Flora of North America species comparison

Varieties 5 (4 in the flora).

Within Philadelphus microphyllus as treated here, P. A. Rydberg (1905) recognized nine species, C. L. Hitchcock (1943) one species with eight subspecies, and Hu S. Y. (1954–1956) 11 species and four varieties, based on vestiture, leaf size and shape, and floral differences. Four varieties are recognized here within the flora area, with a fifth restricted to Mexico and without the needed varietal combination.

Two characters are particularly important in distinguishing the varieties of Philadelphus microphyllus: adaxial leaf blade cuticle thickness and vestiture. Adaxial leaf blade cuticles can be thin and papillate, closely reflecting the adaxial epidermis cells (as seen at 30–40\x) or can be thick and smooth. Leaves with thin cuticles dry brown due to brownish granules developing in the epidermis; those with thick cuticles dry gray-green, olive green, or yellowish green without granules in the epidermis cells. Sometimes both types occur in a leaf in either a tight or bold mosaic pattern or the leaf blade may be papillate and brown only along its margins.

Vestiture is mostly sericeous-strigose on leaves, stems, hypanthia, and sepals. The appressed hairs can be slender, short or long (0.2–1.5 mm), appressed, loosely appressed, ascending, or erect. The larger hairs have slender bases that allow them to be strictly appressed, but in some taxa, the base (upon drying) lifts and twists the hair upward, leaving the hairs oriented in many different directions; we refer to this condition as chaotic vestiture. In more densely vestitured plants, very slender, elongate, wavy-curved hairs form an understory beneath the more or less dense sericeous-strigose vestiture.

(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.)