Thesium L. is the largest member of the family Santalaceae (sensu APG IV 2016 and Thesiaceae sensu Nickrent et al. 2010; sandalwood family), tribe Thesieae (Germishuizen 2000; Jordaan & Burgoyne 2000), and currently includes 305 ‘accepted’ species concepts (The Plant List 2017). The genus is distributed across temperate and tropical regions of both hemispheres but its centre of diversity is in southern Africa. It is a genus of hemiparasitic shrubs or subshrubs with reduced or scale-like leaves, and nut-like fruits (Forest & Manning 2013; Leistner 2000; Nickrent & García 2015). The southern African species are currently divided among four sections (according to Hill 1925): (1) Sect. Imberbia A.W.Hill, (2) Sect. Barbatae A.W.Hill, (3) Sect. Penicillatae A.W.Hill and (4) Sect. Annulatae A.W.Hill or two subgenera (according to Hendrych 1972): (1) Subgen. Thesium and (2) Subgen. Frisea (Rchb. ex Endl.) Peterm. Hill’s study focussed only on the southern African species. As the type of the genus (Thesium alpinum L.) is of Eurasian origin (included in De Candolle’s Sect. Euthesium [=Thesium]) (De Candolle 1857; Hill 1915; Moore, Verboom & Forest 2010), Hill excluded this section from his classification system and created Sect. Imberbia to accommodate the African species that initially belonged to the type section.

Sect. Barbatae is characterised by perianth segments with a dense apical beard (Figure 1c) and anthers attached to the segments or lobes by hairs. Thesium confine Sond., T. durum Hilliard & B.L.Burtt and T. spartioides A.W.Hill (hereafter referred to as T. confine species complex) form part of the section. These three species occur in the summer rainfall region in South Africa and share a unique combination of morphological characters, which include monotelic, spike-like inflorescences, scale-like leaves, anthers inserted in the tube, perennial habit and placental column twisted (Figure 1), and are often misidentified in the herbarium.

The most recent taxonomic revision for the southern African species dates back to 1925 (Hill 1925) and included only T. confine (Sonder 1857) and T. spartioides (Hill 1915) as T. durum (Hilliard & Burtt 1983) was only described later. Thesium confine was distinguished from the Cape species, Thesium capituliflorum Sond. (indicated by ‘proximum’), mainly by having scale-like leaves and 4–6 flowers in spike-like inflorescences, while the latter species has leaves at the bottom and scales towards the top, and 3–4 flowers in capitellate inflorescences (Sonder 1857). A single collection of T. confine was cited by Sonder (1857) in the protologue. When Hill (1915) described T. spartioides, he distinguished it from T. confine based on the slender, rush-like stems, shorter inflorescences and longer styles (stout stems, longer inflorescences and shorter styles in T. spartioides). Hill (1915) cited only a single collection in the protologue of T. spartioides. In Hill’s (1925) revision, T. confine and T. spartioides were distinguished from one another, respectively, in the key based on slender versus stout stems, ascending versus flexuous or prostrate habit and brown versus grey colour in dried plants. The measurements for the styles given in the descriptions are the same for both species (1 mm in length), providing conflicting information in the key. Three collections were listed for T. confine and a single collection for T. spartioides. In the protologue of T. durum, Hilliard and Burtt (1983) distinguished between T. durum and Thesium flexuosum A.DC. on the basis of dense, lateral spike-like inflorescences in T. durum and elongate spikes in T. flexuosum and listed two collections. According to the limited number of specimens given in all of the treatments until 1983, these species are allopatric in their distribution. The latest checklist of Thesium (Winter 2006), however, indicates that T. confine occurs in the Eastern Cape province and Lesotho, overlapping with T. spartioides, which occurs in the Eastern Cape, Free State, Gauteng, Mpumalanga and North West provinces and Lesotho. Thesium durum is partially sympatric with T. spartioides, occurring in the Free State and KwaZulu-Natal provinces and Lesotho. Although only limited material was available to the authors, they realised that these species should be formally recognised. Today, more specimens are available displaying a larger range of variation, which is not reflected in the current literature. This has led to some confusion and misidentifications in the herbarium, and as a result, species delimitations need to be re-evaluated. An urgent revision of the genus is required, indicated in A Biosystematics Research Strategy for Plant Taxonomic Research in South Africa (Victor, Smith & Van Wyk 2015). The aim of this research is to revise the T. confine species complex, contributing towards a comprehensive revision, and provide an update of the nomenclature, typification, descriptions, distribution ranges and conservation statuses.

