Flora of North America species comparison

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 grasslands (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).

(Discussion copyrighted by Flora of North America; reprinted with permission.)