Philadelphus

Shrubs. Stems erect, ascending, arching, or spreading, decussately branched. Bark tight or exfoliating in grayish, brown, or reddish brown sheets. Branches erect, ascending, or spreading, often arching; twigs glabrous or with simple trichomes. Leaves winter- or drought-deciduous, opposite; petiole present; blade ovate, elliptic-ovate, elliptic, suborbiculate, lanceolate, or linear-lanceolate, herbaceous, subcoriaceous, or coriaceous, margins entire or serrulate to serrate, often irregularly and variably so, plane or revolute; venation acrodromous, secondarily and distally pinnate. Inflorescences terminal, sometimes appearing axillary when 1-flowered, cymes, cymose racemes, or cymose panicles, or flowers solitary, 1–49-flowered; peduncle present. Pedicels present. Flowers bisexual; perianth and androecium perigynous to epigynous; hypanthium completely adnate to ovary, turbinate, obconic, or hemispheric, weakly or strongly 4- or 8-ribbed in fruit; sepals usually persistent, 4, spreading or reflexed, deltate to triangular-acuminate, villous, strigose, or glabrous; petals 4 (or 8+ in some horticultural forms), imbricate, spreading to ascending, white to cream colored, rarely purple-maculate, drying yellowish, oblong-obovate, obovate, or orbiculate, base sessile and tapered, or minutely clawed, surfaces glabrous [rarely hairy]; stamens (11–)13–90; filaments distinct or irregularly connate into groups proximally, dorsiventrally flattened proximally, gradually or abruptly tapered from base to apex, apex not 2-lobed, although sometimes slightly notched; anthers depressed-ovate or transversely oblong; pistil 4-carpellate, ovary inferior to 1/2 inferior, 4-locular; placentation axile proximally, parietal distally; styles persistent, 1 or 4, connate proximally to completely; stigmas 4. Capsules turbinate, obconic to obovoid, hemispheric, subglobose, or oblong-ovoid, coriaceous, persistent and gradually deteriorating, dehiscence loculicidal. Seeds 10+ per locule, rusty brown, fusiform, sometimes caudate. x = 13.

Distribution

w, se North America, Mexico, Central America, Europe, Asia.

Discussion

Species ca. 25 (9 in the flora).

Philadelphus has a relictual distribution in western and southeastern North America, Mexico, and Central America (from southwestern Canada south in the western cordillera to Panama); southern Europe (perhaps only by human introduction); the Caucasus; and eastern Asia. It is naturalized elsewhere, including most temperate areas of the western and eastern hemispheres, and in Hawaii, where P. karwinskianus Koehne is invasive.

Hu S. Y. (1954–1956) relied on vestiture of the leaves, twigs, pedicels, flowers, and fruits to distinguish species in Philadelphus. She extended and expanded the traditional use of such characters by C. D. Beadle (1902), P. A. Rydberg (1905), and C. L. Hitchcock (1943). However, with more specimens now available, it is clear that these characters are variable and poorly correlated with one another. Basing taxa on permutations of such characters often has resulted in sympatric taxa that lack geographic and/or ecologic coherence. Herein, the recognition of biologically meaningful taxonomic units is attempted; this means that many previously named taxa are being combined in general agreement with recent, more local or regional, assessments in North America, which generally recognize fewer taxa (C. L. Hitchcock et al. 1955–1969, vol. 3; W. C. Martin and C. R. Hutchins 1980, vol. 1; C. F. Quibell 1993; N. H. Holmgren and P. K. Holmgren 1997; C. K. Frazier 1999; B. L. Turner 2006).

Hu S. Y. (1954–1956) recognized four subgenera in Philadelphus: Deutzioides S. Y. Hu, Gemmatus (Koehne) S. Y. Hu, Macrothyrsus S. Y. Hu, and Philadelphus (as Euphiladelphus). A preliminary phylogenetic study of Philadelphus based on ITS sequences provided cladistic support for three of the four subgenera (with the reassignment of P. microphyllus from subg. Philadelphus to subg. Gemmatus) and no support for recognition of subg. Macrothyrsus (A. E. Weakley 2002). Guo Y. L. (2013) conducted a more detailed phylogenetic study of Carpenteria and Philadelphus utilizing both nuclear and chloroplast markers and found three lineages: two species sampled from subg. Deutzioides (P. mearnsii and P. texensis var. texensis); Carpenteria; and the remainder of Philadelphus (32 accessions, including P. hirsutus, which had been previously considered to be a component of subg. Deutzioides). These results suggest the potential inclusion of Carpenteria in Philadelphus and also that characters (such as buds exposed versus in nodal pouches) that have been used to distinguish subg. Deutzioides and subg. Macrothyrsus from subg. Philadelphus may be plesiomorphic. In this treatment, species one through four belong to subg. Deutzioides, the basal clade in Philadelphus, which is restricted to southeastern North America and southwestern North America into northern Mexico; species five (P. microphyllus) belongs to subg. Gemmatus, which occurs from southwestern United States south to Panama; and species six through nine belong to subg. Philadelphus, of eastern North America, northwestern North America, southern Europe, the Caucasus, and eastern Asia.

