1. Diodia teres Walter (rough buttonweed)
Diodella teres (Walter) Small
Pl. 545 g, h;
Map 2533
Plants annual,
sometimes becoming woody at the base. Stems 15–35(–80) cm long, usually erect
or strongly ascending, minutely roughened and becoming more or less glabrous to
sparsely or moderately pubescent with short, straight to wavy hairs of various
lengths. Stipules 1–2 mm long, the bristles 5–11 on each side, variously 1–10
mm long. Leaf blades 1.0–3.5 cm long, 2–5 mm wide, narrowly lanceolate to
narrowly elliptic or nearly linear, angled to somewhat rounded at the base,
angled to a sharply pointed tip, the margins minutely pubescent with stiff,
spinescent hairs, frequently becoming curled under (especially in plants from
particularly dry microsites), the surfaces glabrous or nearly so, the venation
with the midvein and usually 2 or 3 pairs of pinnate, secondary veins visible.
Flowers usually solitary in the leaf axils, produced at most of the nodes.
Calyces deeply 4-lobed, the lobes 1–2 mm long, triangular, often unequal.
Corollas 6–7 mm long, pink to less commonly mauve or pale purple, externally
glabrous or hairy, internally glabrous, the tube 4–5 mm long, the lobes 2.0–2.5
mm long, lanceolate. Styles unbranched, with a 2-lobed, capitate stigma. Fruits
3.0–3.5 mm long, 3.0–3.5 mm wide, broadly obovoid, smoothly rounded to broadly
angled on the sides. 2n=28. June–October.
Scattered nearly
throughout the state, but apparently absent from most of the western half of
the Glaciated Plains Division (eastern U.S. to Kansas and Texas, also New
Mexico, Arizona, and California; Mexico, Central America, South America,
Caribbean Islands). Glades, thin-soil areas of upland prairies, sand prairies,
ledges and tops of bluffs, and banks of streams and rivers; also pastures, old
fields, fallow fields, old quarries and mines, railroads, roadsides, and open,
disturbed areas; often in sandy or acidic soils.
As is common in
annual species of weedy habit and habitat, D. teres shows considerable
variation in plant size, leaf size, and pubescence, among other characters.
This variation is continuous and mostly found throughout the range of the
species, often within populations. Some authors have attempted to classify this
variation (Fernald and Griscom, 1937), but, as was noted by Steyermark (1963),
these intergrade greatly. With more collections and information on genetics now
available, the five North American varieties of D. teres are now
considered to represent only minor variants within a variable species, and
morphological expression also is thought to be strongly influenced by the
plant’s environment.