Rhynchospora nervosa (Vahl) Boeckeler subsp. nervosa
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Authority
Thomas, William W. 1984. The systematics of Rhynchospora section Dichromena. Mem. New York Bot. Gard. 37: 1-116.
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Family
Cyperaceae
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Scientific Name
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Description
Subspecies Description - Rhizomatous perennial; rhizomes (Fig. 37a) ascending or horizontal, slender and straight to knotted and clumped, to 7 cm long, 1-2 mm in diameter, black or dark brown, often covered with fibrous remains of rhizome scales; rhizome scales ca. 2-8 mm long, wrapping at least halfway around rhizome, nerved, acute; culm erect to arching, 8-50 cm tall, 0.6-1 mm in diameter, obtusely triquetrous, green, often puberulent at summit; leaves ca. 3-10 per culm, canaliculate to flat or involute, green, usually ciliate basally along margins of blades; basal leaves 621 cm long, 1.1-2.6 mm wide; cauline leaf blades 0-l(-2) per culm, 6-13 cm long, 1-2.6 mm wide; leaf sheaths 2-6 cm long, green, often ciliate, especially along edges of inner band; summit of inner band concave, hyaline near summit, nerved; inflorescence bracts (Fig. 37b) 3-5 exceeding inflorescence, tapering gradually to a point, usually ciliate basally along margins, hirsute to glabrous abaxially; basal bract longest, 4.4-11 cm long, 1.1-2.7 mm wide at base of green portion; white portion 1.4-3.2 cm long, smooth textured, thicker than green portion; inflorescence a congested, conical head of 4-12 spikelets; spike lets ovate to widely so, 5.2-8 mm long, 1.9-3 mm wide, consisting of 9-20 scales; scales creamy white, sometimes lightly ferrugineously streaked, ovate to widely so, ca. 3.3-5.2 mm long, 2-5 mm wide, carinate distally, cymbiform; filaments 3.7-4.6 mm long; anthers 1.9-2.4 mm long; stigma and style ca. 2.8-3.7 mm long; stigma branches 2-3 mm long; achenes widely obovate to widely depressed obovate, 1.2-1.6 mm long, 1-1.7 mm wide, stramineous to almost black, usually stramineous orange; summit of achene arched; shoulders of achene at edge of style base very seldom confluent with achene margin, usually forming a noticeable angle; style base shallowly to very shallowly triangular, 0.3-0.5 mm high, 0.6-1.4 mm broad, brownish to greyish; achene surface transversely rugulose, sometimes faintly so, occasionally appearing pebbly; epidermal cells of achene elliptic, indistinct. Flowering and Fruiting: This subspecies of R. nervosa shows a distinct periodicity in flowering. In Mexico, mature specimens have been collected from April to October, but the plants flower predominantly in June, July and August. In Central America, flowering and fruiting takes place from February to October although most mature specimens are found between May and August. In the Greater Antilles, mature plants develop between April and December. In Colombia there seem to be two peaks of flowering, one from November to February and another from June to August. In Venezuela, Trinidad and the Guianas, the pattern is not as clear, although most plants mature between March and August. In southern Brazil, Paraguay and Argentina, it occurs predominantly between October and February. It should be remembered that in any one locality, the flowering and fruiting period is likely to be more restricted than the range given for that region. Distinguishing Features: The species R. nervosa is unique because of its conspicuous bracts which are white above only, its pubescent leaves and bracts and its triangular style base. Rhynchospora colorata, R. latifolia and R. floridensis are virtually glabrous and have bracts that are white above and below. Rhynchospora reptans, although it does have bracts that are white above only, is stoloniferous and has a lunate style base. Rhynchospora nervosa ssp. nervosa can be distinguished from ssp. ciliata by its rhizomes, long slender culms and narrow leaves which are seldom hirsute abaxially. The distinction between the two subspecies is presented in more detail in Table VI and in Fig. 36. The latter shows how the size and shape of the bracts differ in many specimens of the two subspecies. The bracts of ssp. nervosa have a larger proportion white, although they are narrower.
