Poaceae tribe Andropogoneae
The tribe Andropogoneae includes about 87 genera and 1060 species, of which 31 genera and 102 species have been found in the Flora region; some of these have not become established. The tribe is common in tropical and subtropical regions, particularly in areas with significant summer rains, such as the central plains of North America. Two of the grasses that used to dominate the prairies of central North America, Andropogon gerardii and Schizachyrium scoparium (Big and Little Bluestem, respectively), are member of the Andropogoneae. The reddish-purplish coloration that characterizes the culms and leaves of many Andropogoneae gives a striking aspect to grass¬lands (and lawns) dominated by its members.
Members of the Andropogoneae differ from those of Paniceae in the reduced lemmas and paleas of their florets and, usually, in their paired, unequally pedicellate spikelets, disarticulating inflorescence branches (rames), and the manner in which these branches are aggregated into inflorescences. Unequally pedicellate spikelet pairs are found in many other tribes, but they are more common, and the pedicels more strikingly unequal in length, in the Andropogoneae. Recent molecular work supports recognition of the tribe with one modification of its traditional limits, the incorporation of Arundinella and Tristachya (Kellogg 2000). There is less agreement on the tribe's internal structure and its relationship to the Paniceae (Clayton and Renvoize 1986; Kellogg 2000; Spangler 2000; Guissani et al. 2001).
Inflorescence Structures Describing inflorescence structures in the Andropogoneae is not simple. There is a basic pattern, but its many modifications have resulted in great structural diversity. The following paragraphs provide an overview of this diversity and explain the words and phrases used in describing it. Diagrammatic representations of many of the structures mentioned are presented on pages 604 and 605.
Spikelets Members of the Andropogoneae, like those of the Paniceae, generally have two florets per spikelet, the lower floret usually being reduced in size and sterile or staminate, and the upper floret bisexual (p. 604). Despite this similarity, spikelets of the two tribes are easy to distinguish. In the Paniceae, the lowest glume is usually much shorter than the floret, and the upper florets usually have lemmas that are thicker and tougher than the glumes and lower lemmas. In the Andropogoneae, the glumes usually exceed and enclose both florets, and are thicker and tougher than the lemmas. The florets of the Andropogoneae contrast strongly with the glumes, having hyaline or thinly membranous lemma bodies and hyaline paleas, or, in many cases, no palea. They are almost always completely concealed by the glumes, except that the upper floret often has an awn that projects beyond the glumes.
In some Andropogoneae, the glumes are merely thickly membranous, but most genera have coriaceous or indurate glumes. The lower glumes are sometimes tougher and larger than the upper glumes, and may even conceal the upper glumes as, for example, in Heteropogon (p. 681). In such genera, the lower glumes may be mistaken for lemmas. In dioecious species, or monoecious species with strongly differentiated staminate and pistillate spikelets, the staminate spikelets usually have softer glumes than the pistillate spikelets.
Spikelet Units The basic element of the inflorescence structure in the Andropogoneae is the spikelet unit. These units usually consist of pairs of spikelets, one sessile and one pedicellate (e.g., Saccharum bengalense, p. 615), but they may consist of a pair of unequally pedicellate spikelets (e.g., Miscanthus sacchariflorus, p. 619) or of three spikelets (e.g., Chysopogon fulvus, p. 635). If there are three spikelets in the unit, one is usually sessile and the other two pedicellate, but a few genera, such as Polytrias, have two sessile spikelets and one pedicellate spikelet.
