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Andropogon Linnaeus. Subfamily: Panicoideae. Tribe: Andropogoneae. Subtribe: Andropogoninae. supertribe: Andropogonodae. clade: "PACMAD". Broomsedge, Bluestem.

Key to Andropogon

A genus of about 100-110 species, mainly tropical. Campbell's work (1983, 1986, 2003) has greatly clarified the taxonomy of Andropogon in e. North America. Great confusion and disagreement were previously the rule in dealing with the Andropogon virginicus-A. glomeratus complex. Campbell's careful morphologic work has provided workable technical characters which distinguish the taxa he recognizes. I have generally followed Campbell (1983, et seq.) in his circumscriptions of taxa, but have differed in decisions of rank; see Weakley et al. (2011) for discussion. Taxa differing in numerous morphologic characters, with different (though overlapping) geographic ranges, with different ecological preferences (often rather narrowly segregated by hydrology), and (when they do occur in proximity to one another) showing little or no sign of introgression or hybridization are probably better treated as biological species. Thus, I have treated a number of Campbell's varieties as species. Several of his "variants" also warrant taxonomic recognition, at varietal or specific rank (Campbell 1986; Weakley et al. 2011). The issue of the separation of Schizachyrium from Andropogon, the membership of each genus, the morphological characters that are key and diagnostic (when characters are in conflict), and the reciprocal monophyly of each genus remain very uncertain. For now, I retain the predominant usage of recent decades of separating the two genera, and allocate the species to genera on the pragmatic character of single racemes (Schizachyrium) vs. 2 or more racemes (Andropogon), but that may well prove erroneous.

While the dispersal units are still attached to one another, the rachis internodes form a continuous and more-or-less straight rachis. The dispersal units attached together in an unbranched sequence are termed a raceme, whose length is a useful character. Two or more racemes are attached digitately at the summit of the peduncle (in Schizachyrium only a single raceme is found). The number of racemes attached is an important character. A raceme sheath subtends the peduncle, often more or less surrounding the peduncle and the racemes. The length of the peduncle (distance between the points of attachment of the raceme sheath and the racemes) is an important character. The length and width (at its widest point, and spread and flattened) of the raceme sheath are very useful characters, used throughout the key. The racemes, peduncle and subtending raceme sheath make up an inflorescence unit. The overall inflorescence is more-or-less complexly branched; its overall size and shape are very useful in recognizing the various taxa, but variation in such a subjective (and environmentally plastic) character has added to the taxonomic confusion in Andropogon. The use of inflorescence shape in the key has been minimized, but is often mentioned in the discussion of each species. The number of inflorescence units per plant varies from species to species, in some species rarely exceeding 10, in others ranging upward to 500 or 600. The absence or presence of hairs immediately below the raceme sheath is useful in some groups.

There are several important characters of the foliage. Andropogon capillipes, A. dealbatus, and A. cretaceus have culm sheaths and leaf blades that are strongly glaucous; this is usually very obvious, but can be tested for by running the finger along the surface of the leaf (a white coating of wax will come off on the finger). The key often calls for the ligule length; measure the longest portion of the undivided portion of the ligule. The ligule often has an erose or ciliate upper margin; measure the length of the cilia. The length of leaf blade is measured from the ligule to the leaf apex; do not include the leaf sheath, which is often long and (especially late in the year) only loosely sheathing the culm or even divergent it. Whether the leaf sheaths are antrorsely scabrous or smooth is better determined by touch than by sight. Choose several mid-culm sheaths, run one’s finger downward and upward along the sheath surface (near the collar is best). If the sheath is antrorsely scabrous one will feel a somewhat greater resistance to moving the finger downward than upward.

ID notes:A thorough understanding of the architecture of the inflorescences of Andropogon is necessary in order to identify them successfully. The parts will be described, beginning from the apex of a branch of the inflorescence. Spikelets occur in pairs, the sessile spikelet (usually just referred to as the spikelet) and the pedicelled spikelet, which is usually vestigial or absent (except in A. gerardi) and sterile (except in A. gerardi, where it is staminate). The first or lower glume of the sessile spikelet has two keels, and the presence and location of antrorse prickle hairs (scabrousness) is an important character in the A. glomeratus complex. The length of the sessile spikelet is an important character; it should be measured exclusive of the awn, borne at the apex of the lemma. Awn length is also a useful taxonomic character. The pedicelled spikelet is borne on the pedicel, which is attached at the base of the sessile spikelet and typically angles away from it at about a 45 degree angle. The rachis internode extends from the base of one sessile spikelet to the next sessile spikelet above, breaking apart (upon dehiscence) just below the next spikelet and remaining attached to the sessile spikelet below. The dispersal unit consists of a sessile spikelet sitting in the V shape formed by (on one side) the pedicel and pedicelled spikelet and (on the other side) the rachis internode. Both the pedicel and the rachis internode are usually pubescent with long hairs, and the color of those hairs and their distribution are useful characters. Sometimes culm sheath scabrousness is discernible by touch only near the sheath summit.

Ref: Bridges & Orzell (2018a); Bridges & Orzell (2020a) In Weakley et al. (2020); Campbell (1983); Campbell (1986); Campbell (2003) In Flora of North America Editorial Committee (2003a); Deshmuke, Shende, & Reddy (2022); Sorrie (2020b) In Weakley et al. (2020); Weakley & Schori (2018) In Weakley et al. (2018a); Weakley et al. (2011); Wipff & Shaw (2018d). Show full citations.

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image of plant© Gary P. Fleming | Andropogon gyrans | Original Image ⭷

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