or absent and represented only by a pedicel (or absent or totally obscured in pistillate inflorescences of Zea or Tripsacum). The sessile spikelet, as in group 8, the tribe Paniceae, contains basically two florets, one sterile and the other fertile. Both the sterile and the fertile lemmas are thin and hyaline; in our species (except in Tripsacum, Zea, and sometimes Sorghum) the fertile lemma bears an elongate awn, usually twisted basally.
1. Summit of plant with a large “tassel” or spike-like raceme bearing staminate florets in pairs, the pistillate florets either sunken in thick, hard joints of rachis below the staminate portion or in separate leafy bracted spikes (ears) lower on the plant.
2. Pistillate flowers several, sunken in thick, hard joints of lower portion of same rachis as the terminal staminate spikelets, at maturity disarticulating into 1-seeded segments.
2. Pistillate flowers aggregated into separate leafy-bracted spikes (ears) in lower axils of plant, not disarticulating into segments.
1. Summit of plant not as above: upper portion of inflorescence bearing pistillate or sterile and pistillate florets; pistillate florets not in leafy bracted spikes nor enclosed in thick, hard joints of the rachis.
3. Inflorescence an open to contracted panicle.
4. Pedicels and lower portions of spikelets with copious silky hairs longer than the spikelets and their awns and largely concealing them; ligule and immediately adjacent portion of the blade conspicuously silky -hairy.
4. Pedicels and lower portions of spikelets with at most short hairs, not concealing the spikelets; Ligule and immediately adjacent portion of the blade membranous and glabrous or ciliate or short-pubescent, but not long-silky.
5. Stalked spikelet absent or represented by a sterile hairy pedicel closely resembling the segments of the panicle axis; ligule stiff, ± cartilaginous, glabrous or at most with minute hairs.
5. Stalked spikelet present, sterile or staminate; ligule at least in part of evident soft hairs.
3. Inflorescence of 1 or several narrow or spike-like simple racemes (some spikelets sessile and some pediceled).
6. Spike-like simple racemes solitary at the ends of the branches (if plant a decumbent-based sprawling annual, see Microstegium).
6. Spike-like simple racemes 2–ca. 20 at the ends of the branches.
7. Racemes 10–20, not disarticulating; spikelets with a dense basal tuft of silky hairs longer than the glumes, the pedicel glabrous or very short hairy.
7. Racemes 2–6 (–10), disarticulating readily; spikelets lacking a dense basal tuft of silky hairs, the pedicel, however, hairy and sometimes with an apical tuft of hairs just below the articulation with the spikelet.
8. Sprawling annuals with decumbent bases; leaves 1.5–6 cm long.
8. Hard based, tufted perennials with erect stems at most slightly curved at the base; leaves; ca. 10–50 cm or more long.
9. Pedicels and rachis of inflorescences with hairs up to ca. 3 mm long, the awns very conspicuous; anthers (2–) 2.5–5 (–5.5) mm long.
Andropogon (in part)
9. Pedicels and rachis of inflorescences with long, silky hairs, many > 5 mm long and largely concealing the awns, these therefore inconspicuous; anthers 0.4-1.5 mm long.
10. Stalked floret usually represented by only a hairy pedicel; ligule of major culm leaves a minutely ciliate scale ca. 0.5–0.6 mm long; base of racemes ± included in spathe-like leaf base even at maturity.
Andropogon (in part)
10. Stalked floret present, rudimentary or staminate; ligule of major culm leaves ca. 0.9–3 mm long; racemes usually fully exserted at maturity.