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mat rock-spirea, rockmat spirea, rockrose, Rocky Mountain rockmat

Photo is of parent taxon

marble rockmat

Habit Shrubs, 1–10+ dm diam.
Stems

prostrate or decumbent, loosely intertwined, 1–4+ cm, and erect or ascending, tightly coalesced, 0.2–0.5 cm, internodes 0.1–1 cm.

Leaves

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 1–1.8 cm, abaxial surface sparsely strigose.

Panicles

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.

Inflorescences

compressed panicles, usually appearing racemose, with scattered bracts along peduncle, branches uncommon, 0.5–1 cm.

Pedicels

0.5–2.5 mm;

bracteoles 1, extending from middle to well beyond apex of sepals.

Flowers

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 acute to acuminate.

Follicles

2 mm.

Short

shoots compressed into tight rosettes.

2n

= 18.

Petrophytum caespitosum

Petrophytum caespitosum subsp. acuminatum

Phenology Flowering Jun–Sep.
Habitat Limestone cliffs
Elevation 1200–2500 m (3900–8200 ft)
Distribution
from FNA
AZ; CA; CO; ID; MT; NM; NV; SD; TX; UT; WA; ne Mexico
[WildflowerSearch map]
[BONAP county map]
from FNA
CA
Discussion

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.)

Of conservation concern.

Subspecies acuminatum is known only from the southern Sierra Nevada in Fresno and Tulare counties.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Key
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
Source FNA vol. 9, p. 413. FNA vol. 9, p. 414.
Parent taxa Rosaceae > subfam. Amygdaloideae > tribe Spiraeeae > Petrophytum Rosaceae > subfam. Amygdaloideae > tribe Spiraeeae > Petrophytum > Petrophytum caespitosum
Sibling taxa
P. cinerascens, P. hendersonii
P. caespitosum subsp. caespitosum
Subordinate taxa
P. caespitosum subsp. acuminatum, P. caespitosum subsp. caespitosum
Synonyms Spiraea caespitosa, Eriogynia caespitosa, Luetkea caespitosa P. acuminatum
Name authority (Nuttall) Rydberg: Mem. New York Bot. Gard. 1: 206. (1900) (Rydberg) Munz: Aliso 4: 92. (1958)
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