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Published In: The Gardeners Dictionary...Abridged...fourth edition vol. 2. 1754. (28 Jan 1754) (Gard. Dict. Abr. (ed. 4)) Name publication detail

Project Name Data (Last Modified On 8/11/2017)
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Project Data     (Last Modified On 7/30/2009)


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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.


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1 1. Petal-bearing flowers 2–10 at each stem tip; sepals pubescent only with stellate hairs; fruits developing from cleistogamous flowers with 1–3 seeds; seeds with the surface with rows of minute pits or an obscure network of ridges ... 1. H. BICKNELLII

Helianthemum bicknellii
2 1. Petal-bearing flowers occurring singly or rarely in pairs at each stem tip; sepals of petaliferous flowers with longer, simple hairs intermixed with the shorter, stellate ones (those of cleistogamous flowers usually only stellate-hairy); fruits developing from cleistogamous flowers with 5–12 seeds; seeds with the surface pebbled to finely tubercled ... 2. H. CANADENSE Helianthemum canadense
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