Unequally pedicellate spikelet pairs or triplets are found in other tribes, but in the Andropogoneae they usually differ in size, shape, and sexuality. Spikelet units with spikelets that differ in their sexuality are described as heterogamous; those with sexually similar spikelets are said to be homogamous. Spikelet units with morphologically dissimilar spikelets are beteromorphic (e.g., Andropogon longiberbis, p. 663); those with morphologically similar spikelets are homomorphic (e.g., Chrysopogon zizanioides, p. 636). In most Andropogoneae, the spikelet units are heterogamous and heteromorphic. The sessile spikelets usually contain a bisexual or pistillate floret, and often exhibit features such as awns and calluses that are related to seed dispersal and establishment (Peart 1984); the pedicellate spikelets are usually staminate, sterile, vestigial, or even absent. In some genera the situation is reversed, the pedicellate spikelets being bisexual or pistillate, and the sessile spikelets staminate or sterile. Sterile and staminate spikelets are sometimes morphologically similar to the pistiallye or bisexual spikelets, but usually lack the features associated with seed dispersal and establishment. A few genera have no staminate or sterile spikelets, merely empty pedicels associated dwith the bisexual sessile spikelets, as in Sorghastrum (p. 632) or even, as in Arthraxon (p. 679), with only a stump where the pedicel and its spikelet would be. Inflorescence Structure Further complexity is introduced to the Andropogoneae inflorescence structure by the manner in which the spikelet units are aggregated and the mode of disarticulation. Three patterns can be identified. The simplest pattern consists of inflorescences similar to those common in other tribes, in which neither the rachis nor the inflorescence branches break up at maturity. Genera with such inflorescences [e.g., Miscanthus (p. 619) and Imperata (p.622)] have unewually pedicellate spikelets, and disarticulation is below the glumes. Such inflorescences are, however, in the minority within the Andropogoneae. A more common situation is for the spikelets to be in sessile-pedicellate pairs and disarticulation to be in the branch axes, immediately below the attachment of the sessile spikelets. The resulting dispersal unit consists of the spikelet pair plus the internode that extends from the sessile spikelet to the next most distal sessile spikelet. These disarticulating inflorescence branches, termed rames in this Flora, form the basic unit of the typical Andropogoneae inflorescence. In other publications, the rames are often called racemes, a word that is restricted in this Flora to an entire inflorescence, not just an inflorescence branch. Rames are usually composed of several spikelet units, but sometimes of only one. The spikelets may be evenly distributed, or the base of the rame axis may be naked. Individual plants may bear few to many rames, and the rames themselves may be aggregated in a wide array of primary and secondary arrangements; they may also be branched.
One or more rames may be borne on a single stalk. If this stalk is attached to a rachis, the unit formed by the stalk and its rame(s) constitutes an inflorescence branch. Such a pattern is seen, for example, in Sorghum halepense (p. 629) and Bothriochloa bladhii (p. 647). A more common sit-uation is for one or more rames to be attached digitately to a common stalk, the peduncle. This peduncle may terminate a culm (as in Dichanthium annulatum [p. 638] or Elionurus [p. 686]) or be axillary to a subtending leaf (as in Andropogon hallii [p. 654] and Hemarthria altissima [p. 686]). Each peduncle and its associated rame(s) constitutes an inflorescence unit.
False panicles represent a further level of complexity. In these, the inflorescence units terminate rays, each of which has a prophyll, a 2-veined structure, in its axil. Several rays may develop within the axil of a single leaf sheath, and rays may themselves give rise to subtending leaves with multiple rays in their axils. The result is a complex, tiered inflorescence in which only the ultimate units are easily described. Such inflorescences are found, for example, in Andropogon glomeratus (p. 663) and Cymbopogon citratus (p. 667). Fortunately, identification of the Andropogoneae does not require analyzing false panicles, merely their ultimate inflorescence units.
In another inflorescence pattern, the rame axes are thick and the pedicels are either closely appressed or even fused to the rame axes. In these genera, the pedicellate spikelets are often highly reduced or absent. Pistillate rames of Tripsacum (p. 697) and wild taxa of Zea (p. 700) represent an extreme example of this pattern. In these genera, the sessile spikelets are completely embedded in the rame axes, the lower glumes being indurate and completely concealing the florets. Less extreme examples are seen in Coelorachis (p. 689) and Hackelochloa (p. 694).
Clayton, W.D. 1987. Andropogoneae. Pp. 307-309 in T.R. Soderstrom, K.W. Hilu, C.S. Campbell, and M.E. Barkworth (eds.). Grass Systematics and Evolution. Smithsonian Institution Press, Washington, D.C., U.S.A. 473 pp.
Clayton, W.D., G. Davidse, F. Gould, M. Lazarides, and T.R. Soderstrom. 1994. A Revised Handbook to the Flora of Ceylon, vol. 8 (ed. M.D. Dassanayake). Amerind Publishing Co., New Delhi, India. 458 pp.
Guissani, L.M., J.H. Cota-Sanchez, F.O. Zuloaga, and E.A. Kellogg. 2001. A molecular phylogeny of the grass subfamily Panicoideae (Poaceae) shows multiple origins of C4 photosynthesis. Amer. J. Bot. 88:1993-2001
Kellogg, E.A. 2000. Molecular and morphological evolution in the Andropogoneae. Pp. 149-158 in S.W.L. Jacobs and J. Everett (eds.). Grasses: Systematics and Evolution. International Symposium on Grass Systematics and Evolution (3rd:1998). CSIRO Publishing, Collingwood, Victoria, Australia. 408 pp.
