INTRODUCTION
Composite Flowers, common name for one of the largest families of
flowering plants, with more than 20,000 species. The common name refers
to the clustering of the flowers into compact heads so that an entire
cluster resembles a single flower, as in the familiar aster or
dandelion.
The composite family is worldwide in distribution except for Antarctica,
where two species of grasses are the only flowering plants known.
Composites are particularly well adapted to semiarid parts of tropical
and subtropical regions, to arctic and alpine tundra, and to temperate
regions. They are poorly represented only in tropical rain forests.
Although the composite family contains nearly 10 percent of all the
flowering plants, its direct economic importance is relatively small.
Lettuce is the most important crop; other notable food plants are
artichoke, endive, salsify, chicory, and tarragon. Sunflower and
safflower are important sources of vegetable oils, and some members such
as guayule have been studied as potential sources of rubber. Weedy
members of the family include dandelion, thistle, cocklebur, and
ragweed, which is a major source of the airborne pollen that affects hay
fever sufferers. Horticulturally important members of the composite
family include marigold, dahlia, zinnia, daisy, cosmos, chrysanthemum,
tansy, and aster.
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II STRUCTURE
The flowers (see Flower) are always grouped into an inflorescence (flower
cluster) called the head, or capitulum, which resembles and functions as
a single flower. In more than half the species, the flowers at the
margin of the head have outwardly extended corollas, called ray florets,
that resemble numerous petals and make the flower head more conspicuous
to pollinators. The flowers within the head are called disk florets.
Both types are surrounded by bracts (specialized leaves) at the base.
The calyx (floral envelope, or pappus) that surrounds the individual
flowers in a head consists of scales or bristles, which often aid in
distribution of seeds. In the common dandelion the pappus consists of
fine bristles that enable the fruit to float through the air. In the
beggar-ticks it is barbed, causing the fruit to stick to passersby.
The petals of the florets are fused along the base, forming a somewhat
tubular corolla. The anthers (pollen-producing flower parts) are joined
to form a tube through which the style (part of the female flower part)
extends. The anthers release the pollen into the tube, and as the style
elongates, it pushes the pollen upward out of the tube, making it
available to insect pollinators or to wind dispersal. The stigmatic
(pollen-receiving) areas of the style are located on two branches of the
style tip, and these branches separate after elongation. Thus,
self-pollination is usually avoided, although in some cases the
stigmatic branches recurve backward into the pollen-bearing anther tube.
The pistil (female flower part), which has a single ovary, bears the
other flower parts on its apex. After fertilization it matures into a
hard-coated fruit that bears a single seed.
III FLOWER TYPES
Although the flowering heads of composites are fundamentally similar,
variation in their structure, particularly in the corolla, accounts for
much diversity. The simplest head is the discoid, consisting of all disk
florets with tubular corollas. Both stamens and pistils are functional.
Thistle, eupatorium, and ageratum have heads solely of disk florets.
A second type of head is the radiate, which has disk florets in the
center and ray florets at the margin. The corollas of the marginal
florets are tubular at the base but flatten and elongate as they grow
outward. Ray florets are usually pistillate (with functional pistils but
nonfunctioning anthers) or sterile. Such familiar flowers as sunflowers,
black-eyed Susans, and daisies have radiate heads.
A third common head is the ligulate, in which all the florets resemble
ray florets but have both pistil and stamens functioning. The flowering
heads of dandelions and chicory are ligulate. Other kinds of heads also
occur in the family.
Scientific classification: Composite flowers make up the family
Asteraceae (formerly Compositae) of the order Asterales.
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