2. Diodia virginiana L. (large buttonweed)
D. virginiana var. attenuata Fernald
Pl. 545 c, d; Map
2534
Plants annual,
perhaps sometimes short-lived perennial herbs. Stems 20–40(–80) cm long,
prostrate or loosely ascending from a spreading base, sparsely to moderately
pubescent with short or sometimes longer, straight to wavy hairs. Stipules
2.0–4.5 mm long, the bristles 3–7 on each side, variously 2–7 mm long. Leaf
blades 1.2–7.0 cm long, 2–15 mm wide, narrowly elliptic to oblong-lanceolate,
angled at the base, angled to a sharply pointed tip, the margins minutely
pubescent with stiff, spinescent hairs, flat, the surfaces glabrous
short-hairy, the undersurface finely glandular-hairy along the midvein, the
venation with the midvein and usually 2–4 pairs of pinnate, secondary veins
visible. Flowers solitary or paired in the leaf axils, produced at most of the
nodes. Calyces deeply 2-lobed, the lobes 2–8 mm long, lanceolate to triangular,
often unequal. Corollas 10–13 mm long, bright white, externally glabrous or
nearly so, internally bearded in the throat with the pubescence usually
extending onto the lobes, the tube 6–7 mm long, the lobes 5–6 mm long,
lanceolate. Styles 2-branched, with long, linear stigmas. Fruits 6–7 mm long,
4–5 mm wide, ellipsoid to broadly ellipsoid, with well-developed longitudinal
ridges on the sides, becoming corky, eventually splitting longitudinally into
halves. 2n=28. June–September.
Scattered in the
Mississippi Lowlands Division and the southern portion of the Ozarks north
locally to Lincoln and Hickory Counties (eastern [mostly southeastern] U.S.
west to Kansas and Texas). Margins of ponds, lakes, and sinkhole ponds,
bottomland prairies, mesic swales of sand prairies, fens, banks of streams and
rivers; also levees, ditches, roadsides and moist, open, disturbed areas.
The corky fruits
of this species apparently float and are probably dispersed at least in part by
water. As in most weedy plants growing in seasonally wet sites, the plants are
notably variable in overall size as well as the sizes of the leaves and
stipules. And, as in most Rubiaceae, there is a range of variation in
pubescence among different plants. Noticeably hairy plants have been called f. hirsuta
(Pursh) Fernald.