Aquatic and Wetland Vascular Plants of the Northern Great Plains

Asteraceae - The Aster Family

Annual, biennial or perennial herbs (those included here) of various growth habits. Leaves simple or compound, exstipulate, opposite, alternate or both, sometimes whorled, occasionally the principal leaves basal. Flowers reduced in size, crowded into involucrate heads and sharing a common receptacle, the disk, the heads each resembling a single flower and arranged in various kinds of inflorescences; flowers of the head usually of 2 types, ray (or ligulate) florets simulating petals around the outside of the disk, and the usually less conspicuous disk (or tubular) florets occupying the central portion of the disk (to form a radiate head), or the head comprised entirely of ray florets (ligulate head), or disk florets (discoid head, Fig. 93c), the disk florets of a radiate or discoid head sometimes intermixed with bracts so that the receptacle is chaffy, otherwise the receptacle naked or pitted; involucral bracts sepaloid or foliaceous, surrounding and subtending the disk in 1-several series, often imbricate. Individual flowers perfect or imperfect*, regular (disk florets) or irregular (ray florets), lacking a definite calyx; a pappus of capillary bristles, scales or awns often encircling the summit of the ovary outside the corolla, commonly accrescent and persistent in fruit, often functioning in dispersal; corollas of 2 types, those of the ray florets tubular only at the base, expanded into the flat, petaloid ray or ligule, entire or toothed at the tip, the disk corollas tubular, with 5 equal lobes or teeth at the summit; stamens 5, epipetalous, usually with the elongate anthers united into a tube around the style; style branches usually 2, ovary inferior, 2-carpellary, 1-celled and 1-ovuled, ripening into an achene.

*Imperfect flowers in composite flower heads are commonplace. The ray florets of radiate heads are pistillate or sterile, whereas the disk florets are perfect or functionally staminate. In ligulate and discoid heads, usually all the florets are perfect, but in some discoid types, e.g., Xanthium, staminate and pistillate florets are borne in 2 very different types of discoid heads.

Lead	Characteristic	Go To
1	Heads ligulate, comprised entirely of yellow ray florets; plants with milky juice.	Lead 2
1	Heads radiate or discoid; plants with watery juice.	Lead 3
2	Leaves in a basal rosette like those of a dandelion; mature achenes terete.	Genus Crepis
2	Leaves cauline; mature achenes flattened.	Genus Sonchus
3	Leaves with sharp spines.	Genus Cirsium
3	Leaves spineless.	Lead 4
4	Leaves all opposite or whorled.	Lead 5
4	Leaves alternate or partly so, or the leaves chiefly basal.	Lead 8
5	Receptacle chaffy.	Lead 6
5	Receptacle naked.	Genus Eupatorium
6	Pappus none; leaves, or at least the upper ones, connate-perfoliate, forming a cup around the stem.	Genus Silphium
6	Pappus of awns or awn-tipped scales; leaves sessile or petiolate, sometimes slightly connate, but not forming a cup around the stem.	Lead 7
7	Pappus of 2-4 retrorsely barbed awns.	Genus Bidens
7	Pappus of 2 awn-tipped scale.	Genus Helianthus
8	Heads radiate with yellow rays.	Lead 9
8	Heads discoid or radiate with rays colored other than yellow.	Lead 13
9	Pappus of 2-several awns or scales.	Lead 10
9	Pappus of numerous capillary bristles.	Lead 11
10	Receptacle chaffy; leaves short-petioled, not decurrent on the stem.	Genus Helianthus
10	Receptacle naked; leaves tapered to the base, decurrent as wings on the stem.	Genus Helenium
11	Involucral bracts in one series, not imbricate, sometimes with a few reduced bracts below.	Genus Senecio
11	Involucral bracts in few to several series, imbricate.	Lead 12
12	Inflorescence corymbiform; leaves linear to linear-lanceolate or linear-elliptic, 2-10 mm wide, entire, glandular-punctate.	Genus Euthamia
12	Inflorescence paniculiform; leaves lanceolate to elliptic, 10-40 mm wide, serrate, not glandular.	Genus Solidago
13	Heads unisexual and dimorphic, the male florets in small heads above the larger female heads; involucres of the female heads spiny, completely enclosing the pistillate florets to form a bur.	Genus Xanthium
13	Heads bisexual or rarely unisexual, all alike; involucres not spiny.	Lead 14
14	Principal leaves basal, sagittate or palmate, white-woolly at least on the lower surface; flowering in spring or early summer.	Genus Petasites
14	Principal leaves cauline, shaped other than sagittate or palmate, not white-woolly; flowering late summer or autumn (except Erigeron philadelphicus).	Lead 15
15	Heads discoid.	Lead 16
15	Heads radiate (the rays very narrow and only slightly, if at all, exceeding the involucre in Erigeron lonchophyllus and Conyza canadensis, therefore inconspicuous).	Lead 18
16	Leaves pinnately dissected; pappus none.	Genus Artemisia
16	Leaves simple, entire or toothed; pappus of numerous capillary bristles.	Lead 17
17	Perennial with lanceolate to ovate-lanceolate leaves; involucral bracts purple-tipped.	Genus Vernonia
17	Annual with linear leaves; involucral bracts green.	Genus Aster
18	Pappus of 2 awns and several minute bristles.	Genus Boltonia
18	Pappus of numerous capillary bristles.	Lead 19
19	Plants taprooted annuals blooming in late summer or fall; involucre 3-4(5) mm high.	Genus Conyza
19	Plants fibrous-rooted perennials, often with rhizomes, blooming in early or late summer or fall; involucre usually more than 4 mm high.	Lead 20
20	Involucral bracts green, often chartaceous at the base; rays wider than 0.5 mm.	Genus Aster
20	Involucral bracts hyaline at the tip and on the margins above, green in the middle and at the base; rays 0.1-0.6 mm wide.	Genus Erigeron