Herbarium specimens from NU, PRE and PRU, digitised specimens from Global Plants (JSTOR 2017) and images photographed in K were studied (acronyms according to Index Herbariorum; Thiers 2011), including the type specimens. Specimens were divided into three operational taxonomic units (OTUs) based on their morphological characters (habit and growth form, stem diameter and inflorescence structure) and distribution ranges. Three specimens from each OTU were selected for measurements of the vegetative and reproductive morphology. Flowers were placed in boiling water for 2 minutes and dissected under a Zeiss Discovery V8 stereo microscope. Measurements were taken and recorded using the Zeiss stereo microscope and camera and Zeiss ZEN software (Carl Zeiss Microscopy GmbH).

Fieldwork was conducted in October 2016 and 2017 in Mpumalanga province, South Africa, to observe the plants in their natural environment. Specimens were collected and deposited in PRE. Distribution and habitat information were gathered from herbarium specimens and during the field trip.

All three species are perennials with scale-like leaves and monotelic, spike-like inflorescences. Single flowers within the inflorescence are sometimes replaced by 2- or 3-flowered cymes.

Thesium confine and T. spartioides are both herbaceous, with thin, wiry, decumbent stems up to 3 mm diam. and greyish green in colour (Figure 1d and e). Thesium durum is shrubby, with relatively thick and erect stems (3 mm – 6 mm diam.) that are dark brown in colour (Figure 1). The inflorescences are mostly terminal, but may infrequently also be axillary in T. confine and T. spartioides and are invariably axillary in T. durum. In T. confine and T. spartioides, the number of flowers ranges from five to eight (rarely nine) per inflorescence and involucral bracts are usually absent, while in T. durum the number of flowers per inflorescence ranges from one to five (rarely six) and involucral bracts are invariably present. Protologue descriptions were compared to the type and additional specimens in the listed herbaria and the variation observed in T. confine and T. spartioides are overlapping. No characters could be found to separate the two species from one another as was done by Hill (1915, 1925). Hill studied four specimens, but here 22 specimens were available which revealed more variation than was previously thought to exist. In addition to their similar morphology, T. confine and T. spartioides are sympatric, occurring in the northern parts of South Africa (in the Mpumalanga, Gauteng, North West, Free State and Eastern Cape provinces; Figure 2). We therefore propose to subsume T. spartioides in T. confine. Thesium durum has a more southerly distribution, occurring from Bethlehem to Cradock (Free State, Northern Cape, Eastern Cape and KwaZulu-Natal provinces and Lesotho). A summary of the morphological characters studied is provided in Table 1.

1. Thesium confine Sond. in Flora 40: 363 (1857); DC., Prodr. 14: 665 (1857); A.W.Hill in Fl. Cap. 5(2): 176 (1925). Type: SOUTH AFRICA, Free State: ‘Trans-Garipina, Nieuwejaarspruit, zwischen Garip und Calendonrivier, am Fuss der Witbergen’ [Nieuwejaarspruit, between Gariep River and Calendon River, at the foot of Witberg], October, Zeyher 114 (S, holo. – digital image!; K,S – digital image!, iso.).

Thesium spartioides A.W.Hill, in Bull. Misc. Inform. Kew 1915 (1): 41 (1915); Hill in Fl. Cap. 5(2): 176 (1925), syn. nov. Type: SOUTH AFRICA, Mpumalanga: ‘in collibus pr. Brug Spruit’ (on the hills near Brug Spruit), 19 November 1893, Schlechter 3754 (K, holo. – digital image!; BOL – digital image!, HBG – digital image!, NBG – digital image!; PRE [four sheets]!, S [two sheets] – digital image!, iso.).

