Hydrangea panamensis Standl., J. Wash. Acad. Sci. 17(1): 10. 1927., Standl., J. Wash. Acad. Sci. 17 (1): 10. 1927.

Samain, Marie-Stephanie, Granados Mendoza, Carolina & Martinez Salas, Esteban Manuel, 2021, On Hydrangea peruviana, an endangered species from Ecuador, and Hydrangea oerstedii, very common in Costa Rica and Panama, and seven threatened Central and South American Hydrangeas, which have been confounded with these, PhytoKeys 171, pp. 91-153: 91

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Hydrangea panamensis Standl., J. Wash. Acad. Sci. 17(1): 10. 1927.


5. Hydrangea panamensis Standl., J. Wash. Acad. Sci. 17(1): 10. 1927.   Figures 8 View Figure 8 , 9 View Figure 9


Panama. Colón, along Río Fato, 10-100 m, floral buds, H.F. Pittier 3919 (holotype: US! [00097000], isotypes: A!, BM! [BM000028808], C!, F! [V0066620F], GH! [00042780], K! [K000486137], NY! [00007170]) floral buds.


Root-climbing liana of up to 20 m high, never reaching above the lower branches of its host tree canopy (Fig. 9A View Figure 9 ), sometimes bending downwards, functionally dioecious; leaves decussate, petiole sulcate adaxially, terete abaxially, color reddish brown, densely pubescent with caducous, appressed, reddish, stellate hairs, 1-2 cm long, leaving a nearly triangular scar on the branch when leaves fall; lamina obovate to slightly elliptic, 5-12 cm long, 3-6 cm broad, base cuneate to rounded, slightly asymmetric, apex rounded with a very small acumen, leaf margin very slightly glandular-dentate, slightly revolute, venation brochidodromous, veins 4-6 pairs, adaxial leaf side with midvein and primary veins marked, primary veins join to form submarginal vein, pubescent with caducous, appressed, stellate hairs, the basal stalk reddish, the rest whitish, opaque pale green, abaxially with protruding midvein, primary veins alternately protruding and marked, secondary veins marked, regularly with concave cleavages on the junction of primary and secondary veins, opaque pale reddish brown, densely pubescent with appressed, stellate hairs, the basal stalk reddish, the rest whitish, acarodomatia very rare, 0-2 per leaf, consisting of a simple cavity in the axil of the midvein and primary veins; inflorescence axis pubescent with appressed, small, yellow-reddish, stellate hairs, shedding in older specimens, 2-10 cm long, quadrangular, with 1 decussate leaf and 1 kataphyll pair below the inflorescence, rapidly deciduous, apex of the floral axis woody, quadrangular, elongated bract scars visible, thickening at the top, 2-3 mm broad, 1-1.5 mm high in functionally female plants, 2-3 mm broad, 1 mm high in functionally male plants, inflorescence bracts cucullate, membranous, densely pubescent with reddish stellate hairs, veins palmate, bracts increasing in size, lowermost bract 1.5 cm large, 1.1 cm broad, higher bracts up to 2 cm large, 1.6 cm broad, inflorescences lateral, decussate, 1-3 pairs of inflorescences per flowering branch, flowering branch only continues growing very rapidly during inflorescence development, with up to 7 leaf pairs above the inflorescences, giving a very homogeneous aspect, inflorescence axes with basal lignified parts of inflorescences of previous years not observed, leaves or kataphylls present at the base of the inflorescence, inflorescence umbellate (Fig. 9B View Figure 9 ), buds up to 2 cm broad and 1.5 cm high before opening, in flowering stage 3-10 cm diameter, 1.5-5 cm high, with 5-8 main axes in functionally male plants, 5-8 main axes in functionally female plants, partial inflorescences racemoid, secondary and tertiary inflorescence axes with yellowish white stellate hairs, pubescence slightly decreasing towards flower insertion; enlarged marginal flowers always present (Fig. 9B View Figure 9 ), terminally placed in a raceme, sepals white, yellow or pink, fertile or reduced, when fertile stamens 8, pistils 2, fruit developing, when reduced only sepals developed, central part of the flower amorph, 0.5-0.7 cm diameter when flowering, 1.5-2 cm diameter with mature fruits, pedicel 1.2 cm long when flowering, 1.5-2 cm long with pedicel 1.5-2 cm long with mature fruits; flower pedicel of reduced flowers 0.5-2 mm long in functionally male flowers, 0.5-2 mm long in functionally female flowers, receptacle broadly triangular in functionally male flowers, semiglobose in functionally female flowers, ovary inferior, calyx lobes 4, broadly triangular, 0.2-0.3 mm long, petals 4, white, valvate, cucullate, membranous, 1.5-2 mm long, 1.5-1.8 mm broad; functionally male flowers: hypanthium 1.2-1.8 mm diameter, 0.8-1 mm high, stamens 8, well-developed, filaments 1 mm long, anthers 0.5 mm long, 0.2 mm broad, pistils 2, reduced, 0.1-0.2 mm long, stigmas not penicellate; functionally female flowers: hypanthium 1.5-1.6 mm diameter, 1-1.5 mm high, stamens 8, reduced, filaments 0.5 mm long, anthers 0.1 mm long, 0.1 mm broad, translucent, pistils 2, 1 mm long, enlarging up to 1.8 mm during fruit maturation, stigmas apically clavate and shortly penicellate; fruit a semiglobose capsule, apically with a conspicuous, slightly revolute border, 6-8 well marked ribs, dark reddish brown, 1.5 mm high, 3-3.5 mm broad above, 2 mm diameter, opening between the two pistils to release seeds, seeds not seen.


