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The grass genera of the world

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

Holcus L.

The Latin form of the Greek holkos (attractive, also some species of grass.

Fog grasses.

Type species: Type: H. lanatus L., type cons..

Including Arthrochloa R. Br., Ginannia Bub., Homalachna Kuntze, Nothoholcus Nash, Notholcus Hitchc., Sorghum Adans.

Habit, vegetative morphology. Annual (rarely, e.g. H. setiger), or perennial; rhizomatous to stoloniferous, or caespitose. Culms 8–150 cm high; herbaceous; unbranched above. Culm nodes hairy, or glabrous. Culm internodes hollow. Leaves not basally aggregated; non-auriculate. Leaf blades linear to linear-lanceolate; apically cucullate, or apically flat; broad, or narrow; 2–16 mm wide; flat; not pseudopetiolate; without cross venation; persistent; rolled in bud. Ligule an unfringed membrane to a fringed membrane; not truncate (rounded); 1–5 mm long.

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

Inflorescence. Inflorescence paniculate; fairly open to contracted; when contracted spicate, or more or less irregular. Primary inflorescence branches borne distichously. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund.

Female-fertile spikelets. Spikelets 3–8 mm long; compressed laterally; falling with the glumes; usually with a distinctly elongated rachilla internode above the glumes and with distinctly elongated rachilla internodes between the florets (with the lower floret elevated on a markedly curved internode, which in H. setiger is U-shaped, thickened and attached to the main axis at the middle of the U - i.e. highly modified). Rachilla prolonged beyond the uppermost female-fertile floret (and sometimes prolonged beyond the upper, sterile or male-only floret); hairy, or hairless; the rachilla extension usually with incomplete florets (in the sense that the upper floret is usually male-only or sterile, regardless of whether there is an extension beyond it). Hairy callus present, or absent. Callus very short.

Glumes two; more or less equal (the upper sometimes somewhat longer); about equalling the spikelets to exceeding the spikelets (enclosing them); long relative to the adjacent lemmas; pointed; awned (both, or only the upper), or awnless; carinate; very dissimilar to similar (membranous, complanate, the upper often broader and longer-awned). Lower glume 1 nerved. Upper glume 3 nerved. Spikelets with incomplete florets. The incomplete florets distal to the female-fertile florets (spikelets 2-flowered, the lower hermaphrodite, the upper usually male-only or sterile). The distal incomplete florets 1; clearly specialised and modified in form (male or sterile, falling entire); awned (with a conspicuous short, hooked or geniculate dorsal awn from above the middle). Spikelets without proximal incomplete florets.

Female-fertile florets 1 (very rarely, both florets hermaphrodite). Lemmas decidedly firmer than the glumes (leathery or cartilaginous, shiny); entire, or incised; when entire pointed, or blunt; when incised, 2 lobed; not deeply cleft (bidentate); awnless, or awned. Awns when present, 1; dorsal; from near the top, or from well down the back; geniculate. Lemmas hairy, or hairless; carinate; indistinctly 3–5 nerved. Palea present; relatively long; tightly clasped by the lemma; textured like the lemma; not indurated (thin); 2-nerved; 2-keeled. Lodicules present; 2; free; membranous; glabrous; toothed, or not toothed. Stamens 3. Anthers not penicillate. Ovary apically glabrous. Styles free to their bases. Stigmas 2; white.

Fruit, embryo and seedling. Fruit adhering to lemma and/or palea, or free from both lemma and palea; small; longitudinally grooved (rarely), or not grooved; compressed laterally. Hilum short, or long-linear (rarely). Embryo small; not waisted. Endosperm liquid in the mature fruit, or hard; with lipid. Embryo with an epiblast; without a scutellar tail; with a negligible mesocotyl internode. Embryonic leaf margins meeting.

Seedling with a short mesocotyl, or with a long mesocotyl; with a loose coleoptile. First seedling leaf with a well-developed lamina. The lamina narrow; erect; 3 veined.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells similar in shape costally and intercostally (but the intercostals much larger); of similar wall thickness costally and intercostally. Mid-intercostal long-cells rectangular; having straight or only gently undulating walls. Microhairs absent. Stomata common. Subsidiaries parallel-sided. Guard-cells overlapped by the interstomatals. Intercostal short-cells absent or very rare. Prickles abundant. Costal short-cells neither distinctly grouped into long rows nor predominantly paired. Costal silica bodies horizontally-elongated crenate/sinuous, or horizontally-elongated smooth, or rounded.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with radiate chlorenchyma (rarely), or with non-radiate chlorenchyma. Leaf blade with distinct, prominent adaxial ribs, or ‘nodular’ in section; with the ribs more or less constant in size. Midrib not readily distinguishable; with one bundle only; with colourless mesophyll adaxially (sometimes, in H. lanatus), or without colourless mesophyll adaxially. Bulliforms present in discrete, regular adaxial groups; in simple fans. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; nowhere forming ‘figures’. Sclerenchyma all associated with vascular bundles.

Culm anatomy. Culm internode bundles in one or two rings.

Phytochemistry. Tissues of the culm bases with little or no starch. Leaves without flavonoid sulphates (1 species).

Cytology. Chromosome base number, x = 7 (sometimes + 1B), or 4. 2n = 7, 8, 14, 28, 35, 42, and 49. 2, 4, 5, 6, and 7 ploid. Chromosomes ‘large’’.

Classification. Watson & Dallwitz (1994): Pooideae; Poodae; Aveneae. Soreng et al. (2015): Pooideae; Poodae; Poeae; Holcinae. 9 species.

Distribution, phytogeography, ecology. 8 Canary Is., North Africa, Europe to Asia Minor & Caucasus; 1 South Africa.

Commonly adventive. Mesophytic; shade species and species of open habitats; glycophytic. Grassland, open woodland, disturbed ground.

Economic aspects. Significant weed species: H. lanatus, H. mollis. Cultivated fodder: H. lanatus.

Rusts and smuts. Rusts — Puccinia. Taxonomically wide-ranging species: Puccinia coronata, Puccinia striiformis, and Puccinia hordei. Smuts from Tilletiaceae and from Ustilaginaceae. Tilletiaceae — Entyloma and Tilletia. Ustilaginaceae — Ustilago.

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

Illustrations. • H. lanatus, general aspect: Eng. Bot. (1872). • H. mollis, general aspect: John Curtis, 1824. • H. mollis, general aspect: Eng. Bot. (1872). • General aspect and spikelet details (H. lanatus). • Inflorescence detail (H. lanatus). • H. lanatus spikelet showing details of both florets: otiginal. Holcus lanatus. Two florets, the upper male-only with an awned lemma. • Details of spikelet tip (H. lanatus). Holcus lanatus. Spikelet tip, showing upper male-only floret, and a smaller incomplete one distal to it. • Pollen antigens: Watson and Knox (1976)


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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. delta-intkey.com/grass’.

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