This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Shrubs or trees, mostly with edible fruits, the white or pink flowers umbellate or corymbose, the leaves conduplicate or convolute in vernation. Petals spreading. Stamens 15-20, distinct; filaments filiform. Style terminal; stigma peltate or truncate. Exocarp of the drupe fleshy, glabrous, the endocarp bony, smooth or a little roughened, globose or oval, or oblong and compressed. [Ancient Latin name of the Plum-tree.]
About 95 species, natives of the north temperate zone, tropical America and Asia. Besides the following, some 15 others occur in the southern and western parts of North America. The genus is often divided into Prunus proper, the plums, and Cerasus, the cherries; but other than flavor, there appears to be no salient feature separating the two groups. Type species: Prunus domestica L.
* Flowers in lateral scaly umbels or fascicles, expanding with or before the leaves. † Inflorescence umbellate, the clusters sessile or nearly so.
Leaves convolute in vernation; fruit mostly large; pit more or less flattened. (Plums.)
Leaves abruptly acuminate; drupe red or yellow.
Calyx-lobes entire, pubescent within; fruit globose.
Calyx-lobes glandular-serrate; fruit subglobose or oval.
Calyx-lobes glabrous within; leaves oval or obovate.
Calyx-lobes pubescent on both sides; leaves ovate-lanceolate.
Leaves acute or obtusish; drupe red or purple.
Leaves glabrous when mature.
Fruit red, with little bloom or none.
Fruit dark purple, with a bloom; leaves ovate.
Leaves pubescent, at least on the lower surface, when mature.
Drupe 8"-12" in diameter; coast plants.
Leaves ovate or oval, acute; stone pointed at both ends.
Leaves orbicular, very obtuse; stone pointed at base.
Drupe 3"-s" in diameter; prairie plant.
Umbels only 1-2-flowered.
Leaves conduplicate in vernation; fruit mostly small; pit mostly globose. (Cherries.)
Flowers 3"-6" broad; low shrubs.
Leaves oblanceolate or spatulate; northern.
Leaves oval, oblong, or slightly obovate.
Petioles 4"-10" long; drupe 4"-5" in diameter; eastern.
Petioles 2"-3" long; fruit 6"-8" in diameter; western.
Flowers 9"-15" broad; trees; leaves ovate.
Leaves glabrous; pedicels short; fruit sour.
Leaves pubescent beneath, at least on veins; pedicels long; fruit sweet.
†† Inflorescence more or less corymbose; leaves shining.
** Flowers corymbose, terminating twigs of the season.