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This web site deals with 34
species of Asteraceae, which have showy flowers, and may be encountered
by the visitor to the Saylorville Lake Recreation Area.
The Aster family, or Asteraceae, is
the largest family of flowering plants. This group includes perhaps
20,000 species worldwide (see Technical Reference
# 6 ). In North America there are about 2500 species (North American
Guide # 1 ), and in Southcentral Iowa, 141 (Technical
Reference # 13 ).
The "flower" of this family
is typically aster-like. The flower we see is actually a "flowering
head" made up of a cluster of several, small flowers. The
small flowers are of two types: disc flowers and ray flowers.
In sunflowers, for example, the disc flowers are clustered together
in a round "disc," in the center of the flowering head.
Ray flowers surround this disc, and extend their showy petals
outwardly, in radial fashion, away from the disc. While disc
flowers do have petals, they are small and inconspicuous compared
to the those of ray flowers. The color of disc flower petals
contributes to the color of the disc.
The flowering heads of some Asteraceae have ray flowers only.
Wild Lettuce and Chicory are examples. Those of other species,
like Thistle, have only disc flowers. Sunflowers have both ray
and disc flowers.
The flowering head, to which the ray and disc flowers are attached,
is called a receptacle. On the underside of the receptacle are
small leaf-like structures called involucral bracts. Their shape
and color are useful in identifying the different taxa.
Some disc flowers are in the form of buds and will not have opened.
Buds, or "immature" disc flowers are usually green
or gray, and the part of the disc having these will show this
color. "Mature" disc flowers that have opened, following
the bud stage, usually have 5 petals, 5 stamens, a pistil, and
a calyx made of several, fine, hair-like bristles that surround
and envelop the petals. Collectively these bristles are referred
to as the pappus.
The petals of the disc flower are fused to each other forming
a short, open-ended tube. However, at the end of the tube, the
tips of the petals may remain distinct to show 5 lobes, which
in some species, are well developed.
Within the petal tube lie the stamens. These are comprised of
5 anthers, each of which sits upon a slender filament.
The anthers, like the petals, are also fused to each other, forming
an anther tube that, in turn, surrounds the pistil. The petal
tube encloses both the anther tube and the pistil. However, the
tube of fused petals may be shorter than that of the anthers,
leaving the anthers somewhat exposed.
The stigma, at the tip of the pistil, extends beyond the anther
tube, where it divides, typically, into two visible, terminal
branches.
The ovary is located at the base of the disc or ray flower, in
the inferior position. Near the upper surface of the ovary emerge
the bristles of the pappus. Following fertilization the ovary
develops into an achene-like fruit, which becomes the "seed."
The attached bristles of the pappus serve as a cotton-like tuft,
which promotes dispersal of the seeds by the wind.
Ray flowers are constructed in much the same way as disc flowers,
except that the petals of the ray flower extend well beyond the
level of the stigma to form an elongate, conspicuous, brightly-colored
"ray." While petals of the ray flower are fused to
each other, the fusion is incomplete on one side of a "would-be"
tube, so that an actual tube is not formed. Instead, the effect
is that of an "unfurled" tube, in which.the fused petals
occur together as a single, flat, elongate ray.
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