Flora of North America species comparison

Genera 17, species ca. 240 (9 genera, 25 species in the flora).

A. Cronquist (1981) placed Hydrangeaceae among a group of woody families traditionally allied with Saxifragaceae. Phylogenetic studies consistently place Hydrangeaceae in the Cornales and sister to Loasaceae (A. L. Hempel et al. 1995; D. E. Soltis et al. 1995; L. Hufford et al. 2001; Hufford 2004). Within Hydrangeaceae, the western North American genera Fendlera and Jamesia form a clade (subfam. Jamesioideae L. Hufford) that is sister to the rest of the family (subfam. Hydrangeoideae Burnett) (Hufford et al.; Hufford). Subfamily Hydrangeoideae comprises two tribes: Philadelpheae de Candolle ex Duby and Hydrangeeae de Candolle. North American genera in the former are Carpenteria, Deutzia, Fendlerella, Philadelphus, and Whipplea. A molecular phylogenetic study by Y. De Smet et al. (2015) clarified relationships within Hydrangeeae, found Hydrangea to be polyphyletic, and promoted adoption of a broader concept of Hydrangea that includes the eight other genera in the tribe. The two North American genera in the tribe, Decumaria and Hydrangea, are circumscribed here in their traditional senses.

The Hydrangeaceae are well represented in the paleobotanical record dating back to the Upper Cretaceous but best represented in the Tertiary (L. Hufford 2004). Some genera are sources of popular introduced or native ornamentals, including Carpenteria, Deutzia, Hydrangea, and Philadelphus. Some ornamentals have become established outside of cultivation in the flora area. A few North American Hydrangeaceae have reputed medicinal (D. E. Moerman 1998) or toxicologic (G. E. Burrows and R. J. Tyrl 2001) properties.

Trichomes in most Hydrangeaceae consist of a long, unicellular portion, often borne on a multicellular base. The unicellular portion often bears tubercles on its surface. Sometimes instead of tubercles, it bears long extensions, making the trichome appear branched or dendritic. Such trichomes are here referred to as branched.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

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.

(Discussion copyrighted by Flora of North America; reprinted with permission.)