|1||Leaves smelling of lemon oil or citronella, the sheaths without glandular depressions on the keel; plants perennial, not reaching reproductive maturity in the Flora region when grown outdoors||Cymbopogon|
|1||Leaves usually not aromatic or, if aromatic and smelling of citronella, the sheaths with glandular depressions along the keel and plants annual; plants reaching reproductive maturity in the Flora region.||> 2|
|2||All spikelets unisexual, the pistillate and staminate spikelets in separate inflorescences or the pistillate spikelets below the staminate spikelets in the same inflorescence.||> 3|
|3||Pistillate spikelets completely concealed within a hard, globose, beadlike structure (a modified leaf sheath) from which the staminate rames protrude||Coix|
|3||Pistillate spikelets exposed or enclosed by 1 or more subtending leaf sheaths and a hyaline prophyll; staminate spikelets either distal on the same branch or in a separate inflorescence on the same plant.||> 4|
|4||Staminate and pistillate spikelets in the same inflorescence and on the same branch, the staminate spikelets distal to the pistillate spikelets||Tripsacum|
|4||Staminate and pistillate inflorescences usually separate; staminate inflorescences terminal on the culms and branches; pistillate inflorescences terminal on axillary peduncles, sometimes aggregated in false panicles||Zea|
|2||Some spikelets bisexual (usually the sessile or more shortly pedicellate spikelet of each spikelet pair or triplet).||> 3|
|5||Spikelets apparently solitary and sessile, the pedicellate spikelets absent; pedicels absent or present.||> 6|
|6||Culms decumbent, scrambling; leaf blades ovate to ovate-lanceolate; pedicels absent or shorter than 3 mm||Arthraxon|
|6||Culms erect; leaf blades lanceolate to linear-lanceolate; pedicels always present, usually longer than 3 mm.||> 7|
|7||Inflorescences terminal and axillary, composed of digitate clusters of 1-13 rames on a common peduncle; peduncles subtended by, and often partially included in, a modified leaf||Andropogon|
|7||Inflorescences terminal, with elongate rachises and brancheswith several to many rames; peduncles and branches not subtended by a modified leaf||Sorghastrum|
|5||Spikelets in sessile-pedicellate or unequally pedicellate pairs or triplets, the pedicellate spikelets often smaller than the sessile spikelets, sometimes rudimentary.||> 6|
|8||Pedicels strongly appressed or fused to the thick rame axes, or the rames with only 1 spikelet unit, this a triplet with 2 unequally pedicellate spikelets; bisexual spikelets usually unawned; inflorescences of rames.||> 9|
|9||Lower glumes of the sessile spikelets rugose, pitted, tuberculate, or alveolate or the keels winged or with spinelike projections at the base.||> 10|
|10||Keels of the lower glumes with spinelike projections on the base, sometimes winged distally, the surface between the keels smooth; spikelets unawned||Eremochloa|
|10||Keels of the lower glumes winged throughout or not winged, the surface between the keels rough, rugose, pitted, tuberculate, or alveolate; spikelets unawned or awned.||> 11|
|11||Sessile spikelets awned||Ischaemum|
|11||Sessile spikelets unawned.||> 12|
|12||Plants perennial; sessile spikelets ovate, the lower glumes smooth, rugose, or pitted||Coelorachis|
|12||Plants annual; sessile spikelets hemispherical, the lower glumes alveolate||Hackelochloa|
|9||Lower glumes of the sessile spikelets smooth or scabrous, not sculptured, the keels without spinelike projections.||> 10|
|13||Inflorescences false panicles; individual rames to 1 cm long, with 1 spikelet unit; spikelet units composed of 1 sessile and 2 unequally pedicellate and dissimilar spikelets||Apluda|
|13||Inflorescences usually solitary rames, sometimes with 2 rames in a digitate cluster; individual rames 2-15 cm long, with more than 1 spikelet unit; spikelet units composed of sessile pedicellate pairs, the pedicellate spikelets often rudimentary or absent.||> 14|
|14||Pedicels appressed, but not fused, to the rame axes.||> 15|
|15||Pedicellate spikelets 1-3 mm long||Coelorachis|
|15||Pedicellate spikelets 4-8 mm long||Elionurus|
|14||Pedicels at least partially fused to the rame axes.||> 15|
|16||Plants perennial; sheaths mostly glabrous, sparsely ciliate basally||Hemarthria|
|16||Plants annual; sheaths with stiff, papillose-based hairs 1-3 mm long||Rottboellia|
|8||Pedicels free; rame or branch internodes slender, sometimes thickened distally; bisexual spikelets usually awned; inflorescences of rames with the spikelets in sessile-pedicellate pairs or of non-disarticulating branches with the spikelets in unequally pedicellate pairs. 17. All spikelet units homogamous, frequently also homomorphic.||> 9|
