alternate. The root is generally thickened,
sometimes, as in dahlia, tuberous; root and stem contain oil passages,
or, as in lettuce and dandelion, a milky white latex. The flowers are
crowded in heads (_capitula_) which are surrounded by an involucre of
green bracts,--these protect the head of flowers in the bud stage,
performing the usual function of a calyx. The enlarged top of the axis,
the receptacle, is flat, convex or conical, and the flowers open in
centripetal succession. In many cases, as in the sunflower or daisy, the
outer or ray-florets are larger and more conspicuous than the inner, or
disk-florets; in other cases, as in dandelion, the florets are all
alike. Ray-florets when present are usually pistillate, but neuter in
some genera (as _Centaurea_); the disk-florets are hermaphrodite. The
flower is epigynous; the calyx is sometimes absent, or is represented by
a rim on the top of the ovary, or takes the form of hairs or bristles
which enlarge in the fruiting stage to form the pappus by means of which
the seed is dispersed. The corolla, of five united petals, is regular
and tubular in shape as in the disk-florets, or irregular when it is
either strap-shaped (ligulate), as in the ray-florets of daisy, &c., or
all the florets of dandelion, or more rarely two-lipped. The five
stamens are attached to the interior of the corolla-tube; the filaments
are free; the anthers are joined (syngenesious) to form a tube round the
single style, which ends in a pair of stigmas. The inferior ovary
contains one ovule (attached to the base of the chamber), and ripens to
form a dry one-seeded fruit; the seed is filled with the straight
embryo.

[Illustration: FIG. 1.

1. Flower head of Marigold. 3. Head of fruits, nat. size.
2. Same in vertical section. 4. A single fruit.]

The flower-heads are an admirable example of an adaptation for
pollination by aid of insects. The crowding of the flowers in heads
ensures the pollination of a large number as the result of a single
insect visit. Honey is secreted at the base of the style, and is
protected from rain or dew and the visits of short-lipped insects by the
corolla-tube, the length of which is correlated with the length of
proboscis of the visiting insect. When the flower opens, the two stigmas
are pressed together below the tube formed by the anthers, the latter
split on the inside, and the pollen fills the tube; the style gradually
lengthens and carries the polle