Flora of North America species comparison

Species ca. 400 (34, including 1 hybrid, in the flora).

A marked concentration of Cotoneaster species is found in the Himalaya and western China. The majority of species (ca. 90%) are apomictic, tetraploid (G. H. Kroon 1975; I. V. Bartish et al. 2001), and breed true when raised from seed.

Cotoneaster was long placed near Crataegus. Based on a reevaluation of flower and fruit characters, it is now believed to be more closely related to Heteromeles and Pyracantha (K. R. Robertson et al. 1991; J. R. Rohrer et al. 1991), but lacks thorns and toothed leaf margins. Recent molecular studies also distance Crataegus from Cotoneaster; as of yet, there is no consensus about the closest allies of Cotoneaster (C. S. Campbell et al. 1995, 2007; R. C. Evans and Campbell 2002). D. Potter et al. (2007) grouped Cotoneaster with Chamaemeles and Malus, but further study is needed to resolve its placement. The genera of Maleae are difficult to organize in a phylogeny, due in part to interfertility, lack of divergence, and reticulate evolution (Campbell et al. 2007).

Plants of Cotoneaster are used by apiarists, as the flowers have abundant nectar and are much loved by bees; the resulting honey is pale golden with a delicate flavor. In Asia, leaves and fruits are used in tea, and the wood for implements. Cotoneaster is used in India and Iran as the source of a sweet mannalike substance high in dextrose. The fruit of C. integerrimus Medikus has been used in the treatment of diarrhea. Many species are ornamentals, with brilliant fall foliage or colorful fruit persisting throughout the winter. The genus is bird-disseminated, mainly by American robins (Turdus migratorius Linnaeus) and cedar waxwings (Bombycilla cedrorum Vieillot) in North America. A few orange- or red-fruited species (C. franchetii, C. lacteus, C. pannosus, and C. simonsii) are weedy on the Pacific coast (J. M. Randall and J. Marinelli 1996; C. C. Bossard et al. 2000). Only C. divaricatus and the black-fruited C. lucidus are widely escaped in the interior.

More than 70 species of Cotoneaster are cultivated in North America, and additional escapes might be expected. Reports of several species in the wild appear to be unfounded. These include C. acutifolius Turczaninow and C. apiculatus Rehder & E. H. Wilson from Pennsylvania (A. F. Rhoads and W. M. Klein 1993; Rhoads and T. A. Block 2000), C. bullatus Bois from British Columbia (J. Pojar 1999), C. hupehensis Rehder & E. H. Wilson from New York (R. S. Mitchell and G. C. Tucker 1997), and C. multiflorus Bunge and C. racemiflorus (Desfontaines) K. Koch from Quebec (J. Cayouette et al. 1983). Without reference specimens it is often difficult to name adventives, given the lack of North American literature.

As with many plants of Maleae, Cotoneaster foliage is variable, yet mature leaf size, shape, texture, and luster on vigorous sterile shoots are usually diagnostic. The leaves of fertile shoots are usually smaller and less typical than those on sterile ones. Shaded plants have larger leaves than individuals in full sun. Deciduous species in flower have smaller leaves than when fruiting. A few species, such as C. gamblei, often hold leaves into January (at least on vigorous sheltered shoots) and key as evergreen shrubs, although naturalized populations in North America will eventually drop most leaves in late winter, especially where exposed. Several species are reported in the literature as semievergreen shrubs (for example, C. horizontalis, C. simonsii), but most of their leaves color and drop by midwinter where they are naturalized on the Pacific coast, so they are keyed as deciduous here. Shrubs with tomentose leaves can be sorted into evergreen and deciduous species by checking for the presence of darkened and weathered tomentum on the abaxial surfaces of overwintered leaves. Sunken (versus superficial) veins are mentioned in the keys and descriptions, always referring to the adaxial leaf surfaces. Collectors should observe this on fresh material and note it on herbarium labels. Sunken veins and wrinkling or bulging between the lateral veins can be obscured when pressing thin-leaved species. The number of veins in the descriptions represents the major lateral veins on one side of the leaf or midvein, most easily observed on the adaxial surface.

The pyrenes are usually pubescent at the apex, if exposed by the navel between the sepals on the summit of the pome. Extracted pyrenes are planoconvex or trigonous (like orange segments). Potentially useful taxonomic characters such as pyrene size, outline, and surface texture are not described here but warrant investigation. The style remnant is usually visible as a small peg or scar on the distal half of the inner surface or central keel of the pyrene; its location is easy to assess if fresh material is available. At least five pomes should be opened to determine the number of pyrenes present.

