Stobaea rigida Thunb., 1800

Karis, Per Ola & Mckenzie, Robert J., 2024, Second-step lectotypification of Stobaea rigida and reassessment of heterotypic synonyms of Berkheya rigida (Asteraceae, Arctotideae), Phytotaxa 641 (4), pp. 267-276 : 269-271

Stobaea rigida Thunb.

Thunberg (1800: 141) published the name Stobaea rigida accompanied only by the brief diagnostic phrase “ S. foliis cordatis pinnatifidis spinosis tomentosis ”. Subsequently, Thunberg (1823: 621) provided a morphological description of S. rigida together with the following locality information: “ Crescit in campis extra urbem et supra littus maris, pone Duyvels kopp et alibi. Floret Martio, Aprili. ”

The former personal herbarium of Thunberg, containing material that he collected while resident at the Cape of Good Hope, is housed in Herb. UPS-THUNB, although specimens collected by Thunberg are found in numerous other herbaria ( Stafleu & Cowan 1986). The Herb. UPS-THUNB holdings unequivocally represent original material for the names published by Thunberg. Roessler (1959: 188) cited the type for S. rigida as “Typus specie: in Herb. Thunberg (UPS)”. Although Roessler did not cite a collection number or specific herbarium sheet number in Herb. UPS, under ICN Art. 7.11, in using the term ‘typus’, Roessler (1959) effectively designated a lectotype for S. rigida . A single sheet in Herb. UPS-THUNB (UPS-THUNB 18601; Fig. 2 ) is annotated in Thunberg’s handwriting in the lower-right corner with the name “ Stobaea rigida ”. The sheet is annotated on the back with the locality information “ in arvis extra Knocken supra littus in Cap. B. Spei ” [Caput bonae spei]. This sheet is interpreted to be original material for Stobaea rigida and the lectotype for that name as designated by Roessler (1959).

Three fertile shoots are mounted on the sheet UPS-THUNB 18601. Each shoot is unbranched, ranging in length from ca. 23 cm (left specimen) to ca. 29 cm (central specimen), and has a terminal capitulum and one to several capitula on short, lateral, axillary branches at the upper nodes. The underground plant parts are not represented in the specimens and were not described by Thunberg (1800, 1823) in his morphological description of S. rigida . In the central specimen, leaves are absent from the lower nodes and only incomplete leaf remnants are present at the middle nodes on the stem, whereas leaves are present at all nodes of the left and right specimens. The upper leaves are morphologically similar in the three specimens, i.e., sessile, pinnatifid, the lateral lobes longer than the undivided portion of the lamina along the midrib, similar in length and width of the lateral lobes, and the apex of all lamina lobes is acute and drawn into a conspicuous spine. In the three specimens, the lamina is discolorous, with dense appressed tomentum clothing the adaxial surface, but the abaxial surface is more densely tomentose in the lateral specimens.

Crucially, the central specimen differs in capitulum type and medial involucral bract morphology from the lateral specimens on the sheet UPS-THUNB 18601 ( Fig. 3 ). The lateral specimens have radiate capitula, containing peripheral ray florets with a short limb, whereas the central specimen has discoid capitula that lack ray florets. The medial involucral bracts are notably longer in the lateral specimens (ca. 8–12 mm in the lateral specimens vs ca. 4–8 mm in the central specimen). Accordingly, the medial bract shape differs (lanceolate in the lateral specimens vs ovate-lanceolate in the central specimen). However, in all three specimens, the bract margins are indicated to be revolute (conferring a convex shape to the bract lamina) and the medial involucral bract width is similar (ca. 2–4 mm).

Harvey (1865) is the only previous author to have noted the heterogeneity of the capitula in the original material for S. rigida , but he expressed uncertainty as to its significance. Harvey (1865: 499) stated, “I have only seen rays on one sp. (Hb. Th.); are they generally present? They scarcely exceed the disc flowers in length.” Thunberg (1800, 1823) did not mention whether the capitula of S. rigida were discoid or radiate. Lessing (1832: 68) described the capitula of S. rigida as “ breviter radiata ” and made no reference to discoid capitula in his concept of S. rigida . While Roessler (1959) noted the general morphological variability of B. rigida , he described the capitula of the species as discoid and made no comment that the original material for S. rigida was in any way heteromorphic.

The presence of discoid capitula with convex, ovate-lanceolate medial involucral bracts are consistent characters in B. rigida . All other characters accessible on the central specimen on the sheet UPS-THUNB 18601 (i.e., stem, leaf and other involucral bract characters) are congruous with the range of morphological variation in B. rigida as circumscribed by Roessler (1959). In contrast, radiate capitula and exclusively lanceolate medial involucral bracts are incompatible with B. rigida . These features, together with all other accessible morphological characters for the left and right specimens on the sheet UPS-THUNB 18601, especially the discolorous pinnatifid leaves with the adaxial surface glabrous and non-spinescent and the abaxial surface covered with dense woolly indumentum, are compatible with certain elements included by Roessler (1959) within a broadly circumscribed, polymorphic Berkheya heterophylla var. radiata ( Candolle 1838: 517) Roessler (1959: 193) .

Based on the foregoing interpretation, a second-step lectotypification (under ICN Art. 9.17) is required to fix the application of the name Stobaea rigida and the combination Berkheya rigida . When a type comprises elements that belong to more than one taxon (e.g., a gathering of mixed material), the name must remain attached to the specimen (as defined in ICN Art. 8.2) that corresponds most nearly with the original description or diagnosis (ICN Art. 9.14). In addition, in the lectotypification process, all aspects of the protologue and which of the specimens correspond most closely with the protologue must be considered (ICN Art. 9.14).

The three specimens on the sheet UPS-THUNB 18601 are equally concordant with Thunberg’s (1800) original diagnosis for S. rigida and with Thunberg’s (1823) more detailed morphological description of S. rigida . Notably, Thunberg (1800, 1823) did not specify the capitulum type for S. rigida . However, designation of either of the left or right specimens of UPS-THUNB 18601 as the second-step lectotype would conflict with the predominant usage of the name Berkheya rigida for the last century, and require the publication of a new name for the species currently known as B. rigida . Therefore, we designate the central specimen of the sheet UPS-THUNB 18601 as the second-step lectotype for Stobaea rigida , consistent with the predominant usage of the combination Berkheya rigida .

Figures

FIGURE 2. Original material for Stobaea rigida Thunb. (UPS-THUNB 18601). Photograph by R.J. McKenzie.

FIGURE 3. Capitula on original material for Stobaea rigida Thunb. (UPS-THUNB 18601). A. Terminal capitula on middle specimen. Scale bar = 1 cm. B. Radiate capitulum on right specimen.Arrows indicate individual ray florets. Photographs by R.J. McKenzie.