Petrophytum caespitosum
Petrophytum caespitosum subsp. caespitosum
mat rock-spirea, rockmat spirea, rockrose, Rocky Mountain rockmat
mat rockspirea, rock spiraea, Rocky Mountain rockmat
prostrate or decumbent, loosely intertwined, 1–4+ cm, and erect or ascending, tightly coalesced, 0.2–0.5 cm, internodes 0.1–1 cm.
blade spatulate, 0.4–1.8 × 0.2–0.4 cm, venation rarely visible except on long-shoot leaves, 1(–3)-veined, apex acute, abaxial surface pilose to sericeous, rarely sparsely strigose on lamina and veins.
blades 0.4–1.4 cm, abaxial surface pilose to sericeous.
widely branched to compact and racemose, [0.5–]1–15[–20] × 0.5–6 cm, sericeous;
bracts subulate to narrowly obtrullate, 3–8 mm, pilose to sericeous.
compressed panicles, proximal 1/2 of panicle varying from unbranched with scattered bracts along peduncles to ± branched with branches 1–3(–4+) cm, distal 1/2 of panicle often appearing racemose with no or relatively few branches.
0.5–2.5 mm;
bracteoles 1, extending from middle to well beyond apex of sepals.
3–6 mm diam.;
hypanthium 1 mm, densely sericeous;
sepals erect, ovate-lanceolate, 1–1.5 mm, margins sericeous, abaxial surface sericeous to tomentose;
petals narrowly oblanceolate, 1–2.5 mm, apex obtuse to slightly cleft or acute to acuminate;
stamens 20, lengths 1.5–2(–2.5) times petals (1.5–3 times sepals);
carpels (3–)5(–6), adaxially connate.
petal apex obtuse to slightly cleft.
2 mm.
shoots usually compressed into tight rosettes, sometimes internodes extended and rosette structure lost.
= 18.
Petrophytum caespitosum
Petrophytum caespitosum subsp. caespitosum
Subspecies 2 (2 in the flora).
Petrophytum caespitosum primarily inhabits arid rocky outcrops and talus slopes at high elevations in mountain ranges of the western United States and northeastern Mexico.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Subspecies caespitosum is frequently found with relatively small leaves that are pilose to densely sericeous. Sometimes plants growing in moister conditions will have larger leaves and reduced indument, and they may develop visible primary and secondary venation.
For this treatment, var. elatius has been subsumed in subsp. caespitosum, as the author is uncertain as to whether it should be recognized as a variety within this subspecies or merits elevation to subspecies status. Variety elatius has short shoots with extended internodes (1–3 cm), larger panicles (10–20 × 8–15 cm), widely branched with 4–10 cm branches, with bracteoles linear to narrowly oblanceolate, 8–16 mm; it is found on dry rock ledges and outcrops, often on limestone, between 1100 and 2800 m in Arizona, Nevada, and Utah.
Variety elatius appears to intergrade with subsp. caespitosum (freely intergrades; T. H. Kearney and R. H. Peebles 1942, 1960) but does not appear to be merely an ecological variant of it. Morphological work on the carpel of Petrophytum by C. Sterling (1966) indicated that, for the carpel characters examined, the character state distribution of P. elatius (here cited as var. elatius) was most similar to that of P. hendersonii. Variety elatius was dissimilar to both P. caespitosum (infraspecific taxon not identified, most likely subsp. caespitosum) and P. cinerascens. If var. elatius was only an ecological variant of P. caespitosum, it would be expected that carpel morphology and anatomy would be more similar than found by Sterling. Variety elatius may merit taxonomic recognition.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Leaf blades 0.4–1.4 cm, abaxial surfaces pilose to sericeous; petal apices obtuse to slightly cleft. | subsp. caespitosum |
1. Leaf blades 1–1.8 cm, abaxial surfaces sparsely strigose; petal apices acute to acuminate. | subsp. acuminatum |
CA
WA
Flora NW
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