Description: Slender perennial with slightly woody rootstock and decumbent habit, branching from rootstock, 70 mm – 280 mm high; stems wiry, 1 mm – 3 mm diam. Leaves scale-like, closely adpressed to stem, imbricate on young twigs, lanceolate, acuminate, 1.2 mm – 3.3 mm × 0.2 mm – 0.6 mm, margins ciliate. Inflorescences terminal (rarely axillary), monotelic, of 5- to 8(9)-flowered spike-like cymes (with single flowers sometimes replaced by 3-flowered monochasia), peduncles 5 mm – 38 mm long; involucral bracts usually absent; bracts ovate, acuminate, 1.4 mm – 1.5 mm × 1.0 mm – 1.1 mm, adpressed to flower; bracteoles 2, lanceolate, acute, 1.4 mm – 1.7 mm × 1.0 mm – 1.2 mm. Flowers white, tubular, subsessile, perianth tube 2.0 mm – 2.4 mm long, lobes ± 1 mm long, densely bearded with white hairs. Stamens inserted at base of the perianth tube; filaments 0.1 mm – 0.2 mm long, obscured by anthers; anthers 0.4 mm – 0.5 mm long. Style and stigma 0.5 mm – 0.7 mm long, stigma opposite middle or top of anthers. Placental column twisted. Fruit subglobose, brown, with 10 longitudinal ribbed veins, reticulate between main veins, 2 mm – 3 mm diam., shortly stiptitae, stipe 0.5 mm – 0.7 mm long. Flowering time: August to October and December to January.

Distribution and ecology: Thesium confine is endemic to northern and interior South Africa, extending from near Wolmaransstad in North West province and Spitskop in the Steelpoort area of Mpumalanga province southwards through Gauteng and Free State to Middelburg in Eastern Cape province (Figure 2). It grows in grassland in sand, clay or loam.

Diagnostic characters and relationships: Thesium confine differs from T. durum in its herbaceous versus woody habit, slender, decumbent stems up to 3 mm diam. versus thicker, erect stems (3 mm – 6 mm diam.), inflorescences terminal and sometimes axillary versus axillary only, and in usually lacking involucral bracts versus invariably with involucral bracts.

Conservation status: Thesium confine is listed as Least Concern because it is widespread and abundant (Foden & Potter 2005b).

Additional specimens seen: SOUTH AFRICA. MPUMALANGA. – 2430 (Pilgrim’s Rest): ± 2 km from Spitskop turn-off off Steelpoort and Lydenburg road, (–CB), 2 Apr. 1997, M. Jordaan 3143 (PRE). 2529 (Witbank): Doornkop 273 JS, vlei wes van Eerstekamp [marsh west of Eerstekamp], (–CB), 29 Oct. 1968, C.J. du Plessis 883 (PRE, PRU); El Shaddai, Zeekoeiwater plot 152, Emalahleni, next to the Olifants River, (–CD), 9 Oct. 2016, N. Visser & M. le Roux 195 (PRE); next to the road on the R104, 18 km east of Middelburg, (–DC), 26 Oct. 2016, N. Visser & M. le Roux 208 (PRE). 2730 (Vryheid): north of Dirkiesdorp, south-east of main road, (–AB), 13 Dec. 1995, K. Balkwill 9398 (PRE).

GAUTENG. – 2528 (Pretoria): Pretoria, (–CA), 15 Oct. 1963, H.U. Scheepans & J.C Stauffer 5300 (K); Pretoria, norden-Eingang Irene, zwischen Bahn und Wellington-Strasse [northern entrance in Irene, between Bahn and Wellington streets], (–CC), 15 Oct. 1963, H.U. Stauffer 5300 (PRE). 2627 (Potchefstroom): Leeuwkuil Research Station, (–DB), 17 Oct. 1939, J.P.H. Acocks 16200 (PRE); Vereeniging, (–DB), 1 Nov. 1911, R. Leendertz 3940 (PRE). 2628 (Johannesburg): Modderfontein, (–CB), 1 Dec. 1943, A.P.D. McClean 52808 (PRE).

NORTH WEST. – 2725 (Bloemhof): Leeuwfontein, 10 km wes van Wolmaransstad plaas van J.J. van Wyk op kalkbanken [10 km west of Wolmaransstad, farm of J.J. van Wyk on kalk bank], (–BB), 25 Mar. 1978, A.E. van Wyk 2240 (PRE, PRU); Makwassie, op pad na Wolmaransstad by Leeuwfontein uitdraaipad [Makwassie, on the road to Wolmaransstad at Leeuwfontein turn-off], (–BD), 28 Sep. 1974, A.E. van Wyk 570 (PRE).

