Grasses of the Intermountain Region

Key to Tribes

POACEAE 5 Intermountain Region ecosystems, but it is also welcomed as a source of early spring feed in some parts of the United States. Cynodon dactylon (bermudagrass) is listed as a noxious weed in some jurisdictions; in others it is valued as a lawn grass. Although grasses are widespread and often dominant in open areas, all evidence points to their having originated in forests, probably in the Southern Hemisphere, by the late Cretaceous to early Paleocene (70–55 mya). Recent evidence from phytoliths (isolated silica bodies commonly produced inside the epidermal cells of grasses and some other plants) embedded in fossil coprolites strongly suggests that grasses evolved earlier in the Cretaceous than previously thought. Living representatives of the three earliest lineages of the grass family, together comprising about 30 species, are perennial, broad-leaved plants of relatively small stature, native to tropical or subtropical forests in South America, Africa, southeast Asia, some Pacific Islands, and northern Australia. The major diversification of the family probably occurred in the Oligocene (34–24 mya), and was associated with climatic changes that produced more open habitats. All major lineages of the grass family were present by the middle of the Miocene (24–5.5 mya); C4 photosynthesis in grasses had also evolved by then. Grasses exhibit three variants of C4 photosynthesis. They are associated with Kranz leaf anatomy, in which the vascular bundles are surrounded by a cylinder of cells (bundle sheath cells) that, because they contain starch, stain dark with iodine. When seen in cross-section, the cylinders form rings or wreaths (Kranz in German) around the vascular bundles. C4 grasses also have three or fewer mesophyll cells between adjacent vascular bundles. C3 grasses do not have starch-containing bundle sheath cells and have more than three mesophyll cells between the vascular bundles. Both these features can be seen in handmade cross-sections (Hattersley and Watson 1976); they are, admittedly, more useful in the herbarium than in the field. Key to Tribes Mary E. Barkworth 1. Culms perennial, woody (except in greenhouse plants) often developing complex branching systems from the upper nodes; leaves on the upper portion of the culm or distal on the branches usually pseudopetiolate (see p. 7) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bambusoideae 1. Culms usually annual, sometimes overwintering, rarely woody, sometimes branching from the upper culm nodes but branching not complex; leaves usually not pseudopetiolate. 2. Spikelets almost always with two dissimilar florets, the lower floret sterile or staminate, frequently reduced to a lemma, sometimes missing, the upper floret usually bisexual, sometimes unisexual or sterile; upper floret well developed, usually with a leathery to hard, usually unawned, lemma or reduced, with a hyaline, often awned, lemma, awn attached at or near the top of the lemma; rachilla not prolonged beyond the base of the upper floret (Panicoideae). 3. Glumes flexible, membranous, the lower glume usually shorter than the upper glume, the upper glume subequal to or exceeded by the upper floret; lower lemma similar in texture to the glumes; upper lemma usually leathery to hard; spikelets usually single or in pairs; disarticulation below the glumes . . . . . . . . . . . . . . . . . . . 14. Paniceae 3. Glumes usually stiff, leathery to indurate, often subequal, at least 1 and usually both exceeding the upper floret (excluding the awn); both lemmas hyaline; most spikelets in pairs or triplets, at least one spikelet in the group usually sessile; disarticulation often in the inflorescence branches, below the sessile spikelet, sometimes below the glumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15. Andropogoneae 2. Spikelets with other than 2 florets or, if with 2, the lower floret bisexual or the upper floret awned from the back or base of the lemma or the spikelets bulbiferous; glumes usually membranous; rachilla often prolonged beyond the base of the distal floret. 4. All or most spikelets bulbiferous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Poeae (in part) 4. All or most spikelets sexual. 5. Spikelets with 1 floret; lemma terminating in a 3-branched awn, the lateral branches sometimes greatly reduced; callus well developed; ligules of hairs or a ciliate membrane, the cilia longer than the membranous base. . . . . . . . . . . . . . . . . . . . . . 13. Aristideae 5. Spikelets with more than 1 sexual floret or, if with only 1, the lemma not terminating in a 3-branched awn; callus sometimes well developed; ligules membranous, of hairs, or a ciliate membrane. 6. Spikelets with 1 sexual floret; glumes absent or less than ¼ the length of the adjacent floret; lower glume, if present, without veins; upper glume, if present, without veins or 1-veined. 7. Inflorescence 1-sided, spikelike; spikelets triangular in cross section . . . . . . . . . . . . 3. Nardeae 7. Inflorescence paniculate; spikelets laterally...