Thesium celatum N.E.Br.

Lombard, Natasha & Le Roux, M. Marianne, 2023, Thesium hispidifructum (Santalaceae), a New Hispidulous Species from Limpopo, South Africa and Notes on Enigmatic T. celatum, Taxonomy (Clifton, N. J.) 3 (1), pp. 95-108 : 100-103

Thesium celatum N.E.Br. ( Figure 3 ) in Man. Pl. Transvaal [Burtt Davy] 2: p. xxxii. 461 (1932).

Type: SOUTH AFRICA. Limpopo Province: Zandrivierspoort, Waterberg , January 1920, F.A. Rogers 24973 (holotype K 000431771 !) .

Descriptions: Shrub or subshrub, up to 1.2 m tall, rootstock a woody taproot; stems erect to spreading, up to 6 mm in diameter, virgate, glaucous, glabrous below and glabrous to densely verruculose above. Leaves mostly ascending, linear, 2–22(–30) × 0.1–0.75 mm, glabrous to densely verruculose on the margins and midrib or entire surface. Flowers sessile, single in bract axils, occasionally solitary but mostly arranged in 2–6-flowered determinate spike-like inflorescences. Bracts and bracteoles 3–7, leaf-like but smaller, 2–15 mm long, forming an involucre. Perianth narrowly campanulate, 3.5–5.6 mm long, no “glands” observed on outside, externally glabrous; hypanthium 1.0– 1.4 mm long; corolla lobes very narrowly triangular (almost linear), 1.2–2.5 mm long, apex slightly cucullate or sometimes somewhat pointed, with a dense apical beard and fringed margins. Stamens inserted in the top half of the hypanthium; anthers 0.4–0.8 mm long, included in- or rarely partially exerted from the hypanthium. Style 0.6–0.9 mm long, stigma often below anthers or sometimes reaching the lower half of the anthers. Placental column slightly to prominently undulate but not twisted; ovules ( 2)3( 4). Fruit globose to elliptic, 6.5–8.4 × 2.4–3.2 mm including stipe and persistent perianth, stipe (0) 0.6–2.2 mm long, persistent perianth 2.4–3.1 mm long, longitudinally and reticulately veined when dry, glabrous.

Diagnostic characters: This species is morphologically similar to T. burchellii sharing its tall (up to 1.2 m tall in T. celatum ; up to 1(?) m tall in T. burchellii ) woody and leafy habit, involucral bracts often present, flower lobes bearded at the inner apex and style length (0.6–0.9 mm long in T. celatum ; ± 1 mm long in T. burchellii ). Thesium celatum differs from T. burchellii in the virgate branching pattern, long leaves (2–22(–30) mm long) and bracts (2–15 mm long), 2–6-flowered determinate spike-like inflorescences with occasional solitary flowers, large flowers (3.5–5.6 mm long) with long lobes (1.2–2.5 mm long) and large fruit (6.5–8.4 × 2.4–3.2 mm) (versus short leaves (± 3–10 mm long) and bracts (±3.5–7.0 mm long), inflorescences with one or rarely two smaller flowers (2.6–3.6 mm long) with shorter lobes (±1.0– 1.5 mm long) and smaller fruit (4.5–6.5 × 2.4–2.9 mm) in T. burchellii ).

Note: Further useful morphological characters to consider for T. celatum include its stamens that are inserted in the top half of the hypanthium and the presence of undulate placental columns. These characters are unknown in T. burchellii .

Distribution and habitat: Thesium celatum is near endemic to the Limpopo Province in South Africa. It is known from 11 gatherings in the Waterberg area between Grootwater Nature Reserve, Naboomfontein, and Settlers Pride in Limpopo Province, and one isolated gathering in the vicinity of Brits (Nort West Province) ( Figure 4 ). It grows solitary or in clumps in sandy soils between 1200 and 1590 m a.s.l. The habitat is predominantly open grassy savanna.

