This section is from the book "Wild Flowers Of New York", by Homer D. House. Also available from Amazon: Wild Flowers Of New York.
Asclepiadaceae The Milkweeds are familiar and well-known plants, but in order to distinguish some of the closely related species, a special study of the flower structure is necessary. They are perennial herbs with milky juice and flowers in umbellate clusters. The calyx is small and inferior (below the ovary), five-lobed; its tube short or none. The corolla varies in shape from bell-shaped to urn-shaped, funnelform or saucer-shaped, five-lobed; the lobes or segments commonly reflexed when the flower is fully open. The flowers of the Milkweeds are further characterized by possessing a third floral envelope, consisting of a five-lobed or five-parted crown (corona) between the corolla and the stamens and attached to one or the other. Stamens five, fastened to the corolla, usually near its base, sometimes the filaments of the stamens being attached to one another. The ovary consists of two carpels, with two short styles connected at the summit by a shield-shaped stigma. The fruit consists of two large, fleshy pods (follicles) developing from each flower, but usually only one or a very few flowers of an umbel develop fruit. Seeds flattened and appendaged by a long coma of white or whitish hairs.
Key to the New York Milkweeds Corona hoods unappendagod. entire at the apex; umbels of flowers sessile; flowers green (Green Milkweed) - ------------------------------------------A cerates viridiflora
Corona hoods each with an incurved horn within (Asclepias)
Corolla and corona orange; leaves mostly alternate -----------Asclepias tuberosa
Corolla bright red or purple; leaves opposite
Flowers one-third of an inch broad or broader; corona hoods one-fourth of an inch high; leaves oblong, ovate or ovate-oblong; hoods oblong and pointed. . . .
Asclepias purpurascens Flowers one-third of an inch broad or less; corona hoods one-twelfth to one-eighth of an inch high
Plant nearly or quite glabrous; leaves lanceolate or oblong-lanceolate -----Asclepias incarnata
Plant pubescent; leaves oblong - - - ----------Asclepias pulchra
Corolla greenish, purplish, yellowish or white; leaves opposite Leaves ovate, oblong, ovate-lanceolate, obovate or orbicular
Plants, at least the lower surfaces of the leaves, canescent or tomentose; corona hoods short and blunt - - - -----------------Asclepias syriaca
Plants smooth throughout or minutely pubescent above; umbels on long stalks Leaves wavy-margined and sessile or nearly so
Leaves cordate-clasping ---------Asclepias amplexicaulis
Leaves rounded at the base and short petioled ------------------Asclepias intermedia Leaves petioled and flat
Corolla greenish, umbels loose, pedicels drooping -------------------Asclepias exaltata
Corolla white; umbels dense ---------Asclepias variegata
Corolla pink or white, some of the leaves verticillate in fours...... Asclepias quadrifolia
Leaves narrowly linear, mostly verticillate in threes to sixes: hoods entire....... Asclepias verticillata
 
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