Sedum moniliforme I. García & Costea, 2015

García-Ruiz, Ignacio & Costea, Mihai, 2015, A new species of Sedum (Crassulaceae) from northwest Michoacan, Mexico, Phytotaxa 212 (1), pp. 80-86 : 81-85

Sedum moniliforme I. García & Costea , sp. nov., Figs. 1–2

Like Sedum longipes but differing in the moniliform rhizome-like basal stems and markedly articulate regular stems; whorled leaves, 2–3.5 mm long, 3–4 at each node; flowers 6–8 mm in diameter, with sepals that are not imbricated at the base, 1–1.2 mm wide, petals 3–5 × 1–1.5 mm, pollen grains subspheroidal, 14–17 μm long, and nectaries 1 mm long.

Type: — MÉXICO. Michoacán: Municipio Jiquilpan, Barranca La Gloria, lado oeste del “Medio Sitio” y al nor-noroeste de la localidad El Fresno, sobre paredes de rocas basálticas con orientación oeste y oeste-noroeste. Ubicación aproximada 20°01’15”N, 102°48’54”W, 1790 m, 25 November 2014, I. García et al. 8994 (holotype CIMI! Isotypes IEB! MEXU!, WLU!).

Plants saxicole, perennial, glabrous, with prostrate to ascending stems, pendent to climbing; basal stems are horizontal, rhizome-like, with short and swollen, moniliform internodes, 4–10 mm long and 3–5 mm in diameter; remaining stems are ascending, 10–20 (30) cm long with internodes markedly articulate, 10–30 mm long and 1–2 mm in diameter; all stems are green to pale chestnut brown, maculate with reddish dots or lines. Sterile shoots form lax rosettes, 0.8–12 mm in diameter, farinose on the young leaves. Roots fibrose; particularly basal stems generate brown-reddish adventitious roots at the nodes, 2–3 cm long. Leaves whorled, 3–4 at each node, obovate to sphatulate, 4–9 × 2–3.5 mm, short petiolate, somewhat clasping, spurred, apex rounded; bracts similar to leaves, opposite or 3–4 whorled. Inflorescences cymes terminal or lateral, erect or nutant, lax, 1–2(3)-flowered; pedicels 5–17 mm long and 0.8–1 mm in diameter. Flowers 5-merous, 6–8 mm in diameter; sepals oblong-oblanceolate, not adpressed in bud, 2–3.5 × 1–1.2 mm, free basally, not imbricate at the base, spurred, unequal, green, farinose; petals 3–5 × 1–1.5 mm, basally free, lanceolate-elliptical, acute and somewhat mucronate, white to pale greenish with reddish spots or lines, reflexed as the flowers age and ultimately caducous. Stamens 10; antesepalous filaments 5, 2.2–2.5 mm long; episepalous filaments 5, 2 mm long; anthers orange or yellow; pollen tricolpate, subspheroidal, 14–17 × 11–13 μm, sexine rugulate; nectaries bilobed, 1 mm long and 0.7 mm wide, reddish, with 1–3 teeth on each side. Carpels greenish to reddish, 3 mm long and 2 mm wide at the base. Fruit aggregate of 5 erect or spreading follicles. Seeds 0.5–0.6 × 0.2–0.25 mm long, ellipsoidal to obovoidal, brown-yellowish, reticulate, each seed coat cell with an umbo.

Taxonomic affinities: —Following the classification of Berger (1930: 352–485) emended by Meyrán & López (2003: 21–22), S. moniliforme belongs to Sect. Sedum , “Group” Americana , characterized by flat, broader in the upper half leaves, sterile rosettes, and white flowers. However, as suggested by Clausen (1942) and Clausen & Uhl (1943) “Group” Americana includes a heterogeneous assemblage of taxa. The latter authors transferred the American species to subgenus Gormania (Britton in Britton & Rose 1903: 29) R. T. Clausen (1942: 29) or section Cockerellia R.T. Clausen & Uhl (1943: 34–35) , while the remaining Mexican species require more study for their appropriate placement. Among these Mexican species, S. longipes is closest to S. moniliforme , both biogeographically and morphologically. In its currently accepted delimitation ( Clausen 1959), S. longipes includes two subspecies differentiated morphologically mainly by the compactness of their rosettes (less compact with divergent leaves in subsp. rosulare ) and the ratio between their sepals and petals (petals shorter or equaling sepals in subsp. rosulare versus petals longer than sepals in subsp. longipes ; Figs. 1K–L ). Sedum longipes subsp. rosulare is known only from cultivation in the States of Michoacan ( I. García, unpublished), Puebla, and Veracruz ( Clausen 1959). In contrast, S. longipes subsp. longipes is relatively common in tropical deciduous or oak-pine forest along the Trans-Mexican Volcanic Belt in the States of Mexico, Morelos, Puebla ( Clausen 1959); Chihuahua, Jalisco, Michoacán ( Cházaro & Thiede 1995), and Guanajuato ( Pérez-Calix 2008). As indicated in the diagnose, S. moniliforme and S. longipes are easily differentiated morphologically, and a complete comparison is provided in Table 1.