FREE STATE. – 2725 (Bloemhof): 5 mi [8.1 km] north-east of Hoopstad, (–DD), 21 Feb. 1946, J.P.H. Acocks 12486 (PRE). 2826 (Boshoff): Boshoff, Modder River, Paardeberg, (–CA), 20 Dec. 1937, J.P.H. Acocks 8544 (PRE); Inhoek Krugerdriftdam Nature Reserve, (–CC), 6 Nov. 1974, D.B. Muller 1485 (PRE); Glen Agricultural College, (–CD), 21 Jan. 1900, J.A. van der Berg 3950 (PRE). 2926 (Bloemfontein): Bloemfontein, (–AA), Dec., unknown collector 1906 (K); Townlands, (–AA), 18 Mar. 1975, E.R. Anderson 26 (PRE). 3025 (Colesburg): Bethulie, Tussen die Riviere Wildtuin, (–AD), 14 Nov. 1970, B.R. Roberts 5520 (PRE).

EASTERN CAPE. – 3124 (Hanover): 3 mi [4.8 km] north-west of Middleburg, (–BD), 10 Mar. 1955, J.P.H. Acocks 17970 (PRE). 3125 (Steynsburg): Middleburg; Grootfontein above camp 6, (–AC), 10 Mar. 1955, J.P.H. Acocks 17971 (PRE).

2. Thesium durum Hillard & B.L.Burtt in Notes Roy. Bot. Gard. Edinburgh 41(2): 311 (1983). Type: SOUTH AFRICA, Eastern Cape: Fetcani Pass, 15 Oct. 1980, Hilliard & Burtt 13136 (E – digital image!, holo.; K – digital image!, NBG!, NU!, PRE!, S – digital image!, iso.).

Description: Erect shrub branching above, root structure unknown, 50 mm – 360 mm high; stems relatively thick, 3 mm – 6 mm diam. Leaves scale-like, closely adpressed to stem, imbricate on young twigs, lanceolate, acuminate, 1.5 mm – 2.3 mm × 0.2 mm – 0.6 mm, ciliate margins. Inflorescences axillary, monotelic, of 1- to 4-flowered spike-like cymes (with single flowers sometimes replaced by 3-flowered monochasia), peduncles 6 mm – 85 mm long; involucral bracts present; bracts ovate, acuminate, 1.4 mm – 1.5 mm × 1.2 mm – 1.6 mm, adpressed to flower; bracteoles 2, lanceolate, slightly narrower than bract, acute, 1.4 mm – 1.5 mm × 0.9 mm – 1.2 mm. Flowers white, tubular, subsessile, perianth tube 2.0 mm – 2.5 mm long, lobes ± 1 mm long, densely bearded with white hairs. Stamens inserted at base of perianth tube; filaments 0.1 mm – 0.2 mm, obscured by anthers; anthers 0.3 mm – 0.6 mm long. Style and stigma 0.3 mm – 0.5 mm long, stigma almost reaching to top of anthers. Placental column twisted. Fruit ellipsoid, brown, with 10 longitudinal ribbed veins, reticulate between main veins, 2.0 mm – 3.3 mm × 2.4 mm – 3.0 mm, stipe 0.3 mm – 0.5 mm. Flowering time: August to October.

Distribution and ecology: Thesium durum occurs in eastern South Africa and Lesotho, from Bethlehem in Free State province along the KwaZulu-Natal Drakensberg to Cradock in Eastern Cape province and Hanover in Northern Cape province (Figure 2). It grows on mountainous slopes and rocky outcrops in grasslands, in loam.

Diagnostic characters and relationships: Thesium durum differs from T. confine in its erect habit with thicker, woody branches and stems 3 mm – 6 mm diam., and axillary inflorescences with involucral bracts.

Conservation status: Thesium durum is a Least Concern species because it is widespread and abundant (Foden & Potter 2005a).

Additional specimens seen: SOUTH AFRICA. FREE STATE. – 2828 (Bethlehem): Bethlehem, Golden Gate National Park, Generaalskop, (–BC), 21 Jan. 1965, B.R. Roberts 3432 (PRE).

