Digitaria, (Nees) Henrard emend. A. S. Vega & Rugolo

Medico, Julia Mariela Lo, Tosto, Daniela Sandra, Rua, Gabriel Hugo, Agrasar, Zulma Esther Rúgolo de, Scataglini, Maria Amalia & Vega, Andrea Susana, 2017, Phylogeny of Digitaria Sections Trichachne and Trichophorae (Poaceae, Panicoideae, Paniceae): A Morphological and Molecular Analysis. New Circumscription and Synopsis, Systematic Botany (Basel, Switzerland) 42 (1), pp. 37-53 : 42-45

DIGITARIA sect. TRICHACHNE (Nees) Henrard emend. A. S. Vega & Rúgolo.

Trichachne Nees , Fl. Bras. Enum. Pl. 2 (1): 85. 1829. Panicum L. sect. Trichachne (Nees) Steud ., Syn. Pl. Glumac. 1: 38. 1855 [1853]. Panicum L. ser. Trichachne (Nees) Benth. , Fl. Austral. 7: 464. 1878. Digitaria sect. Trichophorae Henrard , Monogr. Digitaria: 573. 1950. syn. nov.

TYPE SPECIES— Andropogon insularis L.

[= DIGITARIA INSULARIS (L.) Fedde ].

Perennial taxa native to the New World, with knotty or spreading rhizomes. Spikelets in pairs. Rachilla internodes more or less elongated, hence upper florets stipitate. Indumentum of unicellular hairs, hairs smooth walled, apex acute, whitish-silvery, sometimes whitish with purplish tints or ochraceous, abundant in upper glume and lower lemma (scarce in D. laxa ), generally exceeding the length of the spikelet and spreading at maturity.

Morphological Diagnostic Characters of Digitaria sect. Trichachne — HABIT AND VEGETATIVE SHOOTS — Taxa are perennial and all native to the New World. Most of the species exhibit a short rhizomatous knotty base, with internodes slightly developed and coated with pubescent cataphylls, although D. swalleniana and D. catamarcensis form spreading rhizomes because they have more numerous and elongated internodes.

SPIKELETS — Spikelets are arranged on the rachis in groups of two, one sub-sessile and the other one pedicellate. Pedicels are scabrous, with some prickles scarcely exceeding their summit ( Fig. 3A ), never forming a coronula of hairs. Spikelets are ellipsoid ( D. hitchcockii ), ovate and abruptly acuminate ( D. californica ), or, in the majority of species, lanceolate to narrowly lanceolate with an acuminate apex. Rachillas have elongated internodes between the lower and upper glumes, upper glume and lower floret, and between lower and upper floret; for this reason, the upper floret is considered stipitate ( Fig. 3B–C ). Internode elongation was also observed in D. eggersii and D. pittieri ( Fig. 3C ), although Henrard (1950) regarded the upper floret as non-stipitate in both taxa.

PILOSE INDUMENTUM OF THE SPIKELETS — Hairs are unicellular, smooth-walled, spreading at maturity, whitish-silvery, sometimes with purplish tints, or ochraceous, apex acute. In most taxa, the upper glume and lower lemma are densely hairy, with hairs usually longer than the spikelet length, with the exception of D. laxa ( Fig. 3D–E ). In D. laxa the lower glume is glabrous and both the upper glume and lower lemma have short ochraceous hairs along the margin, shorter than the spikelet length.

UPPER GLUME—Most of the species have a 3-nerved upper glume with hairs between the nerves and along the margin, with the exception of D. laxa that is 1-nerved and with some short marginal hairs.

LOWER LEMMA — The midnerve may be distant from lateral ones (not equally spaced in D. californica , D. catamarcensis , D. insularis , D. patens , and D. sacchariflora ; Fig. 3F ) or equally spaced in the remaining taxa ( Fig. 3G ). Between the nerves, hair distribution occurs in one of four different patterns: pilose between the nerves and along the margin (exclusive of D. hitchcockii ; Fig. 3H ), glabrous on both sides of midnerve and pilose in the remaining zones ( D. californica , D. catamarcensis , and D. patens ; Fig. 3F ), glabrous on both sides of midnerve and alternately pilose and glabrous in the remaining zones ( D. eggersii , D. insularis , D. pittieri , D. sacchariflora , and D. tenuis ; Fig. 3G ), and only pilose along the margin ( D. laxa , D. similis , and D. swalleniana ).

