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The grass genera of the world

L. Watson, T.D. Macfarlane, and M.J. Dallwitz

Amphicarpum Kunth

From the Greek amphikarpos (doubly fruit-bearing), alluding to the two kinds of spikelets.

Habit, vegetative morphology. Annual (erect), or perennial (the culms decumbent at the base). Culms 30–100 cm high; herbaceous. Culm nodes glabrous. Culm internodes hollow. Plants unarmed. Leaves mostly basal (A. purshii), or not basally aggregated; non-auriculate. Leaf blades broad, or narrow; 5–15 mm wide (10–15 cm long); flat; without cross venation; persistent. Ligule a fringe of hairs. Contra-ligule absent (but scattered tubercle-based hairs in that position in A. muhlenbergianum).

Reproductive organization. Plants bisexual, all with bisexual spikelets (but the ‘chasmogamous’ spikelets of the conspicuous terminal panicle not fruitful); with hermaphrodite florets. The spikelets all alike in sexuality. Plants exposed-cleistogamous and chasmogamous; with hidden cleistogenes (these subterranean). The hidden cleistogenes subterranean (on slender branches from the base of the culm, and sometimes also from lower nodes).

Inflorescence. Inflorescence (i.e., the obvious, ‘chasmogamous’ but sterile inflorescence) paniculate; espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets solitary; not secund; pedicellate.

Female-sterile spikelets. The exposed, sterile spikelets of the terminal panicle 4–7 mm long, G1 sometimes obsolete, G2 equalling L1; L2 and palea indurated, the lemma margins thin and flat.

Female-fertile spikelets. Spikelets 7–9 mm long (exclusively cleistogamous, plump, acuminate); compressed dorsiventrally. Rachilla terminated by a female-fertile floret. Hairy callus absent.

Glumes present; one per spikelet to two; (when two) very unequal (the lower obsolete or absent); long relative to the adjacent lemmas (G2 equalling L1); without conspicuous tufts or rows of hairs; awnless; non-carinate; (when two) very dissimilar (the lower vestigial). Upper glume strongly nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 1; paleate. Palea of the proximal incomplete florets not becoming conspicuously hardened and enlarged laterally. The proximal lemmas awnless; exceeded by the female-fertile lemmas (at least in fruit); less firm than the female-fertile lemmas (‘sub-rigid’).

Female-fertile florets 1. Lemmas acuminate; decidedly firmer than the glumes; becoming indurated; entire; pointed; awnless; non-carinate; having the margins lying flat on the palea; with a clear germination flap. Palea present; awnless, without apical setae; indurated. Stamens 3.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells markedly different in shape costally and intercostally (the costals narrower, ‘normal’); differing markedly in wall thickness costally and intercostally (the costals thinner-walled). Intercostal zones exhibiting many atypical long-cells (large and short to isodiametric in the middle of the intercostal zone). Mid-intercostal long-cells rectangular to irregular in shape; having markedly sinuous walls. Microhairs present; panicoid-type; 54–60 microns long; 5.4 microns wide at the septum, or 8.4–9.6 microns wide at the septum. Microhair total length/width at septum 6.3–10. Microhair apical cells 31.5–34.5 microns long. Microhair apical cell/total length ratio 0.53–0.58. Stomata common (ranked alongside the veins); 18–21 microns long, or 37.5–43.5 microns long (in different specimens). Subsidiaries dome-shaped (mostly), or triangular (a few). Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common (especially alongside the veins); in cork/silica-cell pairs; silicified. Costal short-cells conspicuously in long rows. Costal silica bodies ‘panicoid-type’; mostly shortish dumb-bell shaped.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with radiate chlorenchyma; Isachne-type (seemingly, in our poor material). Leaf blade ‘nodular’ in section to adaxially flat. Midrib not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups; in simple fans. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’ (with many I’s). Sclerenchyma all associated with vascular bundles.

Cytology. Chromosome base number, x = 9. 2n = 18. 2 ploid.

Classification. Watson & Dallwitz (1994): Panicoideae; Panicodae; Paniceae. Soreng et al. (2015): Panicoideae; Panicodae; Paniceae; Boivinellinae. 2 species.

Distribution, phytogeography, ecology. South-eastern U.S.A.

Species of open habitats; glycophytic. Sandy pinewoods.

References, etc. Leaf anatomical: Metcalfe 1960; this project.

Special comments. Fruit data wanting. Illustrations. • A. purshii: Hitchcock and Chase (1950). • A. muhlenbergianum, abaxial epidermis of leaf blade: original. • A. muhlenbergianum, abaxial epidermis of leaf blade: original. • A. muhlenbergianum, transverse section of leaf blade: original

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Cite this publication as: ‘Watson, L., Macfarlane, T.D., and Dallwitz, M.J. 1992 onwards. The grass genera of the world: descriptions, illustrations, identification, and information retrieval; including synonyms, morphology, anatomy, physiology, phytochemistry, cytology, classification, pathogens, world and local distribution, and references. Version: 13th November 2017.’.