4. Cerastium diffusum Pers. (dark green mouse-ear chickweed)

C. tetrandrum Curtis

Map 1453, Pl. 340 c, d

Plants annual. Stems 12–30 cm long, ascending, sometimes from a spreading base, usually branched toward the tip, densely pubescent with short glandular hairs. Leaves sessile, lacking axillary clusters of leaves. Leaf blades 0.5–1.2 cm long, spatulate (some basal leaves) or ovate-elliptic, angled to a bluntly or sharply pointed tip. Flowers in open panicles or clusters, the stalks 0.4–0.8(–1.2) cm long, 1–3 times as long as the sepals, erect or ascending at flowering and fruiting, stipitate-glandular, the bracts with herbaceous, green margins, but rarely thin and whitened at the very tip. Sepals 4 or 5, (3–)4–7 mm long, lanceolate, angled or tapered to a sharply pointed tip, green, densely pubescent with short glandular hairs, these not extending past the sepal tips. Petals 4 or 5, 3–5 mm long, 0.7–1.0 times as long as the sepals, shallowly 2-lobed at the tip, the veins usually not apparent. Stamens 4 or 5. Styles 4 or 5. Capsules 6.5–8.0 mm long, 1.0–1.2 times as long as the sepals, slightly curved. Seeds 0.5–0.7 mm wide, the surface tuberculate, reddish brown. 2n=72. March–May.

Introduced, uncommon, known thus far only from Crawford and Pemiscot Counties (native of Europe; introduced sporadically in the southern U.S. north to California, Nebraska, Illinois, and Alabama). Lawns, roadsides, and open, disturbed areas.

Apparently first collected in Crawford County in 1957, C. diffusum may be difficult to distinguish from C. pumilum. Both species belong to the C. semidecandrum complex, a group of 5–8 morphologically similar species native to Europe; see Karlsson (2001) for an extremely detailed treatment of part of the complex and for comments on various ways the taxa can be defined. Although Morton (2005a) included Steyermarks (1963) record of C. diffusum in his concept of C. pumilum, the present treatment maintains C. diffusum, in part to draw attention to plants with some or many 4-parted flowers. Most treatments of C. pumilum show that species to have strictly 5-parted flowers.