UPPER FLORET — Most of the species have lanceolate or narrowly lanceolate upper florets with acuminate apices ( Fig. 3I ). Digitaria californica is distinguished by ovatelanceolate upper florets with abruptly acuminate apices ( Fig. 3J ), whereas D. hitchcockii is recognized by its lanceolate upper florets with acute apices. The rough texture of the upper florets is related to their epidermal cells having strongly undulated walls, each cell with a distally located papilla ( Fig. 3K ). These cells are variable in size and number of undulations among species.

KEY TO THE TAXA OF DIGITARIA SECT. TRICHACHNE

1. Lower lemma exposing 5 equally spaced nerves on the back................................................................... 2

2. Lower lemma 7-nerved, glabrous on the back and pilose along the margins. Indumentum of the spikelets ochraceous or whitish..................................................................... 3

3. Spikelets scarcely pilose, with ochraceous indumentum. Upper glume 1-nerved, ca. ½ the length of the upper floret, with some short marginal hairs. Lower lemma with scabrous nerves. Panicle with (14–)20–60 racemes............................................... D. laxa

3. Spikelets densely pilose, with ochraceous or whitish indumentum. Upper glume 3-nerved, ca. ¾ the length of the upper floret, with long hairs between the nerves and along the margins. Lower lemma with smooth nerves. Panicle with 5–16 racemes.................................................. 4

4. Plants with knotty rhizomes with short internodes (1–1.5 mm). Leaf blades glabrous. Spikelets 4.5–6.5 mm, with ochraceous indumentum. Lower glume obtuse. Anthers 1–1.5 mm ...................... D. similis

4. Plants with spreading rhizomes, with long internodes (3–5 mm). Leaf blades pubescent or with long, rigid, tuberculate hairs. Spikelets 3.8–6 mm, with whitish indumentum. Lower glume triangular. Anthers 0.5–0.6 mm ................................... D. swalleniana

2. Lower lemma 5 or 7-nerved, glabrous on both sides of midnerve and alternately pilose and glabrous in the remaining zones, or pilose between the nerves and along the margins. Indumentum of the spikelets whitish-silvery with purplish tints............................................ 5

5. Lower lemma 5-nerved, with prominent nerves, densely pilose between the nerves and along the marginal zones. Spikelets ellipsoid. Leaf blades adaxially shortly pubescent and abaxially glabrous. Ligule 0.2–0.5 mm, truncate, margins ciliate; ligular zone glabrous or with scarce hirsute hairs........................................................... D. hitchcockii

5. Lower lemma 7-nerved, with scarcely visible nerves, glabrous on both sides of midnerve and alternately pilose and glabrous in the remaining zones. Spikelets lanceolate or narrowly lanceolate. Leaf blades pubescent or glabrous on both surfaces. Ligule 1–4 mm, triangular or obtuse, margins erose; ligular zone hirsute.............................................................. 6

6. Plants 0.15–0.32 m tall. Digitate panicle 2–5 cm, with 2–3 alternate racemes. Leaf blades 1.5–3.5 × 0.2–0.4 cm, pubescent. Spikelets 3.7–4 × 0.8–1 mm, lanceolate.............................. D. eggersii

6. Plants 1.5–1.75 m tall. Non-digitate panicle 14.5–18 cm, with 11–20 racemes, verticillate at the base and then alternate on an axis 5–8 cm long. Leaf blades 11–14 × 1.6–2 cm, glabrous. Spikelets 3–3.5 × 0.7–0.8 mm, narrowly lanceolate......................... D. pittieri

1. Lower lemma exposing 3 nerves on the back, evenly spaced or not.............................................................. 7

7. Panicle digitate or subdigitate, 4–6(–20) cm, with 2–8(–11) appressed racemes. Spikelets 2.7–3 mm ............................. D. tenuis

