Crataegus (sect. Macracanthae) ser. Macracanthae in Flora of North America @ efloras.org

64e.1. Crataegus Linnaeus (sect. Macracanthae) ser. Macracanthae (Loudon) Rehder, Man. Cult. Trees ed. 2. 368. 1940.

Crataegus sect. Macracanthae Loudon, Arbor. Frutic. Brit. 2: 819. 1838

Shrubs or trees, 20–70(–80) dm. Stems: 1–2-year old twigs usually very dark; thorns on twigs ± stout. Leaves: petiole glandular or eglandular; blade usually rhombic to rhombic-elliptic or broadly elliptic to ovate, lobes 0 or 3–5(–8) per side, lobe apex acute to obtuse, venation craspedodromous, veins (3–)6–8 per side, adaxial surface scabrous-pubescent young, glabrescent. Inflorescences (5–)12–30-flowered; bracteoles caducous, membranous, margins sessile- or stipitate-glandular. Flowers (12–)13–19(–22) mm diam.; sepal margins glandular-denticulate to glandular-laciniate, rarely nearly entire; stamens 7–10 or 20; styles 2 or 3(–5). Pomes orange to red, broadly ellipsoid to suborbicular, glabrous or hairy; pyrenes 2 or 3(–5), sides eroded.

Species 5 (5 in the flora): North America.

Three common core species comprise ser. Macracanthae: the similar Crataegus macracantha and C. succulenta and the somewhat dissimilar C. calpodendron. These are notable for their mid late to very late anthesis compared to sympatric hawthorns, stout thorns, and eglandular petioles. Crataegus sheridana and C. rubribracteolata are placed here for convenience, may not be closely related, and perhaps would better sit with ser. Anomalae. Series Macracanthae is one of only two series of red-fruited hawthorns in North America that has eroded pyrenes.

Members of ser. Macracanthae form putative hybrids with members of ser. Crus-galli (Crataegus persimilis), ser. Parvifoliae (C. ×vailiae), ser. Pruinosae (C. chadsfordiana Sargent), ser. Punctatae (C. florifera), and ser. Rotundifoliae (C. laurentiana), at least. Some are treated here as interserial hybrid species numbers 154–169; C. laurentiana, C. persimilis, and C. ×vailiae key out in the eighth, first, and second couplets, respectively.

1 Leaves: adaxial surfaces glabrous or glabrate, glossy, lobes 0, or 3 or 4 per side. 166 Crataegus persimilis, p. xxx

+ Leaves: adaxial surfaces pubescent young, matte, lobes 0 or (1–)3–5(–8) per side (2)

2 (1) Inflorescences 2–8(–20)-flowered; sepals ± equal to petals, margins glandular-laciniate. 169 Crataegus ×vailiae, p. xxx

+ Inflorescences (5–)12–30-flowered; sepals shorter than petals, margins usually glandular-denticulate to glandular-laciniate (3)

3 (2) Petioles eglandular, rarely glandular on distal wing (4)

+ Petioles ± sessile-glandular (6)

4 (3) New growth pubescent; thorns on twigs absent or numerous; leaves (often yellowish green, without impressed venation), abaxial surfaces persistently appressed-pubescent; sepal margins nearly entire or shallowly glandular-serrate; (anthesis 1–3 weeks after sympatric species of this series); stamens 20. 24 Crataegus calpodendron

+ New growth glabrous; thorns on twigs usually numerous; leaves (often blue-green, with impressed venation), abaxial surfaces glabrous (except the pertomentosa form of C. macrantha); sepal margins glandular-serrate to glandular-laciniate; (anthesis 1–3 weeks before C. calpodendron in same locality); stamens 10 or 20 (5)

5 (4) Stamens 20; anthers 0.5–0.7 mm. 25 Crataegus succulenta

+ Stamens 10; anthers 0.9–1.2 mm. 26 Crataegus macracantha (in part)

6 (3) Pyrene sides pitted or ± deeply concave (7)

+ Pyrene sides ± roughened with irregular pitting or diagonal scarring (8)

7 (6) Leaf blade sinuses obscure to well-marked, LII 5–15%, lobe apices usually obtuse; pyrene sides pitted; (anthesis late among sympatric hawthorns); bracteoles in young inflorescences ± pallid; anthers white or pink; nw Montana. 26 Crataegus macracantha (in part)

+ Leaf blade sinuses moderately deep, max LII 25–33%, lobe apices acute to acuminate; pyrene sides ± deeply concave; (anthesis one of two earliest sympatric hawthorns); bracteoles in young inflorescences deep orange-red or fading; anthers ivory or cream; Cypress Hills, Alberta, Saskatchewan. 27 Crataegus rubribracteolata

8 (6) Leaf blade lobe apices acute and sinuses shallow, LII 5–15%, or lobe apices subacute, sinuses deeper, LII 15–30%; anthers ivory; Great Plains. 28 Crataegus sheridana

+ Leaf blade lobe apices acute and sinuses moderately deep, LII 20–33%; anthers cream or pale pink; Minnesota, Maine, Michigan, Wisconsin, to Quebec and Newfoundland. 164 Crataegus laurentiana, p. xxx