1. Helianthemum Mill. (rockrose, frostweed)
(Daoud and
Wilbur, 1965)
Plants not
producing specialized overwintering offshoots, the stems all similar. Stems
loosely ascending to erect, pubescent with stellate hairs, sometimes becoming
nearly glabrous with age. Leaves alternate and sometimes also basal, the blade
elliptic-oblanceolate to narrowly elliptic or elliptic, pubescent with stellate
and sometimes also simple hairs, usually with pinnate venation. Inflorescences
first appearing as a solitary petaliferous flower or a small, dense cluster of
petaliferous flowers at the tip of each nearly unbranched stem, later in the
growing season developing short, racemose branches with dense clusters of
nearly sessile, cleistogamous flowers. Cleistogamous flowers appearing similar
to but smaller than mature open-flowering buds and usually with shorter stalks,
lacking petals. Sepals becoming slightly enlarged as the fruit matures. Outer 2
sepals somewhat shorter than to nearly as long as the inner ones, linear to
narrowly lanceolate, sharply pointed at the tip, those of cleistogamous flowers
partially fused to the inner ones. Inner 3 sepals ovate to ovate-elliptic,
sharply pointed at the tip. Petals 5, showy but withering and shed soon after
the flower opens, obovate-obtriangular, broadly rounded to more or less
truncate or shallowly concave at the tip, yellow. Stamens of petaliferous
flowers numerous; reduced to 3(5) in cleistogamous flowers. Style 1 per flower,
short, the stigma 1, capitate, often 3-lobed. Ovules 9 (3 per carpel) to
numerous. Fruits shorter than the persistent sepals, ovoid to ellipsoid,
somewhat 3-angled, glabrous. Seeds 1 to numerous, those of the open-flowering
fruits slightly larger than those developing from the cleistogamous flowers, variable
in shape (dependent on number in fruit), globose or ovoid to strongly 3-angled
or more or less trapezoidal, the surface dark brown, often covered with an
inconspicuous, thin, membranous outer coat, this sometimes turning gelatinous
when moistened. About 110 species, North America to South America, Caribbean
Islands, Europe, Asia, Africa.
The so-called
normal flowers are produced earlier in the growing season (as indicated in the
flowering months cited after the species descriptions) and occur singly or in
small clusters toward the stem tips. Lateral branches elongate at or after the
first round of flowering. These bear small clusters of cleistogamous flowers at
the nodes. The cleistogamous flowers are much smaller, lack petals, and do not
open. As these flowers are often similar in size to those of Lechea,
care must be taken not to misdetermine plants encountered later in the growing
season.