A taxonomic evaluation of the Thesium confine species complex ( Santalaceae )

Background: Thesium L. is the largest genus in the family Santalaceae (sandalwood family). The last taxonomic revision of the southern African species dates back to 1925. An urgent revision of the genus is required as indicated in a recent national biosystematics research strategy for South Africa.Objectives: To revise the Thesium confine species complex (Thesium durum, T. confine and Thesium spartioides) and to update the typification, nomenclature, descriptions and distribution ranges.Method: Morphological characters were studied using own field collections as well as herbarium specimens from K, NU, PRE and PRU and images on Global Plants (JSTOR). Distribution ranges of the studied species were updated.Results: An examination of morphological characters revealed great similarity between T. spartioides and T. confine. Both species have an herbaceous habit, decumbent growth form, terminal (or less frequently axillary) inflorescences and a sympatric northern distribution in South Africa. Thesium durum differs from T. spartioides and T. confine in having a shrubby habit, erect growth form and axillary inflorescences and occurs in the southern part of South Africa.Conclusion: Thesium spartioides is synonymised under T. confine, and T. durum is retained as a separate species.

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 Background: Thesium L. is the largest genus in the family Santalaceae (sandalwood family).The last taxonomic revision of the southern African species dates back to 1925.An urgent revision of the genus is required as indicated in a recent national biosystematics research strategy for South Africa.
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)

Research method and materials
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.

Results and discussion
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 (1915Hill ( , 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.

FIGURE 1 :
FIGURE 1: Morphological characters of Thesium confine (a, d, e) and T. durum (b, c, f) showing (a) the scale-like leaves found in both species; (b) involucral bracts that are invariably present in T. durum; (c) longitudinal section of a flower showing the dense hairs on the perianth lobes, stigma opposite the anthers and the twisted placental column; the habit of (d) T. confine and typical herbarium specimens of (e) T. confine and (f) T. durum.
(Victor, Smith & Van Wyk 2015).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.