Identification of most naturalized species requires fruit characters and mature foliage; unfortunately, floral characters are also necessary to separate some species pairs. Floral and fruit dimensions are from fresh material and will be smaller on dried specimens. Floral diameter is provided throughout in subg. Chaenopetalum (Koehne) G. Klotz (species 1–16), which has rotate blossoms with spreading petals. For subg. Cotoneaster (species 17–34), the cupulate flowers are described by their floral length, including hypanthium and the erect petals.

Cotoneaster is properly treated as a masculine name (Yu T. T. 1954), although horticultural works still can be found with feminine endings applied to species epithets.

Key to Groups of Cotoneaster

Key to Species by Group

Group A. Shrubs, prostrate or long-trailing, rarely a few shoots to 30 cm tall; leaves deciduous or persistent, not tomentose.

Group B. Leaves deciduous, abaxial surfaces tomentose.

Group C. Shrubs or trees, erect; leaves persistent, abaxial surfaces tomentose.

Group D. Stems erect; leaves persistent, glabrous or sparsely hairy abaxially.

Group E. Stems erect; leaves deciduous, glabrous abaxially or hairy, not densely tomentose; pomes black or purple-black.

Group F. Leaves deciduous, abaxial surfaces glabrous or hairy, not densely tomentose; pomes orange or red, dark red, or red-purple.

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

Genera 88, species ca. 3000 (68 genera, 680 species, including 22 hybrids, in the flora).

Three subfamilies and 16 tribes are recognized for the family with representatives of all tribes found in the flora area.

Rosaceae grow most commonly in north-temperate regions and are more or less absent from hot deserts and high-rainfall, low-altitude tropics. The family is large and diverse, characterized by radially symmetric flowers with a fundamentally saucer-shaped hypanthium and peripheral calyx, corolla, androecium, and, usually, superior gynoecium. Considerable variation occurs in important details of flower and fruit, this enhanced by a very adaptable hypanthium, which is discussed in more detail below.

Rosaceous inflorescences vary in the number of flowers from one to about 500. A great variation occurs also in inflorescence form, though a pattern is perceived when they are viewed as reduction series from a terminal, determinate panicle that is fundamentally bracteate, thus generating a range of panicles, racemes, corymbs, cymes, solitary flowers, or other forms. In this treatment, appendages on the inflorescence are distinguished by the terms bract and bracteole. Bract is used for the larger, laminate, usually chlorophyllous leaf homologues that subtend axes and may be indistinguishable from foliage leaves. Bracteoles are scalelike, often membranous, often caducous, and of uncertain homology, in part due to not being restricted to axis-subtending positions.

The floral architecture in Rosaceae is radially symmetric around a disc-shaped to urceolate hypanthium. Flowers normally have a four- or five-merous corolla and calyx; great variation is found in the numbers of stamens and carpels. Pollination is usually entomophilous, the flowers having normally green sepals and showy, often more or less clawed, usually white, yellow, or pink, less commonly red or green, petals. The flowers in some genera are relatively small and anemophilous and may lack one or two of the principal whorls. An unusual feature of some rosaceous flowers is the torus, a pad of receptacular, usually spongy, tissue, sometimes relatively large, in the center of the hypanthium that, when present, bears the gynoecium. Another unusual feature of some genera is an epicalyx that comprises a ring of sepaloid bractlets, usually of the same number as sepals, which is located on the hypanthium proximal to the calyx.

Fruit types are particularly significant both in rosaceous identification, taxonomy, and diversity, as well as for successful dispersal. Fruit in Rosaceae may be either dry, then dehiscent or not, or succulent and indehiscent; it sometimes involves accessory organs such as the torus or a persistent hypanthium.

Dry indehiscent fruits are achenaceous, in certain cases involving modifications to the style. In anemochorous (adapted for dispersal by wind) situations, this may involve a plumose style, for example, Cercocarpus; in epizoochorous (distributed on the outside of animals) cases, the styles may bear stiff hairs or barbs, for example, Geum. Alternatively, achenaceous fruits may lack styles (sometimes due to abscission) or, more commonly, have a relatively short one, for example, Potentilla. In situations where there appears to be no significant post-flowering function for the style, dispersal may be myrmecochorous (by ants), anemochorous, or in the rare torus-borne cases (for example, Fragaria), the fruit may be endozoochorous (eaten by animals and passed through the gut). Sometimes, fruits with such styles are aggregated in acheneta that rely on barbed, persistent hypanthia for epizoochorous dispersion, for example, Acaena and Agrimonia. Dry dehiscent fruits are follicular with seeds normally distributed by air after splitting of the ripe follicle, for example, Gillenia.

