epals, which are generally free, show much variation in size, shape and
covering, and afford valuable characters for the distinction of genera
or sub-genera. The corolla is generally funnel-shaped, more rarely
bell-shaped or tubular; the outer face is often marked out in
longitudinal areas, five well-defined areas tapering from base to apex,
and marked with longitudinal striae corresponding to the middle of the
petals, and alternating with five non-striated weaker triangular areas;
in the bud the latter are folded inwards, the stronger areas being
exposed and showing a twist to the right. The slender filaments of the
stamens vary widely, often in the same flower; the anthers are linear
to ovate in shape, attached at the back to the filament, and open
lengthwise. Some importance attaches to the form of the pollen grains;
the two principal forms are ellipsoidal with longitudinal bands forming
the _Convolvulus_-type, and a spherical form with a spiny surface known
as the _Ipomaea_-type. The ovary is generally two-chambered, with two
inverted ovules standing side by side at the inner angle of each
chamber. The style is simple or branched, and the stigma is linear,
capitate or globose in form; these variations afford means for
distinguishing the different genera. The fruit is usually a capsule
opening by valves; the seeds, where four are developed, are each shaped
like the quadrant of a sphere; the seed-coat is smooth, or sometimes
warty or hairy; the embryo is large with generally broad, folded,
notched or bilobed cotyledons surrounded by a horny endosperm. _Cuscuta_
has a thread-like, spirally twisted embryo with no trace of cotyledons.

[Illustration: _Convolvulus sepium_, slightly reduced.

1. Flower cut vertically.
2. Fruit, slightly reduced.
3. Seed cut lengthwise showing embryo.
4. Embryo taken out of seed.
5. Horizontal plan of arrangement of flower.]

The large showy flowers are visited by insects for the honey which is
secreted by a ring-like disk below the ovary; large-flowered species of
_Ipomaea_ with narrow tubes are adapted for the visits of honey-seeking
birds.

The largest genus, _Ipomaea_, has about 400 species distributed
throughout the warmer parts of the earth. _Convolvulus_ has about 150 to
200 species, mainly in temperate climates; the genus is principally
developed in the Mediterranean area and western Asia. _Cuscuta_ contains
nearly 100 species in the warmer and temperate regions; two