The grass genera of the world
Habit, vegetative morphology. Aquatic annual (producing long, juvenile, floating basal leaves). Culms 5–20 cm high; herbaceous; sparsely branched above, or unbranched above. The branching simple. The shoots aromatic. Leaves not basally aggregated; not clearly differentiated into sheath and blade; non-auriculate. Sheath margins free (but no clear distinction between sheath and blade). Leaf blades narrow (short); exhibiting multicellular glands abaxially. The abaxial leaf blade glands intercostal. Leaf blades without cross venation; persistent. Ligule absent.
Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets all alike in sexuality.
Inflorescence. Inflorescence a single spike, or a single raceme (then spike-like); espatheate; not comprising partial inflorescences and foliar organs. Spikelet-bearing axes persistent. Spikelets solitary; not secund; distichous; subsessile to pedicellate.
Female-fertile spikelets. Spikelets 8–15 mm long; compressed laterally; disarticulating above the glumes; disarticulating between the florets (when wet). Rachilla prolonged beyond the uppermost female-fertile floret; the rachilla extension with incomplete florets.
Glumes two; more or less equal; shorter than the spikelets; shorter than the adjacent lemmas; not pointed; awnless; similar (broad, 2–5 toothed). Lower glume 9–15 nerved. Upper glume 9–15 nerved. Spikelets with incomplete florets. The incomplete florets distal to the female-fertile florets. The distal incomplete florets merely underdeveloped.
Female-fertile florets 5–11 (?). Lemmas firm; not becoming indurated; incised; 5 lobed; deeply cleft; mucronate (with mucronate teeth), or awned (the teeth awn-tipped). Awns when present, 5; when present, median and lateral; the median similar in form to the laterals; non-geniculate. Lemmas hairy, or hairless; 13–15 nerved. Palea present (broad); relatively long (as long as the lemma); 2-nerved; 2-keeled. Palea keels glabrous. Lodicules absent. Stamens 3. Stigmas 2.
Fruit, embryo and seedling. Fruit compressed laterally. Hilum short (large). Embryo large; without an epiblast; with a scutellar tail; with an elongated mesocotyl internode; with one scutellum bundle. Embryonic leaf margins meeting.
Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present; intercostal. Intercostal papillae not over-arching the stomata; consisting of one oblique swelling per cell. Mid-intercostal long-cells having straight or only gently undulating walls (or only slightly so). Microhairs present; elongated; clearly two-celled; chloridoid-type (Neostapfia type: button mushroom shaped, and secretory). Microhair apical cell wall of similar thickness/rigidity to that of the basal cell. Microhairs 24–33(–36) microns long. Microhair basal cells 12 microns long. Microhairs 13.5–18.6 microns wide at the septum. Microhair total length/width at septum 1.3–2.7. Microhair apical cells 10.5–13.5 microns long. Microhair apical cell/total length ratio 0.32–0.44. Stomata common; 21–27 microns long (O. inaequalis), or 30–37.5 microns long (O. californica). Subsidiaries mostly high dome-shaped. Intercostal short-cells absent or very rare. Intercostal silica bodies absent. Costal short-cells conspicuously in long rows. Costal silica bodies present throughout the costal zones; panicoid-type; cross shaped to dumb-bell shaped, or 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; traversed by columns of colourless mesophyll cells. Leaf blade adaxially flat. Midrib not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups; in simple fans and associated with colourless mesophyll cells to form deeply-penetrating fans (linking with traversing columns of colourless cells). Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present, or absent. Sclerenchyma all associated with vascular bundles.
Cytology. Chromosome base number, x = 10 (probably). 2n = 24–32. 2 and 4 ploid (? - and aneuploids).
Classification. Watson & Dallwitz (1994): Chloridoideae; Orcuttieae. Soreng et al. (2015): Chloridoideae; Cynodonteae; Orcuttiinae. 5 species.
Distribution, phytogeography, ecology. California and Mexico.
Rusts and smuts. Rusts Puccinia. Taxonomically wide-ranging species: Puccinia graminis.
References, etc. Morphological/taxonomic: Reeder 1965. Leaf anatomical: Metcalfe 1960 and this project.
Illustrations. • O. californica: Lamson-Scribner (1900). • O. pilosa, O. tenuis: Hitchcock and Chase (1950)
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: 13th November 2017. delta-intkey.com/grass’.