The grass genera of the world
Including Spirotheros Raf.
Habit, vegetative morphology. Annual, or perennial; caespitose. Culms 20–100 cm high; herbaceous; branched above, or unbranched above. The branching simple, or fastigiate. Culm nodes glabrous. Culm internodes solid. Leaves not basally aggregated; non-auriculate. Sheath margins free. The sheaths keeled. Leaf blades linear; narrow; flat; without cross venation; persistent. Ligule a fringed membrane.
Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets, or without hermaphrodite florets. The spikelets of sexually distinct forms on the same plant (heterogamous, but only in the upper parts of the raceme); hermaphrodite, male-only, and sterile, or female-only, male-only, and sterile; overtly heteromorphic; in both homogamous and heterogamous combinations (lower pairs homogamous and homomorphic, male or sterile). Apomictic, or reproducing sexually.
Inflorescence. Inflorescence a single raceme, or paniculate (the single racemes sometimes in false panicles); spatheate, or espatheate; a complex of partial inflorescences and intervening foliar organs (when having axillary fascicles), or not comprising partial inflorescences and foliar organs. Spikelet-bearing axes racemes (of several to many joints); solitary; disarticulating; disarticulating at the joints (between the heterogamous upper spikelet pairs). Articles linear; disarticulating obliquely. Spikelets paired; secund; consistently in long-and-short combinations; in pedicellate/sessile combinations (but the pedicel reduced to a short stump, the spikelet being supported on a long, slender callus). Pedicels of the pedicellate spikelets free of the rachis. The shorter spikelets hermaphrodite (in upper regions of spike-like panicles only), or female-only. The longer spikelets male-only, or sterile.
Female-sterile spikelets. The pedicellate male or sterile spikelets larger, with a pedicel-like callus (the true pedicel represented by a stump); awnless, dorsally flattened, rather asymmetric. G1 herbaceous, many nerved, winged above. The male spikelets with glumes. The lemmas awnless.
Female-fertile spikelets. Spikelets not noticeably compressed to compressed dorsiventrally (subterete); falling with the glumes. Rachilla terminated by a female-fertile floret. Hairy callus present. Callus long; pointed.
Glumes two; more or less equal; long relative to the adjacent lemmas; hairy; without conspicuous tufts or rows of hairs; awnless; very dissimilar (the upper with deep longitudinal grooves). Lower glume not two-keeled; convex on the back; not pitted; relatively smooth, or tuberculate; 5–9 nerved. Upper glume 3 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 1; epaleate; sterile. The proximal lemmas awnless; 0 nerved; similar in texture to the female-fertile lemmas (hyaline); not becoming indurated.
Female-fertile florets 1. Lemmas stipitate-cartilaginous below, produced into the awn; less firm than the glumes (hyaline); not becoming indurated; entire; awned. Awns 1; median; apical; geniculate; hairy; much longer than the body of the lemma. Lemmas non-carinate; 3 nerved. Palea present, or absent; when present, very reduced; nerveless. Lodicules present, or absent; when present, 2; free; fleshy; glabrous. Stamens 0–3. Anthers not penicillate. Ovary apically glabrous. Styles free to their bases. Stigmas 2; red pigmented.
Fruit, embryo and seedling. Fruit free from both lemma and palea; longitudinally grooved (channelled on one side); compressed dorsiventrally. Hilum short. Embryo large. Endosperm containing only simple starch grains. Embryo without an epiblast; with a scutellar tail; with an elongated mesocotyl internode. Embryonic leaf margins overlapping.
First seedling leaf with a well-developed lamina. The lamina broad; curved; 21–30 veined.
Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present (e.g. H. contortus), or absent (e.g. H. triticeus). Intercostal papillae consisting of one oblique swelling per cell (in H. contortus). Long-cells of similar wall thickness costally and intercostally. Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; panicoid-type; 61.5–82 microns long; 7.2–9.6 microns wide at the septum. Microhair total length/width at septum 6.4–8.8. Microhair apical cells 24–38 microns long. Microhair apical cell/total length ratio 0.41–0.51. Stomata common; 42–45 microns long. Subsidiaries triangular. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs, or not paired (many solitary, some twos, some short rows - being an unusual feature in Poaceae); not silicified. Costal short-cells conspicuously in long rows. Costal silica bodies panicoid-type; cross shaped to dumb-bell shaped.
Transverse section of leaf blade, physiology. C4; XyMS. PCR sheath outlines even (the walls thick). PCR sheath extensions absent. PCR cells with a suberised lamella. PCR cell chloroplasts with reduced grana; centrifugal/peripheral. Mesophyll with radiate chlorenchyma; traversed by columns of colourless mesophyll cells (in places, apparently, in H. triticeus), or not traversed by colourless columns. Leaf blade adaxially flat (the furrows scarcely detectable). Midrib conspicuous; having a conventional arc of bundles; with colourless mesophyll adaxially (e.g. in H. triticeus), or without colourless mesophyll adaxially (e.g. in H. contortus). Bulliforms present in discrete, regular adaxial groups; associated with colourless mesophyll cells to form deeply-penetrating fans (sometimes linked with traversing columns of colourless cells, or in places irregularly grouped); associating with colourless mesophyll cells to form arches over small vascular bundles (in places,in H. triticeus), or nowhere involved in bulliform-plus-colourless mesophyll arches. Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present; nowhere forming figures. Sclerenchyma all associated with vascular bundles.
Culm anatomy. Culm internode bundles scattered.
Phytochemistry. Leaves without flavonoid sulphates (2 species).
Cytology. Chromosome base number, x = 10 and 11. 2n = 20, 22, 40, 44, 50, 60, and 80. 2, 4, 6, and 8 ploid.
Classification. Watson & Dallwitz (1994): Panicoideae; Andropogonodae; Andropogoneae; Andropogoninae. Soreng et al. (2015): Panicoideae; Andropogonodae; Andropogoneae. 7 species.
Distribution, phytogeography, ecology. Tropical.
Mesophytic to xerophytic; species of open habitats; glycophytic. Dry places, often on poor soils.
Economic aspects. Significant weed species: H. contortus (with needle-sharp, penetrative callus). Important native pasture species: H. contortus.
Rusts and smuts. Rusts Puccinia. Taxonomically wide-ranging species: Puccinia versicolor and Uromyces clignyi. Smuts from Ustilaginaceae. Ustilaginaceae Sorosporium, Sphacelotheca, and Tolyposporium.
References, etc. Leaf anatomical: Metcalfe 1960; this project.
Illustrations. • H. contortus: Gardner, 1952. • General aspect (H. contortus): Gibbs Russell et al., 1990. • Inflorescence detail of H. contortus. • T.S. leaf blade midrib zone (H. triticeus). • T.S. lateral part of leaf blade (H. triticeus)
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: 12th March 2017. delta-intkey.com/grass’.