Phenology and ecology: This species has been found in flower and fruit in January, June, August, September, and November. While Thesium species from this area normally flower between September and February, it is not uncommon for shrub-like species (e.g., T. procerum ) to flower throughout the majority of the year ([ 8], personal observation).

Conservation status: Thesium celatum has an extent of occurrence (EOO) of 11 560 km 2 and an area of occurrence (AOO) of 36 km 2. It is known from 13 gatherings at seven to nine locations. Several subpopulations of this species have been observed for the first time in the past two years following field expeditions. It is therefore possible that more subpopulations remain undiscovered. Six of the known gatherings (Jacobsen 2432, Popp s.n., Popp s.n., Popp s.n., Van Vuuren 263 and Willemse 10) came from protected areas. The type gathering (Rogers 24973) came from a farm where most of the natural habitat has been transformed for agricultural purposes and it is very probable that this subpopulation has been lost or are at least severely impacted. Three of the known subpopulations (Burgoyne 1746, Fourie 1512, Van Vuuren 263) are potentially threatened by habitat loss and degradation due to urban and crop developments. The population is therefore believed to be declining. The remaining gatherings come from areas that appear not to be threatened. This species qualifies for a threat status of Vulnerable (VU) under Criterion B1ab(iii,iv,v) + 2ab(iii,iv,v) [ 20].

Additional specimens examined: SOUTH AFRICA. Limpopo: Ellisras District. Farm: Voorstandfontein 622 LQ. About 36 km SE of Ellisras and 43 km NNW of Vaalwater on R 33 between these towns , 25 September 2020, S.P. Bester 15137 ( PRE0876507 ), 15139 ( PRE0876509 ). Farm Antjieshoop 266 KR, 2 km S from Jonkmansdrift shop , November 1980, E. Fourie 1512 ( PRE0617759 Pietersburg District, N of Naboomspruit, farm Kaalfontein , 25 June 1993, P.M. Burgoyne 1746 ( PRE0797871 ); Sterkrivierdam Nature Reserve , 9 January 1973, N. Jacobsen 2432 ( PRE0377518 ); Waterberg District, North of Naboomspruit, Farm Heinrichan Lephalala, central part of the farm on hills overlooking the Greater Lephalala river , 23 June 1993, P.M. Burgoyne 1652 ( PRE0801812 ); W of Moepel , 5 February 2022, M. Popp s.n. ( PRE), M. Popp s.n. ( PRE), M. Popp s.n. ( PRE); Woodstock farm, next to the Mokolo river , 2 December 2020, L. Willemse 10 ( PRE). North West: Jacksonstuin, noord liggende kloof Magaliesberg (Jackson’s Garden, N-lying gorge of the Magaliesberg) , 1 August 1957, D. van Vuuren 263 ( K, PRE0379022 ).

Figures

Figure 3. Thesium celatum. (A) Habitat. (B) Habit. (C) Woody rootstock. (D) Woody stem. (E) Sulcate herbaceous upper branches. (F,G) Variation in indumentum from nearly glabrous to densely verruculose. (H) Few-flowered spike-like inflorescence. (I) Solitary flower with involucral bracts. (J) Cross-section of a rehydrated flower showing the anthers inserted in the upper half of the hypanthium and the stigma reaching to the lower half of the anthers. (K) Dry fruit with longitudinal and reticulate veins, and a prominent stipe. (L) Fresh fruit without venation and lacking a stipe. The scale bars represent 1 mm. Photographs taken by L. Willemse (A,I), N. Lombard (F–H,J–K) and M.R. Popp ((B–E,L) via iNaturalist—https://www.inaturalist.org/observations/105570591, https://www.inaturalist.org/observations/103889320, accessed on 17 November 2022).

Figure 4. Geographical distributions of Thesium celatum (green circles) and five morphologically similar species, T. burchellii (pink crosses), T. gracilarioides (red triangles), T. megalocarpum (white pentagons), T. procerum (yellow stars), and T. whyteanum (blue squares).