| # The grass genera of the world |

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

### Isalus J. Phipps

~ Tristachya

Habit, vegetative morphology. Perennial; caespitose. Culms 35–70 cm high; herbaceous; unbranched above; about 4 noded. Culm nodes exposed; glabrous. Culm leaves present. Young shoots intravaginal. Leaves not basally aggregated; non-auriculate. Leaf blades linear to linear-lanceolate; narrow; 1.5–2 mm wide; flat, or rolled; without cross venation; persistent. Ligule a fringe of hairs. Contra-ligule absent.

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

Inflorescence. Inflorescence paniculate; open (with the triads of spikelets terminating the branches); espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets in triplets; not secund; sessile; not in distinct ‘long-and-short’ combinations.

Female-fertile spikelets. Spikelets 9–14 mm long; compressed laterally; disarticulating above the glumes; not disarticulating between the florets; with conventional internode spacings. The upper floret not stipitate. Rachilla terminated by a female-fertile floret. Hairy callus present. Callus pointed (with long hairs).

Glumes two; more or less equal; about equalling the spikelets; long relative to the adjacent lemmas; hairy (at least the lower, with tubercle-based hairs), or hairless; with distinct rows of hairs; pointed (to subulate); awnless; (at least the upper) carinate; similar (lanceolate-acuminate, firmly membranous to leathery). Lower glume about equalling the lowest lemma; tuberculate; 3 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; paleate. Palea of the proximal incomplete florets fully developed. The proximal incomplete florets male (with three stamens), or sterile. The proximal lemmas similar to the upper glume in shape and texture, sometimes with tubercle-based hairs at the summit; awnless; 5 nerved, or 7 nerved; decidedly exceeding the female-fertile lemmas; less firm than the female-fertile lemmas to similar in texture to the female-fertile lemmas (firmly membranous to leathery); not becoming indurated.

Female-fertile florets 1. Lemmas similar in texture to the glumes to decidedly firmer than the glumes; smooth; not becoming indurated; incised; 2 lobed; deeply cleft; awned. Awns 1; median; from a sinus (between the setaceous-pointed lateral lobes); geniculate; hairless; much longer than the body of the lemma; entered by several veins. Awn bases not twisted; not flattened. Lemmas hairy. The hairs in tufts; in transverse rows (the lemma hairy at the base and with 6–8 tufts aligned transversely in the upper third). Lemmas non-carinate; having the margins inrolled against the palea; without a germination flap; 7 nerved, or 9 nerved; with the nerves confluent towards the tip. Palea present; relatively long; tightly clasped by the lemma; apically notched; awnless, without apical setae; thinner than the lemma; not indurated; 2-nerved; 2-keeled. Palea back hairy. Palea keels wingless (but thickened); hairy. Stamens 3. Anthers about 3 mm long (in I. isalensis); without an apically prolonged connective. Ovary sparsely apically hairy. Styles free to their bases; free. Style bases adjacent. Stigmas 2; brown.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells markedly different in shape costally and intercostally (the costals much narrower); of similar wall thickness costally and intercostally (fairly thin walled). Intercostal zones with typical long-cells to exhibiting many atypical long-cells (I. humbertii having very short long-cells in some files). Mid-intercostal long-cells rectangular to fusiform (mostly more or less rectangular, but the ends often fairly rounded); having markedly sinuous walls (the sinuosity fine or coarse, heavily pitted). Microhairs present; elongated; clearly two-celled; panicoid-type (narrow). Stomata common. Subsidiaries non-papillate; low dome-shaped and triangular (often with a small point). Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs and not paired (many solitary); mostly not silicified (in the material of two species seen). I. isalensis with large, cushion based macrohairs in the intercostal zones, and prickles bordering the costae, I. humbertii with neither. Costal short-cells conspicuously in long rows. Costal silica bodies present and well developed, or absent (I. humbertii); in I. isalensis ‘panicoid-type’; consistently short to medium dumb-bell shaped (in I. isalensis, the silica cells dumb-bell shaped in I. humbertii).

Transverse section of leaf blade, physiology. C4. The anatomical organization conventional. XyMS–. PCR sheath extensions absent. Mesophyll without ‘circular cells’; traversed by columns of colourless mesophyll cells (in places). Leaf blade with distinct, prominent adaxial ribs to ‘nodular’ in section, or adaxially flat; with the ribs more or less constant in size (low). Midrib not readily distinguishable; with one bundle only, or having a conventional arc of bundles (depending on interpretation). The lamina symmetrical on either side of the midrib. Bulliforms present in discrete, regular adaxial groups (a large group in every furrow, overlying a small bundle); in simple fans (in places), or associated with colourless mesophyll cells to form deeply-penetrating fans; associating with colourless mesophyll cells to form arches over small vascular bundles (these colourless cells sometimes seeming to make traversing columns). Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present (with all the main bundles); forming ‘figures’ (the large bundles with massive I’s and ‘anchors’). Sclerenchyma all associated with vascular bundles.

Classification. Watson & Dallwitz (1994): Panicoideae; Panicodae; Arundinelleae. Soreng et al. (2015): Panicoideae (?); Tristachyideae (?). 3 species.

Distribution, phytogeography, ecology. Madagascar.

References, etc. Leaf anatomical: studied by us - I. isalensis (Lam) Phipps, I. humbertii (A. Camus) Phipps.

Special comments. Fruit data wanting.

Illustrations. • Isalus spikelets. • I. isalensis, Leaf blade TS: 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’.

Contents