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Published In: Species Plantarum 1: 475. 1753. (1 May 1753) (Sp. Pl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library

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8. Crataegus L. (hawthorn) Contributed by James B. Phipps

Plants shrubs or small trees. Trunks sometimes armed with simple or branched thorns. Branches often producing straight to slightly curved thorns, these usually determinate, persistent for a few years, rarely becoming elongated or leafy. Bark various shades of tan to orangish brown and brown, appearing mottled or checkered, usually peeling in irregular flakes or thin patches, in a few species not flaking, but instead tight and furrowed. Twigs brown to dark brown, becoming gray or grayish brown with age, glabrous or hairy. Winter buds terminal and lateral, small, broadly ovoid to nearly globose, with few to several overlapping scales, these dark red, somewhat fleshy, glabrous. Leaves deciduous, alternate but sometimes appearing clustered at the tips of short branchlets, folded during development, mostly short- to long-petiolate, the petioles grooved on the upper surface, with or without glands along the margins. Stipules relatively large and persistent on rapidly elongating shoots, smaller and shed early on flowering branches, membranous or more commonly leafy, the margins with gland-tipped teeth. Leaf blades simple, those of rapidly elongating branches often larger and more deeply lobed, those of flowering branches smaller, unlobed or lobed, variously shaped, the margins bluntly or sharply and finely to coarsely toothed, the teeth sometimes gland-tipped, the surfaces glabrous or variously hairy at maturity, the upper surface lacking glands. Inflorescences appearing terminal on lateral, short branchlets, dome-shaped to more or less flat-topped, small panicles of several to numerous short- to long-stalked flowers, occasionally reduced to umbellate clusters or solitary flowers, produced as the leaves develop or later, the axis and stalks glabrous, hairy, or stalked-glandular, the stalks each with a small bract at the base, this linear to narrowly oblong-elliptic, membranous to subherbaceous, usually gland-toothed along the margins, often shed early. Flowers epigynous, usually malodorous, the hypanthium fused to the ovaries, globose or slightly urn-shaped, with a greenish yellow nectar disc at the mouth (this usually turning red after pollination has occurred), glabrous or hairy. Sepals 5, spreading to somewhat reflexed at flowering, mostly triangular to narrowly triangular, angled or tapered to a sharply pointed tip, the margins entire to toothed or shallowly lobed, the teeth or lobes gland-tipped, the outer surface glabrous or hairy, persistent until fruiting or shed after flowering. Petals 5, broadly obovate-elliptic to nearly circular, usually slightly concave (cupped) at flowering, the margins entire or sometimes slightly uneven, white or less commonly pinkish-tinged at flowering, usually pinkish-tinged in bud, fading to white with age, overlapping in bud. Stamens (5–)10–20(–25), the filaments often somewhat unequal in length, attached to the margin of the nectar disc, the anthers (0.3–)0.5–1.0(–1.8) mm, ivory to cream-colored or pink to red, rarely dark pinkish purple. Pistil 1 per flower, of (1–)2–5 fused carpels. Ovary inferior, sometimes protruding slightly from the hypanthium at flowering, with (1–)2–5 locules, each with usually 2 ovules (but 1 of these usually abortive). Styles (1–)2–5, free or fused at the base, exserted from the hypanthium, the stigmas club-shaped. Fruits pomes, globose or broadly ellipsoid, glabrous or hairy at maturity, dark to bright red to less commonly orange or yellow, the surface sometimes dotted, with (1–)2–5 stones (usually called nutlets in Crataegus), these hard or bony, wedge-shaped when more than 2, dorsally usually slightly concave or shallowly grooved, indehiscent, 1(2)-seeded, embedded in the fleshy to mealy middle layer of the fruit. About 200 species, most diverse in temperate regions of the northern hemisphere, a few in the montane tropics and temperate southern hemisphere..

Where abundant, hawthorns are important wildlife plants, providing shelter and protection for numerous small mammals, excellent nesting sites for birds, and autumnal food for medium-sized passerine birds, ground-birds, rodents, deer, cattle, and even bears. Cattle and deer browse young hawthorn shoots before the thorns have hardened, but can actually promote the successful growth of hawthorn plants by browsing back many competitors more severely. Bees and other Hymenoptera, as well as syrphid flies and numerous other Diptera, find the mass-flowering of hawthorns a major seasonal source of pollen and/or nectar. The fruits of most species are considered unpalatable in modern times, but those of a few species are used in jams and jellies. Some species of hawthorns are cultivated as ornamentals in Missouri, although there are relatively few disease-resistant cultivars sold at nurseries in the state. The species that is the most popular is C. phaenopyrum, but a few mainly thornless cultivars of C. crus-galli and C. viridis also are grown, and additional species are occasionally cultivated as specimen plants. The very hard wood is sometimes used in handicrafts. A few species have medicinal value, principally in treating hypertension (Kurz, 2003).