Distribution and Ecology - Habitat: This subspecies is typically found in savannas. In Trinidad, the dominants in one savanna where it is found (Thomas 2466) are Byrsonima crassifolia Kunth, Curatella americana L. and Trachypogon ligularis Nees. The soil at this site (St. Joseph’s savanna) had a pH of 5. In Amazonian Venezuela, the dominant species where subspecies nervosa is found include Trachypogon sp., Rhynchospora barbata (Vahl) Kunth, R. rugosa (Vahl) Gale, Fimbristylis sp. and Vellozia tubiflora Kunth. Distribution: (Fig. 38) This subspecies is widespread, occurring from central Mexico and the Greater Antilles through Central America and South America to northeastern Argentina.
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Discussion
Schoenus tenuifolius Kunth in Humboldt, Bonpland and Kunth, Nov. Gen. Spec. 1: 228. 1816. Syntypes: Venezuela, T. F. Amazonas, near Cumaná, and near Mayapures, Humboldt & Bonpland s.n. (P, not seen). Dichromena tenuifolia (Kunth) Sprengel, Syst. 1: 203. 1824. Rhynchospora nervosa var. tenuifolia (Kunth) Pfeiffer, Repert. Spec. Nov. Regni Veg. 38: 92. 1935, non Rhynchospora tenuifolia Grisebach, Cat. Pl. Cub. 244. 1866. Dichromena pilosa Sprengel, Syst. 1: 203. 1824, nom. illeg. Dichromena persooniana Neesin Martius, Fl. Bras. 2(1): 112. 1842. Type: Puerto Rico, Essequebo, Meyer s.n. (GOET, not seen). Rhynchospora persooniana (Nees) Grisebach, Fl. Br. West Ind. Isl. 577. 1864. Rhynchospora exigua Bõckeler, Vidensk. Meddel. Dansk Naturhist. Foren. Kjøbenhavn ser. 5, 5: 238. 1894. Type: Brazil, São Paulo, Campo Seco, Franca, Loefgren & Edwall 2079 (C, holotype, not seen; NY, microfiche of holotype). Dichromena ciliata var. vahliana Clarke in Urban, Symb. Antillanae 2: 101. 1900. Type: Cuba, Jul s.a., C. Wright 3797 (BM?, holotype, not seen; GH, isotype). Pollination: I never saw this subspecies in full flower and, consequently, did not observe pollination. Because of its great similarity to R. nervosa ssp. ciliata, which is visited by insects, it too must be entomophilous. Nomenclature: Although Persoon’s Dichromena [Dichroma] ciliata (1805) is the oldest name available for either subspecies of R. nervosa, it cannot be used as the basis of a combination under Rhynchospora because of Kükenthal’s R. ciliata published in 1921; Kükenthal based this name on the illegitimate later homonym, D. ciliata Vahl (as stated in article 72 of the International Code of Botanical Nomenclature (Stafleu et al., 1978), R. ciliata Kük. is to be treated as a new name). Since Vahl’s D. ciliata is an illegitimate later homonym, the oldest available name is Vahl’s R. nervosa. If Dichromena is recognized as a distinct genus, however, the earliest name is Persoon’s D. ciliata. This causes nomenclatural changes which are discussed under the nomenclature section for ssp. ciliata. Discussion: Although many specimens may exhibit the tendencies listed above, a large number are intermediate and cannot be easily assigned to either subspecies. A scatter diagram (Fig. 36) of the portion of the bract that is white versus the width of the basal bract (these two criteria were chosen in an effort to find a way to identify specimens collected without bases) shows that although most specimens could be categorized on the basis of these two criteria, there are enough intermediate specimens to form a continuum (the specimens used in Fig. 36 identified to subspecies on the basis of all available characters and then measured for the scatter diagram). Because the two taxa intergrade morphologically, yet are often distinct both geographically and morphologically, I choose to recognize them as two subspecies of R. nervosa. The weediness of R. nervosa ssp. ciliata gives rise to two possible explanations as to why the two taxa are imperfectly separated. One explanation, of course, is that the two taxa have never been completely isolated genetically and show the results of this in their intergradation; I was not able to grow ssp. nervosa in the greenhouse to determine if rhizome growth might be affected by environmental factors. It is also possible that the weedy R. nervosa ssp. ciliata followed man’s disturbance into areas previously occupied only by ssp. nervosa. This breakdown in ecological or geographical isolation would have resulted in hybridization and the blurring of taxonomic distinction.