7. Panicle non-digitate, 5–40 cm, with 4–50 stiffly ascending or spreading racemes at maturity. Spikelets 3–6.6 mm ....................... 8

8. Spikelets (3.7–) 4–6.6 mm, with ochraceous indumentum, narrowly ellipsoid, apex acuminate. Lower lemma glabrous on both sides of midnerve and either pilose or alternately pilose and glabrous in the remaining zones............................................................. 9

9. Plants with spreading rhizomes, with long internodes (3–5 mm). Spikelets (5.5–) 6.3–6.6 mm. Lower lemma (5–)7-nerved, midnerve distant from the lateral ones, glabrous on both sides of the midnerve and pilose in the remaining zones................................. D. catamarcensis

9. Plants with knotty rhizomes, with short internodes (1–1.5 mm). Spikelets (3.7–) 4–5.5 mm. Lower lemma 7-nerved, equidistantly nerved, glabrous on both sides of the midnerve and alternately pilose and glabrous in the remaining zones........................ 10

10. Panicle with 20–50 racemes, distal portion pendulous. Plants robust, generally taller than 1 m. Leaf sheaths pilose. Leaf blades 1–2 cm wide. Lower glume acute or truncate................................................ D. insularis

10. Panicle with 4–15 erect, appressed racemes. Plants shorter than 1 m. Leaf sheaths glabrous or sparsely pilose. Leaf blades 0.15–0.8(–1) cm wide. Lower glume obtuse............. D. sacchariflora

8. Spikelets 3–4.5 mm, with hairs whitish-silvery or whitish with purplish tints, narrowly ovate or ovate, apex acuminate or abruptly acuminate. Lower lemma glabrous on both sides of the midnerve and pilose in the remaining zones.................................... 11

11. Spikelets 3–3.2 mm, ellipsoid or linear–lanceolate, apex acuminate. Panicle 10–18 cm, with 6–9 patent racemes, loosely spiculate, on an axis 7–10 cm ................................ D. patens

11. Spikelets 3–4.5 mm, ovate, apex abruptly acuminate. Panicle 5–19 cm, with 4–10 appressed racemes, densely spiculate, on an axis 5–7 cm .................................................. 12

12. Culms filiform, occasionally branched. Leaf sheaths and blades glabrous or sparsely pilose. Leaf blades filiform, 5–13 × 0.1–0.3 cm. Indumentum exceeding spikelets by 1 mm .............................................. D. californica var. californica

12. Culms robust, generally branched. Leaf sheaths and blades densely hirsute. Leaf blades narrowly lanceolate, with acuminate apex, 6–15 × 0.3–0.6 cm. Indumentum exceeding spikelets by 2.5–3 mm .......................................... D. californica var. villosissima

Figures

FIG. 3. Spikelets, pedicel, and upper lemma epidermis of taxa of D. sect. Trichachne, SEM micrographs. A. Pedicel. B–C. Spikelets in longitudinal section showing rachilla internodes and bract insertion. D. Pilose indumentum of the spikelets. E. Spikelet with pilose indumentum spreading at maturity. F–H. View of spikelets from lower glume and lower lemma. F. Lower lemma with midnerve distant from lateral, contiguous nerves. G. Lower lemma with equidistant nerves. H. Lower lemma densely pilose between the nerves and along the margin. I–J. Upper floret, back of the lemma. K. Upper lemma epidermis, detail. Bars: A = 100 μm; B–C = 200 μm; D = 50 μm; E–F = 1000 μm; G–J = 500 μm; K = 20 μm. A, F: D. patens, Swallen 10027 (US); B: D. similis, Hitchcock 9008 (US); C: D. pittieri, Hitchcock 19949 (US); E: D. tenuis, Swallen 4235 (SI); D, H: D. hitchcockii, Silvens 832 (K); G: D. pittieri, Chase 7826 (US); I: D. patens, Silveus 2719 (US); J–K: D. californica var. villosissima, Rodriguez 241 (SI), Parodi 14052 (BAA). Abbreviations: lg = lower glume; ll = lower lemma; p = papilla; r = rachilla; ug = upper glume; ul = upper lemma; up = upper palea.