In 1923, the “white hawthorn blossom” was designated as Missouri’s official state floral emblem by the state legislature. This was not without controversy, as some botanists complained that establishing a state flower based on such a taxonomically cantankerous genus and without designation of a particular species was inappropriate (Bush, 1927). However, intense lobbying, mainly by the Daughters of the American Revolution, swayed the politicians. Because the precise wording of the bill mentioned red haw, most of the few botanists and other natural history enthusiasts who care about such details have interpreted C. mollis to be the species involved, but this interpretation stands on shaky legal ground.

In Missouri, hawthorns are much less numerous in nature than they were 100 years ago, probably due to several interacting factors, including urbanization, conversion of land to crop fields, fire suppression, fencing of open range, and other changes in land-management practices, as well as the increasing abundance of eastern red cedar (see below) and possibly even increasing populations of nonnative songbirds (which can serve as seed dispersal agents for junipers). In general, hawthorns grow, flower, and become established less well under increasing canopy density in woodlands. The decline in hawthorn abundance in the state is reflected in the fact that the vast majority of the more than 4,000 herbarium specimens of Crataegus accessioned in various herbaria were collected before 1940. However, in spite of the major ecological changes in the state since colonization by settlers of European descent, the genus continues to be moderately common in Missouri.

Hawthorns are susceptible to cedar apple rust (caused by Gymnosporangium juniperi-virginianae Schwein. and related species), a fungus with a complex life cycle involving species of Juniperus L. (in Missouri, mainly J. virginiana L., eastern red cedar) as the alternate host. Symptoms of infection by cedar apple rust in hawthorns include deformation of branches and leaves, the presence of powdery orange patches on the plant surfaces, a reduction in the amount of flowering (correlated with malformation of branchlets), and abnormal development of fruits (resulting in amorphously shaped fruits with small, orange, columnar outgrowths on the surface and usually aborted seeds). Although small hawthorns can become so weakened that the infection proves fatal, a main effect of this fungal disease is the reduction in seed production, which is detrimental to recruitment of new individuals over time. It is worthy to note, that of the nearly 1,600 specimens of Missouri Crataegus accessioned at the Missouri Botanical Garden herbarium up to and including 1937, very few show evidence of infection by rust, but many of the more recently collected specimens show signs of such damage. For more information, see the treatment of Juniperus (Cupressaceae) in Yatskievych (1999).

There is not strong consensus on the species-level taxonomy in Crataegus. Missouri is especially problematic in this regard, because some of the premier collectors of hawthorn specimens in the country during the period of 1880 to 1930 resided in the state, notably B. F. Bush, John Davis, Henry Eggert, John H. Kellogg, Kenneth Mackenzie, and Ernest J. Palmer (see biographical notes on these botanists in Yatskievych [1999]). The specimens collected by these botanists fueled the studies of hawthorn specialists of the era, notably Charles S. Sargent (1908, 1912), who eventually named about 130 species based on type specimens from Missouri (Phipps et al., 2007). Palmer (1963) regarded some of these as hybrids and reduced many of the older species to varietal status or synonymy, and accepted 50 species in the state’s hawthorn flora. At the opposite extreme, treatments such as those of E. L. Little (1979) or Gleason and Cronquist (1991) include only about a dozen species for the state. Many of the taxonomic ambiguities in species recognition have been attributed to the high incidence of hybridization, polyploidy, and apomixis in the genus. However, in studying specimens mainly from Missouri, Phipps (2005) concluded that putative hybrids are restricted to relatively few, very local, sometimes apparently now extinct entities. Instead, Phipps attributed much of the morphological variation to the rather vigorous formation of local races within mainly the series Crus-galli, Punctatae, Pruinosae, and Virides. On the other hand, in a survey of rangewide cellular DNA content in hawthorns (using flow cytometry), Talent and Dickinson (2005) found that tetraploids far outnumbered diploids, with triploid taxa a relatively small, third group of taxa. Talent and Dickinson further noted that diploids are self-sterile (requiring cross-pollination), whereas most tetraploids are self-fertile and some are apomictic. Thus polyploidy and apomixis have a role in the maintenance of local races and the fixation of minor morphological differences within the reduced gene pools of such potentially inbreeding or asexually (apomictically) reproducing plants.

A number of putative hybrids are suspected among the Missouri Crataegus taxa treated in the earlier literature. They were mostly discovered in the Ozarks during the great period of exploration for Missouri Crataegus, roughly 1890–1935, and most were described with binomials as good species rather than as interspecific hybrids. Hawthorns were more abundant then and therefore the opportunities for interserial hybridization were greater. It is notable that relatively few of these persist to this date. The following is an alphabetical list of these putative hybrids, with notes on their presumptive parentage and abundance historically and presently in the Missouri flora. Crataegus sicca, whose hybrid status is not fully understood, but which is extant at scattered sites in the Ozarks and has been divided into two varieties by some authors, is treated as a species in ser. Rotundifoliae and merely mentioned in the list below. Crataegus rupicola, which may prove to be of hybrid origin in the future, is discussed briefly under C. calpodendron.

