Apuleia rigida Gaertn., 1791

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 : 273-274

Apuleia rigida Gaertn. and Stobaea gaertneri DC.

The name Apuleia rigida , and its replacement name Stobaea gaertneri Candolle (1838: 522) , are presently listed as synonyms of B. rigida in prominent online nomenclator databases (Catalogue of Life 2023, GBIF 2023, POWO 2023, World Flora Online 2023, World Plants 2023). No published taxonomic treatment of Asteraceae subtribe Gorteriinae has considered either name to be synonymous with B. rigida (or a combination in a different genus). When publishing the name Apuleia rigida, Gaertner (1791) illustrated and described the involucre, florets and fruit, but not the vegetative parts of the plant, and cited (p. 439) an unpublished name, “ Gorteria rigida . BANKS. hort. sicc. ” Notably, Gaertner (1791: tab. 171 f. 1) depicted a ray floret with a conspicuous, long corolla limb and all involucral bracts are drawn as plane for A. rigida . Given that the capitula of B. rigida are exclusively discoid and the involucral bracts are strongly convex with revolute margins, it can be concluded with certainty that A. rigida is not conspecific with B. rigida . Therefore, the name Apuleia rigida and its replacement name Stobaea gaertneri are excluded from the synonymy of B. rigida .

Sir Joseph Banks allowed Gaertner to sample fruits extensively from specimens in his personal herbarium and from plants growing at Kew, and further loaned or gifted Gaertner specimens ( Deleuze 1804, Stafleu 1969). A search for original material of A. rigida has failed to trace any potential specimens in Herbs. BM, K, and TUB, the most likely repositories for Banks-sourced material that Gaertner examined ( Stafleu & Cowan 1986). Gaertner’s illustrations of A. rigida are part of the original material for the name, but conclusive identification from the drawings is problematic because of inconsistencies with the morphology of potential species. In the interim, we leave the name Apuleia rigida untypified and unresolved while the search for a physical specimen continues.

Morphological variation in Berkheya rigida

Roessler (1959) noted the morphological variability of the material that he included within his circumscription of B. rigida . He commented that recognition of different “ Rassen ” (races) based on differences in size, especially in growth habit, leaf lobe width, involucral bract length, and the nature of the indumentum, may be warranted upon further investigation. Observations on this variability are provided here.

The most common and geographically widespread form of B. rigida has an erect habit, with young plants often initially monocaulous and unbranched ( Fig. 1A ), and with age develops a woody stem ( Fig. 1B ) and subshrubby habit with branches arising from the lower portion of the stems. In coastal populations on the West Coast and Cape Peninsula, plants may be stunted and compacted forming cushions or caespitose mats ( Figs 1F,G ), or the stems are decumbent and form sprawling mats ( Fig. 1I ). The dimensions of the leaf overall, the apical and lateral lobes and spines, and indumentum on the adaxial surface are highly variable. In the common form, the mature leaves are discolorous, glabrous to subtomentose above and densely tomentose below ( Fig. 1C ). The leaves of dwarfed plants are reduced in size and the adaxial surface may be either shiny-waxy or densely tomentose ( Fig. 1J ). All forms produce terminal and axillary cymose compound inflorescences at the upper nodes of flowering shoots, which are usually more compacted and comprise fewer capitula in the coastal forms ( Figs 1D,E,H,K ). The capitulum and floral parts tend to be smaller, and no distinctive micromorphological characters have been noted, in the coastal forms. The specimen here designated the lectotype for Stobaea rigida is compatible with the sprawling coastal form, based on the curved, relatively slender stem, the relatively small capitula (approximately 1.5–2.0 cm diam.), the apparent premature deterioration of leaves at the lower nodes on the specimen, and also Thunberg’s (1823: 621) description of the stems as “ flexuoso-erectus ”.

In the case of B. rigida , the continuous nature of the variation in growth habit, leaf and capitulum morphology, and quantitative foliar, floral and involucral characters, and absence of notable variation in micromorphological characters, precludes delimitation of discrete groupings of populations. Vascular plants growing in extreme coastal environments typically exhibit a variety of ecophysiological responses and phenotypic adaptations ( Ungar 1991). A prostrate or decumbent growth habit is a common phenotypic adaptation to growth in an exposed windswept or coastal environment or at high elevations, in a species that otherwise has an upright growth habit in more sheltered or less stressful environments or at low elevations ( Ford 1975). The variation in growth habit may represent phenotypic plasticity or, in some instances, the phenotypic differences have a genetic basis (e.g., James et al. 2021). The variation in B. rigida could be considered as intergrading ecotypes within the overall range of phenotypic variability in a single taxon and the coastal morphs might represent early stages in adaptive radiation. A future study could examine genetic diversity in the regulation of growth habit among populations of B. rigida by targeting specific growth habit-associated genes.

Berkheya rigida might be mistaken for certain elements with shortly radiate capitula that Roessler (1959) included within his concepts of B. carlinifolia and B. heterophylla var. radiata . These elements are morphologically distinct from B. rigida and hence are not considered to be radiate ‘morphs’ of B. rigida . A detailed comparison of B. rigida with morphologically similar taxa was beyond the scope of the present study and will be addressed where currently unnamed elements of the two aforementioned taxa are formally named.

Figures

FIGURE 1. Macromorphology of Berkheya rigida. A,F,G,I. Growth habit. B. Stem base. C,J. Leaves. D,E,H,K. Compound inflorescence and capitula. Localities (all in Western Cape province): Swellendam (A,C,E); Riversdale (B,D); Cape Columbine (F–H); Scarborough, Cape Peninsula (I,K); Buffels Bay, Cape Peninsula (J). Photographs by N.A. Helme (I,K), P.O. Karis (B,F–H,J) and R.J. McKenzie (A,C–E).