Within each of these three subgeneric clades, the morphology of Philadelphus is very conservative; relatively few morphological characters are useful in distinguishing taxa in each subgenus. Additionally, W. Bangham (1929) studied the chromosomes of about 40 taxa and found no variation in number and complete compatibility in all hybrids that he studied; E. K. Janaki Ammal (1951) reported some pairing irregularities in hybrids.

A number of horticultural forms have been developed, are planted in temperate areas, and may be found as local escapes. Given the morphological variability of Philadelphus species and the uncertain origins of some of these plants, the horticultural forms are difficult to deal with by conventional taxonomic means. An example is P. ×virginalis Rehder, which is alleged to be a hybrid of P. ×lemoinei hort. (itself alleged to be a hybrid of the Old World P. coronarius and southwestern North American P. microphyllus) and P. ×nivalis Jacques (itself possibly a hybrid of P. coronarius and eastern North American P. pubescens); C. A. Stace (2010b) considered this to be the most widely cultivated, persistent, and presumably established Philadelphus taxon in the British Isles. Philadelphus ×virginalis was reported as escaping locally in Lenawee County, Michigan, by E. G. Voss and A. A. Reznicek (2012). Neither reliable identification of cultivars of this kind nor determination of their genetic origins are currently possible. Some idea of the cultivated entities can be gained from A. J. Rehder (1940), Hu S. Y. (1954–1956), D. Wright (1980), G. Krüssmann (1984–1986, vol. 2), and M. H. A. Hoffman (1996).

Philadelphus tomentosus Royle, a native of the Himalayan region, has been reported as naturalized in the flora area (M. A. Vincent and A. W. Cusick 1998) based on specimens from Allen and Paulding counties, Ohio. Because of the close similarity of all Philadelphus taxa, those specimens cannot be definitely identified as P. tomentosus. They appear to be P. pubescens, a native North American species widely cultivated and naturalized in eastern North America.

In some species of Philadelphus, the axillary buds are enclosed in pouches of cortical and epidermal tissue at the petiole bases. The petiole abscises distal to the pouch so that the bud is hidden and proximal to the leaf scar, only becoming visible as it expands. In other species, the buds are exposed, and the petiole abscises proximal to the bud; these buds are distal to the leaf scars, which is the condition normally seen in most plants. Whether the axillary buds are exposed (species one to four) or hidden in pouches (species five to nine) is best observed at nodes of mature leaves on vigorous long shoots and may not be apparent at nodes of young leaves or on short shoots.

Selected References

Beadle, C. D. 1902. Studies in Philadelphus. Biltmore Bot. Stud. 1: 159–161.

Frazier, C. K. 1999. A taxonomic study of Philadelphus (Hydrangeaceae) as it occurs in New Mexico. New Mexico Bot. Newslett. 13: 1–6.

Hitchcock, C. L. 1943. The xerophyllous species of Philadelphus in southwestern North America. Madroño 7: 36–56.

Hoffman, M. H. A. 1996. Cultivar classification of Philadelphus L. (Hydrangeaceae). Acta Bot. Neerl. 45: 199–209.

Hu, S. Y. 1954–1956. A monograph of the genus Philadelphus. J. Arnold Arbor. 35: 275–333; 36: 52–109, 325–368; 37: 15–90.

Rydberg, P. A. 1905. Philadelphus. In: N. L. Britton et al., eds. 1905+. North American Flora.... 47+ vols. New York. Vol. 22, pp. 162–175.

Turner, B. L. 2006. Species of Philadelphus (Hydrangeaceae) from trans-Pecos Texas. Lundellia 9: 34–40.