Clausen (1959: 201) also described from Mexico State, S. pentastamineum , which he considered “closely related” to S. longipes and possibly derived from the latter through the reduction of one stamen whorl. Sedum moniliforme differs from S. pentastamineum in the same way it does from S. longipes (e.g., moniliform/markedly articulate stems and whorled leaves); in addition, S. pentastamineum has elliptical, suborbicular or orbicular leaves; flowers have 5 stamens; sepals are nearly equal, and nectaries are obovate-subquadrate, laciniate (for a complete comparison of the three species see Table 1). Likewise, S. botterii Hemsley (1878: 10) and S. clavifolium Rose (1911: 297) , the only other species indicated by Clausen (1959) to share possible affinities with S. longipes , are easily differentiated morphologically from S. moniliforme ( Clausen 1959) . As stated by the latter author, this small group of species restricted to the Trans-Mexican Volcanic Belt is quite unique morphologically and comparisons with other species are not feasible ( Clausen 1959). Sedum longipes is the only species of this group sampled in a broader molecular phylogeny of the Sedum clade in Crassulaceae ( Carrillo-Reyes et al. 2009) . Its closest phylogenetic relationships were with S. oxypetalum Kunth (Kunth in Humboldt et al. 1823: 45), a woody species that is also endemic to the Trans-Mexican Volcanic Belt ( Clausen 1959). However, both the evolutionary relationships among the species of S. longipes complex, as well as of this group with other Mexican and American Sedum species require a molecular study with more extensive sampling.

Morphology: —In general, the leaf arrangement of Crassulaceae is alternate or opposite-decussate; only in Sedum leaves can be rarely whorled ( Thiede & Eggli 2007). In Mexico, only one species, S. rhodocarpum Rose (1911: 300) has whorled leaves but this taxon has entirely different morphological characteristics and taxonomic affinities (“Group ” Ternata ; Berger 1930; Meyrán & López 2003). The short, moniliform internodes of the basal stems and the markedly articulate internodes of the remaining stems are remarkably distinctive, unknown in other Sedum species, at least from Mexico ( Fig. 1 ). The basal moniliform stems resemble rhizomes in that they are perennial and exhibit scars of previous inflorescence stems; however, they develop aboveground and they have chlorophyll ( Figs. 1D–E ). Sedum moniliforme plants observed in the field have the ability to grow and expand not only following a pendent habit, but also lateral and vertical ( Figs. 1A–B ) through the adventitious roots formed by the rhizome-like stems and the basal nodes of the “normal” stems ( Figs. 1D–E; G–H ). The elevated ability to form adventitious roots indicates both a good anchoring capacity (if a suitable rock support is present), and an excellent potential for vegetative propagation and cultivation.

With regard to micromorphology, young sterile rosette leaves of both S. moniliforme ( Figs. 1D,F ) and S. longipes are covered with a layer of epicuticular wax that gives them a farinose appearance ( Figs. 2A–B ). Considering the relatively high degree of pollen similarity among other Sedum species in Mexico ( Pérez-Calix 2004), the pollen grains of S. moniliforme and S. longipes are quite dissimilar in size and shape, the latter being 50–60% longer and perprolate ( Figs. 2F–K ; Table 1).

and personal observations; S. pentastamineum characteristics were provided by Clausen (1959). Missing data is indicated with “—“.

Distribution, ecology and phenology: — Sedum moniliforme is currently known only from the type locality where it is represented by one population with ca. 150–200 plants. Its habitat corresponds to tropical deciduous forests with some cloud forest elements. It occurs together with other Crassulaceae and succulents ( García & Nava 1996, García et al. 1999). The species grows on rock ledges and semivertical volcanic rocks with north-northwest exposition, in places with sufficient run-off water. During the dry season (winter-spring), leaf-rosettes gradually shrink, but at the end of June, after the first rains, they begin to develop and will form inflorescences . Flowering takes place from October to mid-November. In general, the ecology of S. moniliforme is very similar to that of S. longipes subsp. longipes ( Table 1) but the latter species flowers for a much longer time, from October to February ( Table 1).

Conservation status: Based on the area of occupancy and the small population size, we assess S. moniliforme as Vulnerable (VU) ( IUCN 2001) based on criteria D. A more exhaustive survey will be undertaken in the future to search for additional populations in the area.

Etymology: —From Latin “monil” (necklace) and “formis” (shape), necklace shaped, alluding to the moniliform internodes of the basal, rhizome-like stems.

Additional specimens examined (paratypes):— MÉXICO. Michoacán: Municipio Jiquilpan: Barranca La Gloria , lado oeste del “Medio Sitio” al nor-noroeste de El Fresno, paredes de rocas basálticas con orientación oeste y oeste-noroeste, 20°01’14”N, 102°48’54”, 1830 m., 21 October 2014, I. García et al. 8960-bis ( CIMI!) ; 29 October 2014, I. García et al. 8964 ( CIMI!, IEB!, WLU!) ; 25 November 2014, I. García et al. 8994 ( CIMI!, IEB!, MEXU!, WLU!) .

Figures

FIGURE 1. Sedum moniliforme and flowers of S. longipes. A–K. Type specimen(s) prior to conservation. L. A–B. Habit. C. Flowering stems and inflorescences. D–E. Rhizome-like basal stems with moniliform internodes (arrow indicates scar left by previous year’s flowering stem). F. Sterile rosette. G–I. Flowering stems. G. General view (note the whorled leaves). H. Internode with four whorled leaves and adventitious roots. I. Stem node after the removal of leaves to show the markedly articulate internodes. J. Flower buds. K. Flower. L. Flower of S. longipes subsp. longipes (right) and subsp. rosulare (left).

FIGURE 2. Micromorphological features of Sedum moniliforme (from holotype material fixed in FAA prior to pressing/drying) and pollen comparison with S. longipes. A.Young leaves of a sterile rosette. B. Cristaloid epicuticular wax deposits on young rosette leaf. C–E. Seed morphology. C–D. Shape variation. E. Surface of testa cells. F–H. Pollen grains of S. moniliforme. I–K. Pollen grains of S. longipes. Note the marked difference in shape and length of pollen grains between the two species. Scale bars and SEM data are embedded in the images.