Succulent, endozoochorous fruits exhibit a similar range of variation. The most common types are: multiple drupelets on the surface of a more or less conic torus (for example, Rubus); individual and sometimes large drupes on a flat receptacular apex (Prunus); pomes, which are berrylike fruits in which a fleshy hypanthium more or less completely surrounds and is generally more or less fully adnate to the carpels (for example, Amelanchier, Crataegus). The result is that a terminal orifice may remain open, often bearing floral remnants around its rim. Pomes with hard pyrenes are sometimes distinguished as polypyrenous drupes, a term not used in this treatment due to antonymic confusion caused by combining the roots for pyrene (hardened mesocarp) and drupe (fleshy mesocarp), although the relevant distinction is well recognized (J. Rohrer et al. 1991).

Some important temperate fruits are members of the Rosaceae: apples (Malus), pears (Pyrus), almonds, apricots, cherries, peaches, and plums (Prunus), blackberries and raspberries (Rubus), strawberries (Fragaria), loquat (Eriobotrya); minor fruits include those of Amelanchier, Crataegus, Cydonia, Mespilus, and others. Some genera are popular in ornamental horticulture in North America, for example, most of the above as well as, especially, Chaenomeles, Cotoneaster, Pyracantha, Rhaphiolepis Lindley, Sorbus, Photinia, Physocarpus, Rosa, and Spiraea among trees and shrubs; and Filipendula, Geum, Potentilla, and Spiraea among plants suitable for flower borders.

Apomixis is a feature of some rosaceous genera and may make taxonomic decisions difficult or equivocal in genera such as Alchemilla, Amelanchier, Crataegus, Rubus, and others. Apomixis always seems to be associated with polyploidy and often with hybridization; in some apomictic genera, sexual species and apomicts are facultatively sexual.

Rosaceae lack alkaloids and blue anthocyanic pigments. Some have foliage or seed that becomes toxic due to hydrolysis of benzaldehyde cyanohydrins.

The great diversity of Rosaceae reflects the age of the family (since at least the mid Eocene) and its evolutionary success. The family has needed no fundamental change in circumscription since Jussieu first recognized it, the sometimes included outgroups, for example, Chrysobalanaceae, Neuradaceae, already being confidently excluded in more recent pre-molecular classifications, while segregate families (for example, Malaceae) have received little currency. This reflects a lack both of close neighbors and of large internal discontinuities. Species limits are often debatable, especially where apomixis is present; generic limits are much more fixed, apart from the familiar historic trend to recognizing smaller, more discrete units. What has principally been fluid in rosaceous taxonomy until modern molecular research are the phylogenetic relationships, both to neighboring families and within the family. Rosales once contained families such as Crassulaceae and Saxifragaceae, which have many morphological similarities to Rosaceae, but Angiosperm Phylogeny Group (2003) indicated instead that in Rosales, Rosaceae is sister to a group of families including Moraceae, Rhamnaceae, Ulmaceae, and Urticaceae.

The internal relationships of Rosaceae have received much study, including morphological, anatomical, cytological, phytochemical, breeding system, fungal pathogenicity (especially rusts), and molecular. Among these studies, that of D. Potter et al. (2007) resulted in the first comprehensive molecular phylogeny of Rosaceae, and the treatment in this volume reflects it. Potter et al. recognized three subfamilies with 15 tribes worldwide. Their classification used the ranks supertribe and subtribe, not used here. The subfam. Dryadoideae, with one tribe and four genera, diverged early and is unique in Rosaceae for its actinorhizal symbiosis; it has achenaceous fruits. Subfamily Rosoideae has six tribes and 29 genera and is only slightly altered from its traditional circumscription by shedding Dryadoideae and three small genera. Most Rosoideae have achenaceous fruit; some have multiple drupelets or achenes borne on a torus. The remainders of Rosaceae are found in the large and heterogeneous subfam. Amygdaloideae of nine tribes and 54 genera. This subfamily has the largest diversity of fruit types. Amygdaloideae now contains the traditional spiraeoid genera with follicular fruit, Maleae with pome fruit, and Amygdaleae with drupaceous fruit.

Key to Subfamilies and Tribes of Rosaceae

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