This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", 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..
Perennial erect or decumbent herbs, with opposite verticillate or rarely alternate entire leaves, and middle-sized or small flowers in terminal or axillary umbels. Calyx 5-parted or 5-divided, usually small, the segments or sepals acute, often glandular within. Corolla deeply 5-parted, the segments mostly valvate, reflexed in anthesis. Corona-column generally present. Corona of 5 concave erect or spreading hoods, each bearing within a slender or subulate incurved horn, either included or exserted. Filaments connate into a tube; anthers tipped with an inflexed membrane, winged, the wings broadened below the middle; pollen-masses solitary in each sac, pendulous on their caudicles. Stigma nearly flat, 5-angled or 5-lobed. Follicles acuminate. Seeeds comose in all but one species. [Dedicated to Aesculapius.]
About 95 species, mostly natives of the New World: besides the following, some 25 others occur in southern and western North America. Known as Milkweed, Silkweed, or Swallow-wort. Type species: Asclepias syriaca L.
* Corolla and corona orange; leaves alternate or opposite.
Stem erect or ascending; leaves nearly all alternate.
1. A. tuberosa.
Stems reclining; leaves, at least the upper, opposite, oblong or oval.
2. A. decumbens.
Leaves lanceolate or linear; hoods oblong, obtuse.
3. A. lanceolata.
Leaves ovate or ovate-lanceolate: hoods lanceolate.
4. A. rubra.
Leaves oblong, ovate or ovate-oblong; hoods oblong, acutish.
5. A . purpurascens.
Flowers 2"- 3" broad; corona-hoods 1"-1 l/2" high.
Plant nearly or quite glabrous; leaves lanceolate or oblong-lanceolate.
6. A. incarnata.
Plant pubescent; leaves oblong.
7. A. pulchra.
*** Corolla greenish, purplish, yellowish or white; leaves opposite or verticillate. Leaves ovate, oblong, ovate-lanceolate, obovate or orbicular. Plants glabrous throughout, or minutely pubescent above. Leaves sessile, clasping or very short-petioled.
Peduncles of the solitary or several umbels short.
Leaves ovate-oblong: hoods 2-auriculate at the base.
Leaves nearly orbicular; hoods truncate.
9. A. latifolia.
Peduncle of the usually solitary umbel elongated. Leaves wavy-margined. Leaves cordate-clasping.
10. A. amplexicaulis.
Leaves rounded at the base, short-petioled.
11. A. intermedia.
Leaves sessile, flat; horn not exceeding the hood.
Leaves manifestly petioled.
Corolla greenish; umbels loose, the pedicels drooping.
13. A. exalt at a.
Corolla white; umbels dense.
Corolla pink: some of the leaves verticillate in 4's.
15. A. quadrifoha.
Plants, at least the lower surfaces of the leaves, canescent or tomentose. Follicles tomentose, covered with soft spinose processes. Corona-hoods obtuse, short.
16. A. syriaca.
Corona-hoods elongated, lanceolate.
Follicles with no spinose processes, glabrous or pubescent. Leaves wavy-margined; corolla-segments 4"-5" long.
18. A. arenaria.
Leaves flat; corolla-segments 2"-3" long.
Leaves lanceolate, oblong-lanceolate or linear.
Leaves opposite, lanceolate or oblong lanceolate.
Leaves thick, short-petioled; inflorescence woolly.
20. A. brachystephana.
Leaves thin, slender-petioled; inflorescence downy.
21. A. perennis.
Leaves mostly verticillate in 3's-6's, narrowly linear. Hoods entire.
22. A. verticillata.
Hoods dorsally hastate-sagittate.
Leaves scattered, densely crowded, filiform-linear.
24. A. pumila.