Weakley, A. E. 2002. Evolutionary Relationships within the Genus Philadelphus (Hydrangeaceae): A Molecular Phylogenetic and Biogeographic Analysis. M.A. thesis. University of North Carolina.

Wright, D. 1980. Philadelphus. Plantsman 2: 104–116.

Lower Taxa

Philadelphus coronarius, Philadelphus hirsutus, Philadelphus inodorus, Philadelphus lewisii, Philadelphus mearnsii, Philadelphus microphyllus, Philadelphus pubescens, Philadelphus serpyllifolius, Philadelphus texensis

Key

1	Axillary buds hidden in pouches; styles 4, connate proximally, cylindric, lobes 0.5–8 mm.	> 2
2	Styles 2.5–5.5(–7) mm, lobes 0.5–2.5 mm; leaf blades (0.5–)0.8–3(–5.5) × (0.2–)0.3–1.3(–3.3) cm; filaments often connivent-connate in irregular clusters; w Texas westward.	Philadelphus microphyllus
2	Styles 4–16 mm, lobes 1–8 mm; leaf blades 1.5–12(–16) × 1–7(–11) cm; filaments distinct; widespread in temperate North America.	> 3
3	Inflorescences cymes or racemes, or flowers solitary, 1–3(–9)-flowered; stamens 60–90; styles 10–16 mm, lobes 0.8–1 mm wide.	Philadelphus inodorus
3	Inflorescences usually cymose racemes or panicles, sometimes flowers solitary, (1–)5–49-flowered; stamens 20–50; styles 4–10 mm, lobes 0.3–0.9 mm wide.	> 4
4	Leaf blade abaxial surfaces moderately to densely strigose, or tomentose to villous, hairs twisted, main vein axils and main veins often more densely strigose-tomentose; hypanthia and sepal abaxial surfaces usually sparsely to densely strigose or villous; bark usually gray, tight; inflorescences (1–)5–9-flowered.	Philadelphus pubescens
4	Leaf blade abaxial surfaces glabrous or sparsely strigose, hairs usually appressed-ascending, not twisted, main vein axils often moderately to densely strigose-tomentose; hypanthia and sepal abaxial surfaces glabrous or sparsely villous or strigose; bark reddish, soon exfoliating in flakes or strips; inflorescences (1–)5–49-flowered.	> 5
5	Inflorescences (1–)7–49-flowered; style lobes 1–4 mm; larger leaf blades usually less than 6 × 2.5 cm; w North America.	Philadelphus lewisii
5	Inflorescences 5–7(–9)-flowered; style lobes 3–8 mm; larger leaf blades usually greater than 6 × 2.5 cm; e North America.	Philadelphus coronarius
1	Axillary buds exposed; styles 1, clavate.	> 6
6	Styles 4–6 mm; leaf blades 2–8 cm; e United States (Arkansas, eastward).	Philadelphus hirsutus
6	Styles 1.9–3.2(–3.5) mm; leaf blades 0.5–3.3(–4.7) cm; sc, sw United States (c Texas, westward).	> 7
7	Leaf blade adaxial surfaces strigose, abaxial surfaces with coarse, appressed hairs only, without understory of coiled-crisped hairs.	> 8
8	Hairs on abaxial leaf surface longer and more dense than those of adaxial surface; leaf blades (1–)1.9–3(–4.1) × (0.4–)0.5–1.1(–1.8) cm; c Texas.	Philadelphus texensis
8	Hairs on adaxial and abaxial leaf surfaces ± equal in length and density; leaf blades 0.5–1.7(–3) × 0.1–0.6(–1.1) cm; New Mexico, w Texas.	Philadelphus mearnsii
7	Leaf blade adaxial surfaces strigose to sericeous-strigose, abaxial surfaces with both coarse, appressed hairs and understory of coiled-crisped hairs.	> 9
9	Leaf blade adaxial surfaces with both scattered coarse, appressed hairs and many shorter, erect, often slender hairs.	Philadelphus serpyllifolius
9	Leaf blade adaxial surfaces with appressed hairs only.	> 10
10	Leaf blade adaxial surfaces with (4–)5–9 appressed hairs per mm of surface width; w Texas.	Philadelphus serpyllifolius
10	Leaf blade adaxial surfaces with 1–3 appressed hairs per mm of surface width; c Texas.	Philadelphus texensis