SunflowerS: Family TraiTs

AsterAceAe • sunFlower Family
A family of diverse attributes, its members range from annuals to biennials
and perennials; delicate herbs to woody vines, shrubs, and trees. Leaves
basal and/or on stems; alternate, opposite, or whorled; undivided to2-pinnate. Their tiny flowers (florets) are densely packed into a cluster andattached to a common receptacle, a unit called a composite flowerhead.
Though each flowerhead superficially resembles a single flower, it actuallycontains few-to-many florets (rarely only one). On the upper surface of thereceptacle in some flowerheads, slender bracts called paleae individuallysubtend (occur below) the florets (paleae are also called pales, chaffy bracts,
chaff scales, or receptacular bracts). A receptacle with paleae is describedas paleate; a receptable without paleae is epaleate. The lower outersurface of the flowerhead is surrounded by leaf-like or sepal-like overlapping
bracts called phyllaries (or involucral bracts). Phyllaries are arranged in
one or more whorls or spirals, sometimes crowded, sometimes spread out.
Together, the phyllaries form the involucre.
Each floret contains parts of both sexes (bisexual), of only one sex(staminate [male] or pistillate [female]), or none (sterile). The calyx
is modified into a pappus, which consists of one or more rings of hairlike
bristles (sometimes with feather-like side branches), narrow to broad flat
membranous scales, or long stiff, smooth or barbed awns (sometimes a
mixture of bristles, scales, and/or awns); in some taxa the pappus is totallyabsent. Pappus units are whitish or brownish, never green or sepal-like.
The corolla is radial or bilateral (rarely absent); lobes (0)3-5. Stamens (3-)5,filament bases fused to the corolla, free above, anthers fused into a cylinder thatsurrounds the style. Ovary inferior, pistil 1, 1-chambered; ovule 1, attached inbase of ovary. Style 2-branched near tip; stigmas are borne on the basal facing (inner)sides of the style branches and generally end below the tip. Fruit a dry 1-seeded
cypsela (think of a sunflower seed still in its case, which is the ovary),sometimes called an achene. The pappus may remain attached to the fruitor may fall off. Fruit features are usually helpful for identification.
In bisexual florets, anthers release pollen into the cylinder they formaround the style. Pollen adheres to the outside of the style and is carried
upward as the style grows through the cylinder and elongates. Pollenadheres to insects as they move among the styles in search of food. Whenthey visit other flowers, the pollen is inadvertently transferred to stigmas ofmature florets. In unfertilized florets, the style branches continue to growand curl back, forcing contact of the stigmas with the pollen-laden style,thus effecting self-pollination.
Sunflowers display quite a number of floret types. Those most commonly
encountered in our area are disk, ray, and ligulate florets. Other types
are filiform, naked, and bilabiate florets. Disk floret – corolla radiallysymmetric (rarely bilateral), slender and tubular with 5 (rarely 4) small equalsized lobes at the tip; each includes both sexes (bisexual disk floret, suchas in scabrid sweetbush, Bebbia juncea var. aspera); in a few genera the pistil
is nonfunctional (staminate disk floret). Ray floret – corolla bilaterally
symmetric, a short to long tube topped on one side with a strap-like bladethat is generally 3-lobed at tip (occasionally unlobed or with a different
number of teeth); ray florets lack stamens; each contains a fertile pistil(pistillate ray floret) or a small infertile pistil (sterile ray floret). Common
tidy-tips (Layia platyglossa) has both disk and ray florets. Ligulate floret– corolla bilaterally symmetric; a short to long tube is topped on one sidewith a strap-like blade that is 5-lobed at tip; ligulate florets are always
bisexual (such as in wreath-plants, Stephanomeria spp.). Filiform floret –resembles a disk floret in that the corolla is radially symmetric and tubular
but differs in the corolla’s being cylindric, very narrow, and blunt-tipped(rarely with minute lobes); it also lacks anthers. The female (pistillate)florets of baccharis (Baccharis spp.) are filiform. Naked florets – similar
to filiform florets but have no corolla. The female (pistillate) florets of bursages (Ambrosia spp.) and cockleburs (Xanthium spp.) are naked. Bilabiate
florets – corolla distinctly two-lipped; one lip has two slender lobes, theother has a single broad three-lobed ray. Our only aster with bilabiate florets
is sacapellote (Acourtia microcephala).
04Asteraceae2012-cs6.indd 63

euDICoTS: AsterAceAe • 63

Floret Anatomyand Major Types
Style
Branches

StigmasInner base ofeach branch

Style

Anthers
Corolla(5 lobes)

Pappus(Calyx)

Cypsela(Ovary,contains
1 seed)Disk Floret

Expanded
BilateralDisk Floret
Disk Floret
Expanded Bilateral Form
NoStamens} 3 lobes

RayRayFloret
Ray Floret} 5 lobes
LiguleLigulate
FloretLigulate Floret

CorollaFiliform
Naked
Filiform
Floret Naked
FloretFloretFloretBilabiate
FloretBilabiate
Floret4/11/13 7:45 AM