|18||Terminal inflorescences a single rame or a digitate or subdigitate cluster of rames. 19. Terminal inflorescences a digitate or subdigitate cluster of (1)2-6 rames; rames 3-7 cm long||Microstegium|
|19||Terminal inflorescences solitary rames; rames 2-3 cm long||Polytrias|
|18||Terminal inflorescences with elongated rachises.||> 19|
|20||Spikelets in unequally pedicellate pairs; disarticulation below the glumes, the branches remaining intact at maturity.||> 21|
|21||Spikelets usually awned; inflorescence branches usually 7-35 cm long||Miscanthus|
|21||Spikelets unawned; inflorescence branches 1-7 cm long||Imperata|
|20||Spikelets in sessile-pedicellate pairs or triplets; disarticulation in the rames, below the sessile spikelets.||> 21|
|22||Culms to 100 cm tall, often decumbent and straggling; terminal inflorescences composed of 2-6 subdigitately to racemosely arranged rames||Microstegium|
|22||Culms 40-600 cm tall, erect; terminal inflorescences panicles, with more than 6 primary branches; branches usuallywith 2 or more rames.||> 23|
|23||Panicle branches alternate, with multiple rames; rames with more than 5 spikelet units||Saccharum|
|23||Panicle branches subverticillate, with 1-3 rames; rames with 2-5 spikelet units||Spodiopogon|
|17||All or most spikelet units heterogamous, usually also heteromorphic, sometimes the proximal units on the rames or racemes homomorphic and homogamous.||> 18|
|24||Terminal inflorescences with elongated rachises.||> 25|
|25||Rame internodes and pedicels with a translucent median line||Bothriochloa|
|25||Rame internodes and pedicels without a translucent median line.||> 26|
|26||Sessile spikelets terete or laterally compressed, calluses usually sharp, sometimes blunt||Chrysopogon|
|26||Sessile spikelets dorsally compressed, calluses usually blunt, sometimes sharp||Sorghum|
|24||Terminal inflorescences and individual inflorescence units without elongated rachises, composed of 1-13 rames, or a raceme in which disarticulation occurs below the pedicellate spikelets.||> 25|
|27||Inflorescences composed of rames, disarticulation being in the rame axes; rame internodes and pedicels with a translucent median groove||Bothriochloa|
|27||Inflorescences composed of rames, with disarticulation in the axes or a spikelike raceme (occasionally of 2 subdigitate spikelike branches) with disarticulation below the pedicellate spiklets; inflorescence internodes and pedicels without a translucent median groove.||> 28|
|28||All spikelet units in the inflorescence heterogamous.||> 29|
|29||Disarticulation occurring below the pedicellate spikelets, not in the inflorescence axes; pedicellate spikelets bisexual and awned, awns (4)6-15 cm long, pilose on the column, the hairs 1-2 mm long; sessile or subsessile spikelets staminate or sterile, unawned||Trachypogon|
|29||Disarticulation occurring below the sessile spikelets, in the inflorescence axes; pedicellate spikelets staminate, sterile, rudimentary, or absent, unawned or with awns to 6 mm long; sessile spikelets bisexual or pistillate, awned. 30. Rames usually solitary on the peduncles, occasionally 2; rame internodes cupulate or fimbriate distally; lower glumes of the sessile spikelets veined between the keels||Schizachyrium|
|30||Rames usually 2-13 on the peduncles, occasionally solitary; rame internodes neither fimbriate nor cupulate distally; lower glumes of the sessile spikelets usually without veins between the keels||Andropogon|
|28||Basal spikelet units on each rame homomorphic and homogamous, sterile or staminate, unawned.||> 29|
|31||Awns 5-15 cm long; rames with 3-10 homogamous spikelet units||Heteropogon|
|31||Awns 1-5(19) cm long; rames with 1-2 homogamous spikelet units. 32. Inflorescences terminal on the culms, axillary inflorescences not present or few in number||Dichanthium|
|32||Inflorescences terminal and axillary, axillary inflorescences numerous.||> 33|
|33||Homogamous spikelets distinctive, forming an involucre around the rame bases||Themeda|
|33||Homogamous spikelets not distinctive, not forming an involucre around the rame bases||Hyparrhenia|
|Author||Mary E. Barkworth +|
|Reference||clayton1969a +, clayton1986a +, clayton1987a +, clayton1994a +, guissani2001a +, kellogg2000a +, peart1984a +, pohl1968a + and r2000a +|
|Taxon name||Poaceae tribe Andropogoneae +|
|Taxon parent||Poaceae subfam. Panicoideae +|
|Taxon rank||tribe +|
|Volume||Volume 25 +|