C. ×atrorubens Ashe (ser. Molles × ser. Virides). Described from the St. Louis area, last collected in 1897, and apparently extinct in the wild; however still extant in cultivation as specimen trees at a number of botanical gardens.

C. coccinioides Ashe × C. mollis (Torr. & A. Gray) Scheele (ser. Dilatatae × ser. Molles). Known from a few historical collections from Jefferson, Ripley, and Shannon Counties; it was last collected in 1910.

C. collina Chapm. × C. margarettae Ashe (ser. Punctatae × ser. Rotundifoliae). Known from a few historical collections from Lincoln and Marion Counties; it has not been collected since 1911.

C. collina Chapm. × C. padifolia Sarg. (ser. Intricatae × ser. Punctatae). Known from a single historical specimen collected in 1924 in Benton County.

C. ×danielsii E.J. Palmer (ser. Crus-galli × ser. Virides [possibly C. engelmannii]). Rickett (1937) regarded this as being quite common near Columbia (Boone County), but it has not been collected since 1952.

C. ×dispessa Ashe (C. treleasei Sarg., C. pyriformis Britton) (ser. Molles × ser. Punctatae) (Pl. 528 f). Scattered historically in southern Missouri north locally to Boone County; it was last collected in 1964 in Dallas County.

C. ×incaedua Sarg. (C. pudens Sarg.) (ser. Crus-galli × ser. Punctatae [C. collina]). Scattered historically in southern Missouri; it was last-collected in 1987 in Camden County.

C. ×kelloggii Sarg. (ser. Molles × ser. Rotundifoliae [C. margarettae]). Originally collected in Marion County and the city of St. Louis; it was last collected in Greene County in 1926. This striking taxon produces yellow fruits.

C. ×latebrosa Sarg. (C. noelensis Sarg.) (ser. Molles × ser. Punctatae) (Pl. 528 h). Collected historically more than 50 times in the southern half of the state; it was last-collected in 1953 in Dade County.

C. ×lawrencensis Sarg. (probably ser. Crus-galli × ser. Virides). Not seen since the few original collections in 1903 and 1908, all from Jasper County.

C. neobushii Sarg. × C. pruinosa (H.L. Wendl.) K. Koch (ser. Intricatae × ser. Pruinosae). Known from a single historical specimen collected in 1907 in Taney County.

C. ×nuda Sarg. (ser. Crus-galli × ser. Macracanthae [perhaps C. succulenta]). Described from a collection made in Taney County; it was last-collected in 1952 in Lawrence County.

C. ×permixta E.J. Palmer (C. intermixta Sarg., an illegitimate name) (ser. Crus-galli × ser. Virides) (Pl. 525 p). Described from the Hannibal area (Marion, Pike Counties), it has not been collected since 1913.

C. ×persimilis Sarg. (C. swanensis Sarg.) (ser. Crus-galli × ser. Macracanthae). 2n=68. Known from a few historical collections from Taney County (the original material upon which C. swanensis was described), but also a Dent County specimen collected in 2004 by Alan Brant.

C. sicca Sarg. (possibly ser. Pruinosae × ser. Rotundifoliae). See the treatment in ser. Rotundifoliae.

C. ×simulata Sarg.(ser. Crus-galli × ser. Macracanthae [possibly C. macracantha]) (Pl. 526 c). Known from a number of historical collections from Jasper and Lawrence Counties; it was last-collected in 1956 in Lawrence County.

C. ×vailiae Britton (ser. Macracanthae [probably C. calpodendron] × ser. Parvifoliae [C. uniflora]). Known from a number of historical collections in the southern half of the state and still extant at several sites; it was last collected in 1997 in Howell County. See the treatments of C. uniflora and ser. Macracanthae for further discussion.

C. ×verruculosa Sarg. (ser. Crus-galli × ser. Punctatae [probably C. collina]) (Pl. 530 i–k). Scattered historically in the southern half of the state and apparently still extant at a few sites; it was last-collected in 2002 in Iron County.

The series Crus-galli, Punctatae, Pruinosae, and Virides mainly appear recalcitrant to any straightforward taxonomy, and the account given here gives priority to the ability to unequivocally recognize taxa. The descriptions provided below for those Missouri series with more than one species apply only to Missouri taxa and mostly also exclude putative interserial hybrids. In using the keys, the leaf shapes referred to are those near the center of variation on the flowering branches, except where extension shoots are specifically indicated. The values given for leaf lobing are characteristic for the deepest lobes on a leaf. Because most hawthorns are considered difficult to identify, botanists should give priority to collecting fresh flowering material or ripe fruiting material with accurate color notes on the anthers or fruits (these can change color upon dessication). When fruits are present, it is also helpful to remove stones (nutlets) from a few representatives, as this is easier to accomplish while the pomes are fresh rather than waiting until they are dried. Serious students of the genus generally attempt to collect matching flowering and fruiting specimens from marked individuals. However, material with spent flowers and immature fruits usually can be determined to species.

As has been traditional, the Missouri species of Crataegus are here classified into series. A key to the determination of series follows.


Lower Taxa
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