keys to the vascular plants of east texas - Botanical Research ...

1070 POACEAE/SPOROBOLUS
SPOROBOLUS R. Br. DROPSEED
Annuals or perennials; ligule minute, largely a ciliate fringe on vestigial membranous base; inflorescence
a contracted spike-like or open panicle, often partly included within a leaf sheath; spikelets
1-flowered, awnless; disarticulation above glumes; glumes usually unequal, shorter than or
equaling the lemma, 1-veined; lemma usually 1-veined (faintly 3-veined in S. ozarkanus and S.
vaginiflorus); fruit usually falling free from lemma and palea; seed coat not fused to the pericarp
(therefore not a true caryopsis—Brandenburg (2003) refers to the fruit as a “modified caryopsis”);
pericarp becoming mucilaginous when moist in most species (Peterson et al. 2003); stamens 3.
AA taxonomically problematic C4 genus of more than 160 species that grow in tropical, subtropical,
and warm-temperate regions throughout the world (Peterson et al. 2003). The genus
has been thought to be related to Eragrostis (Clayton & Renvoize 1986), but recent molecular
data indicate a closer relationship to Muhlenbergia (Hilu & Alice 2000). However, molecular
data (Ortiz-Diaz & Culham 2000) also indicate that the genus is monophyletic if circumscription
is expanded to include four related species (including two previously placed in Eragrostis).
The spikelets are similar to those of Muhlenbergia species, though Muhlenbergia spikelets
have the lemma 3(–5)-veined and usually awned (Hatch et al. 1999). Some have edible grains, a
number of species are important native pasture grasses, and some are considered significant
weeds (Watson & Dallwitz 1992). Some species have part of their inflorescences permanently
enclosed within leaf sheaths; these species “tend to reproduce cleistogamously, that is, pollen
cannot be dispersed and obligately pollinates the same floret” (Yatskievych 1999). The common
name DROPSEED refers to the grain usually falling free from the lemma and palea. Mucilage
from the pericarps is thought to be transferred to the seeds, thus allowing them to stick to various
animals and be dispersed (Davidse 1986; Brandenburg 2003). (Greek: sporos, seed, and bolos,
a throw, referring to the free seeds, which are sometimes forcibly ejected when the mucilaginous
pericarp dries—Peterson et al. 2003) (subfamily Chloridoideae, tribe Cynodonteae)
REFERENCES: Clayton 1965; Riggins 1977; Brown 1993; Wipff & Jones 1995; Peterson et al. 1997, 2003;
Weakley & Peterson 1998; Hilu & Alice 2000; Ortiz-Diaz & Culham 2000; Brandenburg 2003.
1. Inflorescences contracted, very narrow, 2.2 cm or less wide (often much less), the branches appressed;
plants annual or perennial.
2. Plants annual; inflorescences usually 1–5 cm long.
3. Glumes as long as or longer than floret; lemma 3-veined (midvein more conspicuous);
lower leaf sheaths papillose-pilose ______________________________________________ S. ozarkanus
3. Glumes shorter than floret; lemma 1-veined, rarely 3-veined; lower leaf sheaths often glabrous,
usually not papillose-pilose.
4. Lemma and palea glabrous; spikelets (1.3–)1.6–2.8 mm long; species known in East TX
only from w margin of area _________________________________________________ S. neglectus
4. Lemma and palea appressed-pubescent (use hand lens or dissecting scope); spikelets
usually 3–5(–6) mm long; species widespread in East TX ________________________ S. vaginiflorus
2. Plants perennial; inflorescences usually 5–40 cm long.
5. Spikelets 1.4–2(–2.6) mm long; inflorescences mostly exserted from leaf sheaths; lower
glume 1 mm or less long _______________________________________________________ S. indicus
5. Spikelets 3–7.5 mm long; inflorescences at least partly included within leaf sheaths OR not
so; lower glume 0.9–4 mm long.
6. Panicle branches in distinct whorls (3 or 5 branches at lower nodes); lemma � equal in
length to upper glume; inflorescences mostly exserted from leaf sheaths; species known
in East TX only from Bexar Co. at extreme sw corner of area _____________________ S. purpurascens
6. Panicle branches not in distinct whorls (usually 1–2 branches per node); lemma usually
longer than upper glume; inflorescences at least partly included within leaf sheaths;
species widespread in East TX.