| # The grass genera of the world |

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

### Soderstromia Morton

Named for T.R. Soderstrom, American agrostologist and bamboo expert.

~ Bouteloua sensu lato

Including Fourniera Scribn.

Habit, vegetative morphology. Slender annual, or perennial; stoloniferous. Culms 20 cm high; herbaceous; branched above. Culm nodes glabrous. Leaves not basally aggregated; non-auriculate. Leaf blades narrow; 1–2 mm wide (2 to 6 cm long); without abaxial multicellular glands; without cross venation. Ligule a fringed membrane (short, densely fringed). Contra-ligule absent.

Reproductive organization. Plants monoecious with all the fertile spikelets unisexual, or dioecious; without hermaphrodite florets. The spikelets all alike in sexuality (i.e., all male or all female, when dioecious), or of sexually distinct forms on the same plant (when monoecious); female-only, or male-only, or female-only and male-only. The male and female-fertile spikelets in different inflorescences (in separate tufts), or on different branches of the same inflorescence. The spikelets overtly heteromorphic.

Inflorescence. Inflorescence a false spike, with spikelets on contracted axes (when each spikelet is interpreted as a reduced cluster, cf. Bouteloua: see below). Inflorescence with the main rachis terminating in a short abortive apex. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes very much reduced (each cluster comprising 3 basal, shortly-pedicellate, flattened, several-nerved, dentate, opaque ‘bracts’ (reduced spikelets), and a central, longer-pedicellate, glumeless fertile spikelet); disarticulating; falling entire (i.e., the clusters falling from the persistent main axis). Spikelets associated with bractiform involucres (comprising reduced spikelets). The involucres shed with the fertile spikelets. Spikelets shortly pedicellate (including the ‘bracts’).

Female-sterile spikelets. Male spikelets with two fertile florets and glumelike basal bracts, the ‘cluster’ suggesting a single, bizarre spikelet; male flowers 3-staminate. The male spikelets 2 floreted. Male florets 2; 3 staminate.

Female-fertile spikelets. Spikelets unconventional (because of the complicating ‘bracts’); 4–5 mm long; compressed dorsiventrally; the clusters disarticulating. Rachilla prolonged beyond the uppermost female-fertile floret; hairless; the rachilla extension with incomplete florets.

Glumes absent. Spikelets with incomplete florets. The incomplete florets distal to the female-fertile florets. The distal incomplete florets 1; clearly specialised and modified in form (the sterile floret consisting of 3 long, straight awns).

Female-fertile florets 1. Lemmas not becoming indurated (membranous); incised; 3 lobed; deeply cleft (trifid, the lobes subacuminate, the central one longer); awnless; hairless; non-carinate; without a germination flap; 3 nerved. Palea present; relatively long; entire; awnless, without apical setae (glabrous); not indurated (membranous); 2-nerved; 2-keeled. Lodicules present; 2; free; membranous; glabrous; not or scarcely vascularized. Stamens 0. Anthers relatively long.

Fruit, embryo and seedling. Fruit small; ellipsoid; compressed dorsiventrally. Hilum short. Pericarp fused. Embryo large.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells markedly different in shape costally and intercostally (the costals narrower, more rectangular); of similar wall thickness costally and intercostally (thick, pitted). Mid-intercostal long-cells rectangular to fusiform; having markedly sinuous walls. Microhairs present; elongated; clearly two-celled; panicoid-type to chloridoid-type (apical cells narrowing to the thick-walled tip, which is more or less apiculate). Microhair apical cell wall thinner than that of the basal cell but not tending to collapse. Microhairs (30–)31–39 microns long; (7.2–)7.5–8.4(–9) microns wide at the septum. Microhair total length/width at septum 3.6–4.3. Microhair apical cells 16.5–18–19.5 microns long. Microhair apical cell/total length ratio 0.42–0.65. Stomata common; 24–25–27 microns long. Subsidiaries dome-shaped (mostly), or triangular (in some files). Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs; silicified. Intercostal silica bodies present and perfectly developed; tall-and-narrow. Costal short-cells conspicuously in long rows (mostly), or predominantly paired to neither distinctly grouped into long rows nor predominantly paired (paired or in short rows over some veins). Costal silica bodies present in alternate cell files of the costal zones; saddle shaped (predominantly), or crescentic; 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 interrupted; interrupted both abaxially and adaxially. PCR sheath extensions absent. Leaf blade ‘nodular’ in section to adaxially flat (with rounded, low abaxial ribs). Midrib not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups; associated with colourless mesophyll cells to form deeply-penetrating fans. All the vascular bundles accompanied by sclerenchyma (but small bundles with only tiny strands). Combined sclerenchyma girders present (with the primaries); forming ‘figures’ (the primaries). Sclerenchyma all associated with vascular bundles.

Classification. Watson & Dallwitz (1994): Chloridoideae; main chloridoid assemblage. Soreng et al. (2015): Chloridoideae; Cynodonteae; Boutelouinae (as a synonym of Bouteloua). 1 species (S. mexicana).

Distribution, phytogeography, ecology. Mexico, Central America.

Species of open habitats. Short grassland.

References, etc. Leaf anatomical: studied by us.

Illustrations. • Soderstromia mexicana male plant, as Fourniera: Lamson-Scribner (1897), American Grasses, Bull. U.S.D.A.. • Soderstromia mexicana female plant, as Fourniera: Lamson-Scribner (1897), American Grasses, Bull. U.S.D.A.. • Sodertromia mexicana, abaxial epidermis of 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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