KWAZULU-NATAL. – 2830 (Dundee): Drakensberg, Cathkin Peak, (–AB), 1 Jul. 1950, E.E. Esterhuysen 17354 (PRE). 2929 (Underburg): Mpendhle, path from Loteni Nature Reserve to Redi on ridge, (–AD), 26 Dec. 1982, O.M. Hilliard 16104 (PRE, K); locality unknown, (–AD), 1 Oct. 1967, F.B. Wright 220 (NU); Mpendhle, Mulangane Ridge, above Carter’s Nek, (–BC), 13 Mar. 1983, O.M. Hilliard 18372 (PRE); Underberg, upper tributaries south of Mkimazi River, (–CB), 30 Nov. 1982, O.M. Hilliard & B.L. Burtt 15715 (K).

EARTERN CAPE. – 3027 (Lady Grey): Verge of road up Joubert’s Pass, ± 8 km from Lady Grey, (–CB), 25 Feb. 2007, C.L. Bredenkamp 26 (PRE); about 15 km east of Rhodes: Mavis Bank Farm, on Kloppershoek turn-off, (–DB), 9 Dec. 1999, M. Koekemoer 1559 (PRE); north-east of Rhodes, Kloppershoekspruit Valley, Mavis Bank Farm, mountain on western side of valley from house, (–DB), 9 Dec. 1999, L. Smook 10311 (PRE); Barkly East, 6 mi [9.7 km] north-west of Moshesh Ford, (–DC), 14 Nov. 1959, J.P.H. Acocks 20165 (PRE); locality unknown, (–DC), 2 Nov. 1983, O.M. Hilliard 16360 (NU); west of Rhodes, Maartinshoekspruit Valley, Maartindale Farm, (–DD), 12 Dec. 1999, L. Smook 10399 (PRE). 3028 (Matatiele): About 15 km east of Rhodes: Mavis Bank Farm; on Kloppershoek turn-off, (–CA), 8 Dec. 1999, M. Koekemoer 1524 (PRE); Glen Lynden Farm, top of Naude’s Nek Pass, (–CA), 20 Oct. 1994, S.P. Bester 3013 (PRE); north-east of Rhodes, Kloppershoekspruit Valley, Mavis Bank Farm, at house, (–CA), 7 Dec. 1999, L. Smook 10232 (PRE). 3225 (Somerset East): Cradock, 1.5 mi [2.4 km], south-east of Lexton Springs, (–CB), 3 Jan. 1959, J.P.H. Acocks 20123 (PRE).

NORTHERN CAPE. – 3124 (Hanover): Colesberg, 9 mi [14.5 km] east by south of Naauwpoort, (–BB), 30 Oct. 1953, J.P.H. Acocks 17521 (PRE).

LESOTHO. – 2927 (Maseru): Mafeteng, Ribaning stream, hillside above stream, (–AD), 22 Oct. 1946, E.E. Esterhuysen 13207 (PRE); mountain road, Leucosidea forest, (–BD), 1 Oct. 1963, M.O. Schmitz 2713 (PRE); moutain road, 5 km before Bushman Pass, (–BD), 2 Nov. 1977, M.O. Schmitz 8030 (PRE). 2928 (Marakabei): Near Aurey, 45 km east of Blue Mountain Pass on road to Mantsonyane, (–CA), 17 Nov. 1983, A.P.M. de Kruif 1172 (PRE); Orange valley, between Sehlabathebe and Thaba Tseka, (–DB), 29 Oct. 1979, M.O. Schmitz 8859 (PRE). 2929 (Underburg): Locality unknown, (–CC), 20 Nov. 1983, O.M. Hilliard & B.L. Burtt 16817 (NU); Sehlabathebe National Park, Matsa a Mafikeng, (–CC), 5 Nov. 1976, A.C. Beverly & F.K. Hoener 375 (PRE). 3028 (Ramatsaliso): Ramatseliso gate (4 km), (–BB), 1 Nov. 1979, M.O. Schmitz 8830 (PRE); locality unknown, (–CC), 13 Nov. 1983, O.M. Hilliard 16621 (NU).