Amelanchier arborea, A. bartramiana, and A. laevis are diploid, and the remainder are a polyploid mixture including many triploid and tetraploid plants. Hybridization, polyploidy, and asexual vegetative reproduction have created a vast array of forms that thrive (as does Crataegus) in cleared, disturbed, or burned-over areas, where even the growth form may differ from ancestral plants (clumped stems, for example, replacing single destroyed trunks).
It is usually desirable to check several representative leaves before deciding the nature of the preponderant vein and tooth pattern. Likewise, it may be necessary to check the summit of several fruits before concluding that the top of the ovary is not to be considered pubescent. Some of the specimens mapped may well be hybrids that resemble one parent so closely (or are so incomplete) that their origin was not recognized.
1. Pedicels 1–3 in axils of leaves; petals less than twice as long as broad; leaves at least partly open and essentially glabrous (except margins and petioles) at flowering time, the blade tapering trough-like into raised petiole margins, the petioles less than 8 (–15) mm long.
1. Pedicels 4 or more (at least scars if some have fallen with fruit), the inflorescence a raceme (or corymbose); petals at least twice as long as broad; leaves various (glabrous to tomentose) but the blade rounded or truncate to subcordate, not tapered at base, and petioles usually longer than 8 mm.
2. Summit of ovary glabrous, even in flower (or with a few hairs at base of style only); leaf blades short-acuminate, finely and closely serrate with 22–45 (–50) teeth per side.
3. Leaves just beginning to unfold at flowering time, densely white-tomentose beneath, otherwise green, retaining some of the pubescence on petioles and along midrib beneath into maturity.
3. Leaves mostly half-grown at flowering time, usually bronze-red, glabrous or nearly so, completely glabrous at maturity.
2. Summit of ovary tomentose (± densely so in flower, sometimes more sparsely, but evenly, in fruit); leaf blades variously shaped and toothed.
4. Larger leaves with ca. (22–) 25–50 (–55) fine teeth on a side (more than twice as many teeth as lateral veins), acute to short-acuminate, at flowering time open though not fully grown and often glabrous or soon becoming so.
A. interior complex
4. Larger leaves with fewer than 20 (–25) teeth on a side (no more than twice as many teeth as lateral veins), the blades at flowering time ± folded and white-tomentose beneath, when mature the apex acute to rounded.
5. Most leaves coarsely toothed (2–5 teeth per cm toward apex when mature), the veins prominent and running to tips of the teeth (or a principal fork into the teeth) at least toward apex of blade; petals (10–) 11–18 (–20) mm long; plants typically solitary or in tall many-stemmed clumps (though sometimes colonial).
A. sanguinea complex
5. Most leaves finely toothed at least toward apex (5–8 teeth per cm when mature), the veins anastomosing and becoming indistinct near the margin, at most with weak veinlets ending in the teeth; petals ca. 5–9 (–10) mm long; plants typically spreading underground and forming colonies of low shrubs.
A. spicata complex