51. Sporobolus R. Br. (dropseed)
Plants annual or perennial, sometimes with rhizomes, forming
tufts or clumps. Flowering stems mostly erect or ascending (occasionally
spreading in Sporobolus neglectus, S. ozarkanus, and S. vaginiflorus),
glabrous or uncommonly roughened. Leaf sheaths rounded on the back or slightly
angled, glabrous or variously roughened or hairy, the ligule a line or band of
hairs, these often fused at the base into a minute membrane. Leaf blades flat,
folded lengthwise, or with inrolled margins, glabrous or variously roughened or
hairy. Inflorescences open panicles or appearing spikelike, but then at least
the lowermost branches somewhat elongate and noticeable or the inflorescence
linear in outline, terminal or in some species also lateral (if present, the
axillary inflorescences usually all or mostly enclosed by the subtending leaf
sheaths). Spikelets with 1 floret, short‑stalked, slightly to moderately
flattened laterally, disarticulating above the glumes, the glumes sometimes
also shed with age. Glumes slightly to strongly unequal in size, the lower
glume mostly shorter than the other one and often shorter than the rest of the
spikelet, the upper glume often about as long as or somewhat longer than the
rest of the spikelet, 1‑nerved, keeled, glabrous or roughened along the
midnerve, awnless. Lemma 1‑nerved (faintly 3‑nerved in S.
ozarkanus and S. vaginiflorus), awnless, rounded on the back or
keeled, the base glabrous or minutely hairy, otherwise glabrous to roughened
along the midnerve or hairy with short, straight, light‑colored hairs
appressed to the surface between the nerves. Palea sometimes splitting
longitudinally at maturity to expose the fruit. Stamens 3. Fruits obovate to
oblong or nearly circular in outline, the outer coat shed easily when
moistened, sometimes turning gelatinous. One hundred to 160 species, nearly
worldwide, mostly in tropical and warm‑temperate regions.
Those species of Sporobolus in which some or all of
the inflorescences remain enclosed in the subtending leaf sheaths tend to
reproduce cleistogamously, that is, pollen cannot be dispersed and obligately
pollinates the stigmas of the same floret. The percentage of seed set in these
inbred plants is usually quite high.