Flowers in a dense head on a common receptacle, and sur-Vounded by an involucre. Calyx-tube adherent to the ovary, its limb either obsolete or forming a pappus of few or many bristles or chaffy scales. Corolla either tubular or with one side much prolonged (strap-shaped or ligulate). Stamens usually 5, on the tube of the corolla, their anthers united (syngenesious). Style 2-cleft. (See Parti., sections 60-62, for examination of a typical flower.)
The heads of flowers present some variety of structure. All the flowers of a head may be tubular; or only the central ones or disk-flowers, as they are then called, may be tubular, whilst those around the margin, then known as ray-flowers, are ligulate or strap-shaped. Or again, all the flowers may be strap-shaped. It is not unusual also to find a mixture of perfect and imperfect flowers in the same head.
The bracts which are often found growing on the common receptacle among the florets are known as the chaff. When these bracts are entirely absent the receptacle is said to be naked. The leaves of the involucre are called its scales.
Artificial Synopsis of the Genera.
Heads either altogether without strap-shaped corollas, or the latter, if present, forming only the outer circle (the ray). Ray-flowers, when present, always without stamens, and often without a pistil also.
A. Ray-flowers entirely absent.
Scales of the involucre in many rows, bristly-pointed, or fringed.
+ Florets all perfect.
1. Cni'cus. Leaves and scales of the involucre prickly. Pappus of long plumose bristles. Receptacle with long soft bristles among the florets. Flowers reddish-purple.
2. Car'duus. Resembling Cnicus, but the bristles of the pappus are not plumose. Atl. Prov.
3. Onopordon. Leaves and scales of the involucre prickly.
Heads much as in Cnicus, but the receptacle naked, and deeply honeycombed. Pappus of long bristles, not plumose. Stem winged by the decurrent bases of the leaves. Flowers purple.
4. Arc'tium. Leaves not prickly, but the scales of the globular involucre tipped with hooked bristles. Pappus of many short rough bristles. Receptacle bristly. Flowers purple.
Verno'nia. One species has scales of the involucre somewhat bristly. See No. 15.
++ Marginal florets often sterile, and much larger than the others, forming a kind of false ray.
5. Centaure'a. Leaves not prickly. Scales of the involucre fringed. Pappus double and bristly, or very short or none. Receptacle bristly.
+++ Sterile and fertile florets in separate heads, i.e., monoecious. Fruit a completely closed involucre (usually bristly) containing only one or two florets, these heads sessile in the axils of the bracts or upper leaves. Sterile heads with more numerous florets in flattish involucres, and forming racemes or spikes. Pappus none.
6. Xan'thium. Fertile florets only 2 together in burs with hooked prickles, clustered in the axils. Sterile heads in short spikes above them, the scales of their involucres in one row only, but not united together.
7. Ambro'sia. Fertile florets single, in a closed involucre armed with a few spines at the top. Sterile heads in racemes or spikes above, the scales of their involucres in a single row and united into a cup.
8. Franseria. Inflorescence much like that of Ambrosia, but the fruiting involucre is armed with a number of long flat and thin spines. - N.W.
* * Scales of the involucre without bristles of any kind. + Marginal florets without stamens.
++ Pappus none or minute. Receptacle naked. Very strong-scented herbs.
9. Tanacetum. Flowers yellow, in numerous corymbed heads. Scales of the involucre dry, imbricated. Pappus 5-lobed. Leaves dissected.
10. Artemis'ia. Flowers yellowish or dull purplish, in numerous small heads which are panicled or racemed. Scales of the involucre with dry and scarious margins, imbricated. Achenes with narrow top.
++++Pappus of all the florets bristly. Receptacle naked.
11. Erechti'tes. Flowers whitish. Scales of the involucre in a single row, linear, with a few bractlets at the base. Corolla of the marginal florets very slender. Pappus copious, of fine soft white hairs. Heads corymbed. Erect and coarse herbs.
12. Gnapha'lium. Flowers whitish or yellowish. Scales of the involucre yellowish-white, in many rows, dry and scarious, woolly at the base. Outer corollas slender. Pappus a single row of tough bristles. Flocculent-woolly herbs.
13. Antenna ria. Very much like Gnaphaliumin appearance, being white-woolly, but the heads are usually diaecious, and the bristles of the pappus thicker in the sterile florets.
+ + All the florets in the head perfect.
13. Antennaria, with dioecious heads, may be looked for here. See previous paragraph.
Bidens. One or two species have no rays. See No. 41. Sene'cio. One species is without rays. See No. 19.
14. Lia'tris. Flowers handsome, rose-purple. Receptacle naked.
Pappus of long and slender bristles, plumose or rough. Achenes slender, 10-ribbed. Lobes of the corolla slender. Stem wandlike, leafy, from a corm or tuber. Leaves narrow or grass-like.
15. Verno'nia. Flowers purple. Scales of the involucre (with us) with slender bristly tips. Receptacle naked. Pappus, double, the outer minute, the inner capillary. Branches of the style long and slender, minutely bristly.
16. Eupato rium. Flowers white or purple. Receptacle naked.
Pappus of slender hair-like bristles, smooth or nearly so. Achenes 5-angled. Heads in corymbs. Leaves whorled, or connate, or opposite.
+++ Marginal florets pistillate and fertile, those in the centre of the head staminate and sterile. Receptacle chaffy. Pappus wanting.
17. Caca'lia. Flowers white or whitish. Heads 5-flowered, rather' large, in flat corymbs. Involucre 5-leaved. Receptacle naked, bearing a scale-like pointed appendage in the centre. Corolla deeply 5-cleft. Pappus of numerous capillary bristles.