This species is known from Costa Rica and Panama (Fig. 8 View Figure 8 ).


Hydrangea panamensis   grows in tropical rainforest between 200 and 1000 m elevation. It usually occurs near water streams at lower elevations. Of all the species of the present study, this is the one that grows at the lowest elevation.


Hydrangea panamensis   has been collected with flowers and fruits between June and September.


Not to be considered a synonym of H. peruviana   from which it can be distinguished by the rounded leaf apex with a very small acumen and the very few acarodomatia (0-2/leaf) on the abaxial leaf side vs. the acute to acuminate, rarely mucronate leaf apex and the many acarodomatia (present in both axils of midvein and primary veins and those of primary and secondary veins) of H. peruviana   . Moreover, their known distribution areas are far away from each other, with H. panamensis   only growing in Central America and H. peruviana   being endemic to Ecuador.

The label of the specimen Gentry 5569B mentions that the flowers are red, whereas the label of the specimen Mori & Bolten 7674 says "summit of ovary of fertile flowers reddish-pink". It seems there is some color variation in this species in both the marginal and reduced flowers. Both flower types are generally white, and this is also the color we have observed in the field ourselves.

According to our molecular study, Hydrangea panamensis   is closely related to H. peruviana   (Granados Mendoza et al. unpublished results).

Preliminary conservation status.

Although this species has an EOO of about 50,123 km2, it is Endangered according to the IUCN categories and criteria ( IUCN 2012), with an AAO of 60 km2, as well as an extensive reduction in both EOO and AOO as a consequence of habitat destruction and fragmentation.

Additional specimens examined.

Costa Rica. Puntarenas: Golfito, Playa Cacao, Cuenca Superior de Quebrada Nazareno, 8°37'50"N, 83°11'30"W, 200 m, 11 Jun 1994, ♂, flowers, Herrera & Rivera 7152 (CR, F, K, MO); Golfito-La Gamba (km 37), gap by the waterfall - Sendero Ozelot near the Rainforest Lodge, 8°41'N, 83°13'W, 70-300 m, 20 Jun 1997, inflorescence buds, Weissenhofer W105 (CR, WU); Golfito, Bosque de los Austriacos, Fila Gamba, Avilán woodcutter way, 8°41'N, 83°13'W, 250 m, 3 Jun 1997, ♂, old flowers, Huber & Weissenhofer 747-3.6.97 (CR); Osa, al lado del camino Rincon-Bahía Drake, 2.23 km al ONO de Rancho Quemado, 8°41'35.39"N, 83°35'7.42"W, 237 m, 8 Aug 2012, ♀, fruits, Samain & Martínez 2012-063 (CR, GENT, IEB, MEXU); Cerro Anguciana, cabeceras del Río Piedras Blancas, 4.96 km al NE de Piedras Blancas, 8°48'27.49"N, 83°12'18.05"W, 676 m, 9 Aug 2012, ♀, fruits, Samain & Martínez 2012-064 (CR, GENT, IEB, MEXU).

Panama. Darién: Cana and vicinity, 2000-6500 ft, 7 Apr 1908-8 Jun 1908, old inflorescence branches, Williams 775 (NY); Coclé: between Cerro Pilón and el Valle de Antón, 700-900 m, 15 May 1967, ♀, flowers, Duke & Dwyer 13952 (MO); region north of El Valle de Anton, 1000 m, 27 Sep 1946, ♀, fruits, Allen 3712 (E, GH, F, MO, P, UPS); alrededores de El Valle de Antón, Altos de La Mesa, 1.3 km al N de la bifurcación hacia Río Indio, 8°38'38" N, 80°06'51"W, 735 m, ♂, flowers, 13 Aug 2014, Galdames et al. 7676 (PMA); Distrito La Pintada, cerca de las cabañas de ANAM, 8°40'05"N, 80°35'34"W, 800-900 m, 14 Aug 2007, ♀, fruits, Hernández et al. 528 (PMA); Panama: Canal Zone, Barro Colorado Island, E. Of Wheeler 13, 18 Aug 1970, ♀, flowers, Foster 1797 (DUKE, F, PMA, UC); Barro Colorado Island, forest 200 m E of Wheeler Trail 1300, 18 Aug 1970, ♀, fruits, Croat 11850 (AAU, DUKE, E, F, MO); pipeline road, 14 Sep 1971, ♀, fruits, Gentry 1792 (DUKE, F, GH, MO); Panamá, North of El Llano, 500-800 m, 25 Jul 1972, ♀, flowers, Gentry 5569B (MO); Cerro Campana, 45 km SW of Panama City on Inter-American Hwy, 8 Aug 1975, ♀, fruits, Mori & Bolten 7674 (AAU, MO); Distrito de Capira, Parque Nacional Altos de Campana, Sendero de Interpretación, 8°40'54"N, 79°55'40"O, ca. 750 m, 23 Aug 1990, ♀, fruits, Galdames 880 (PMA); Parque Nacional Altos de Campana, Sendero de interpretación, 1 km al este del campamento de los guardaparques de INRENARE, parcela 10-8, 8°40'N, 79°55'W, 800-900 m, 19 Aug 1993, ♂, flowers, Correa & Montenegro 9780 (PMA, UCH).