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.

** Corolla bright red or purple (sometimes greenish in A. rubra) leaves opposite. Flowers 4"-6" broad; corona-hoods 2" - 3" high.

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.

8. A.Sullivantii.

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.

12. A.Meadii.

Leaves manifestly petioled.

Corolla greenish; umbels loose, the pedicels drooping.

13. A. exalt at a.

Corolla white; umbels dense.

14. A.variegata.

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.

17. A.speciosa.

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.

19. A.ovalifolia.

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.

23. A.galioides.

Leaves scattered, densely crowded, filiform-linear.

24. A. pumila.