| # The grass genera of the world |

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

### Swallenia Soderstrom & Decker

Named for J.R. Swallen, agrostologist.

Including Ectosperma Swallen

Habit, vegetative morphology. Perennial; rhizomatous. Culms about 10–60 cm high (?); herbaceous; branched above (with sterile and fertile branches). Plants unarmed. Leaves not basally aggregated; non-auriculate. Leaf blades narrow; 3–6 mm wide (4–14 cm long); flat; without abaxial multicellular glands; without cross venation. Ligule a fringe of hairs. Contra-ligule absent.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets.

Inflorescence. Inflorescence paniculate; contracted (sparse, 4–10 cm long); spicate (the branches short, appressed, with 1–3 spikelets); espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund; pedicellate.

Female-fertile spikelets. Spikelets 10–15 mm long (and almost as wide); compressed laterally; not disarticulating (in material seen, even in the presence of fruit); not disarticulating between the florets; with conventional internode spacings. Rachilla prolonged beyond the uppermost female-fertile floret; the rachilla extension with incomplete florets. Hairy callus present.

Glumes two; more or less equal; long relative to the adjacent lemmas (almost as long as the spikelet, spreading); hairless; glabrous; pointed (acuminate); awnless; similar (thinly membranous). Lower glume 5–7 nerved. Upper glume 7–11 nerved. Spikelets with incomplete florets. The incomplete florets distal to the female-fertile florets. The distal incomplete florets merely underdeveloped.

Female-fertile florets 3–7. Lemmas very broad; similar in texture to the glumes to decidedly firmer than the glumes (membranous or papery); not becoming indurated; entire; pointed (acute), or blunt (but mucronate); awnless, or mucronate; hairy (densely villous below, on the margins and sometimes intercostally); non-carinate (dorsally rounded); without a germination flap; 5–7 nerved. Palea present; relatively long (often exceeding the lemma); entire to apically notched (erose-truncate); with apical setae to awned (the 2 nerves shortly excurrent); not indurated (thin, villous below and marginally); 2-nerved; 2-keeled. Lodicules present (large); 2; fleshy; glabrous; each with a thin, auriculate lobe; heavily vascularized. Stamens 3. Anthers 3.5 mm long. Ovary apically glabrous. Styles free to their bases. Stigmas 2; probably white.

Fruit, embryo and seedling. Fruit free from both lemma and palea (and falling free readily); small to medium sized (about 4 mm long); ellipsoid; compressed dorsiventrally (rectangular in section); smooth (brown). Hilum short. Pericarp fused. Embryo large; with an epiblast; with a scutellar tail; with an elongated mesocotyl internode. Embryonic leaf margins meeting.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present; intercostal. Intercostal papillae papilliform cells on the sides of the intercostal grooves extensively obscuring the stomata; many intercostal cells swollen-papilliform. Long-cells markedly different in shape costally and intercostally (costal long-cells ‘normal’); differing markedly in wall thickness costally and intercostally (costals thicker-walled). Intercostal zones exhibiting many atypical long-cells (these being rather short). Mid-intercostal long-cells having straight or only gently undulating walls. Microhairs present; elongated; clearly two-celled; chloridoid-type (large, with distending basal cell - but inconspicuous, because both cells thin-walled and tending to collapse); 27–33 microns long; 19.5–21 microns wide at the septum. Microhair total length/width at septum 1.38–1.57. Microhair apical cells 7.2–7.5 microns long. Microhair apical cell/total length ratio 0.22–0.28. Stomata common (but mostly obscured by prickles and distended intercostal cells); 21–27 microns long. Subsidiaries where visible, high dome-shaped to triangular. Intercostal short-cells absent or very rare (infrequent pairs); in cork/silica-cell pairs (but rare). Intercostal silica bodies absent. Prickles common alongside the intercostal zones. Costal short-cells conspicuously in long rows. Costal silica bodies present in alternate cell files of the costal zones; horizontally-elongated smooth and rounded (these predominating, including potato shapes and large, supine crescents), or ‘panicoid-type’ (a few); the panicoid form somewhat nodular; not sharp-pointed.

Transverse section of leaf blade, physiology. Lamina mid-zone in transverse section open.

C4; XyMS+. PCR sheaths of the primary vascular bundles complete. PCR sheath extensions absent. Mesophyll with radiate chlorenchyma. Leaf blade ‘nodular’ in section (opposing, rounded adaxial and abaxial ribs); with the ribs more or less constant in size. Midrib not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups (and in irregular groups); sometimes in simple fans. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders absent (most bundles with heavy adaxial and abaxial strands only). Sclerenchyma all associated with vascular bundles. The lamina margins with fibres.

Classification. Watson & Dallwitz (1994): Chloridoideae; main chloridoid assemblage. Soreng et al. (2015): Chloridoideae; Cynodonteae; Scleropogoninae. 1 species (S. alexandrae).

Distribution, phytogeography, ecology. California.

Xerophytic; species of open habitats. Sand dunes.

References, etc. Morphological/taxonomic: Swallen 1956. Leaf anatomical: Metcalfe 1960, and studied by us.

Illustrations. • Swallenia alexandrae, as Ectosperma: Hitchcock & Chase (1950). • Swallenia alexandrae, TS leaf blade: this project.

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, and classifications. See also Guidelines for using data taken from Web publications.

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: 15th June 2025. delta-intkey.com’.

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