DEPARTMENT OF BOTANY
Grass Herbarium at US
Grasses occur on all continents. They dominate many open habitats from arctic tundra and alpine, steppes and prairies and tropical savannas, in deserts, saltpans, and high mountains all around our planet. Many species occur in forests, woodlands, and jungles. The grass family includes over 11,500 accepted species, some grass taxonomists (Agrostologists) estimate there are as many as 13,000 species, considering their taxonomy is still a work in progress.
The family includes many crop and major forage and turf species. Grains and sugars of barley, rye, wheat, oats, millets, sorghums, sugarcane, corn or maize, tef, and rice, are food staples around the world, and some provide us with interesting beverages. There are many grasses cultivated for forage, and wild grasses provide rangeland forage and grains for wildlife and domestic livestock. Thus, the old saying, “All flesh is grass”. Woody bamboos provide structural and ornamental uses, and sometimes the young shoots are eaten. Many turf and ornamental grasses grace our landscapes. Some provide biofuels and pulp for paper.
As in many large plant families, genera and their species are arranged in a hierarchical system of subfamilies, tribes, and subtribes. Classifications serve two purposes. Firstly, ease of identification, and secondly, understanding of evolution and biogeography. Past classifications of grasses depended mostly on similarities of vegetative structures and arrangements of spikelets, circumscribing genera and higher groups base on combinations of similarities in these structures. Many of the character combinations used in the past have since proven to have evolved through convergence in form. Studies of anatomy, embryology, cytology, and physiology, exposed much of the convergence and put our classifications on a modern footing (Clayton & Renvoize 1986, Tzvelev 1989, Watson and Dallwitz 1992), prior to the advent of molecular systematics. However, many finer nuances of phylogenetic evolution remaine obscure. Molecular systematics, combining DNA sequencing with powerful computation methods, have provided us with many new insights on phylogeny, resulting in more refined classifications. Our goal is a robust classification based on the principle of monophyly, which is essential to interpreting character evolution and biogeography. We are making good progress, but much more work remains ahead. The grasses are currently classified in 12 subfamilies, 52 tribes, 90 subtribes, and more than 768 genera (Soreng et al. 2017 - https://onlinelibrary.wiley.com/doi/abs/10.1111/jse.12262).
The subfamilies are often divided into Cool Season (Pooideae) or Warm Season grasses (Aristidoideae, Arundinoideae, Micrairoideae, Danthonioideae, Chloridoideae, Panicoideae; also known as the PACMAD clade). Grasses with C4 photosynthesis predominate in dry to humid climates with warm season precipitation. Those with only C3 photosynthesis predominate in cool to frigid climates with cool season precipitation, and few C3 grasses thrive in warm season climates, apart from those of Danthonioideae, Arundinoideae, Micrairoideae and basal Panicoideae.
There are many specialized terms for grass vegetative and floral structures. Grass morphology is streamlined, with flowers so reduced in size that they are not readily recognized as such by us or pollinators. Grasses, apart from a few primitive genera, are primarily wind pollinated. The flower perianth is reduced to 2 or 3 (rarely more), small, colorless, membranous or fleshy, bracts (lodicules), often less than 2 mm long, that are produced below the whorl of stamens (usually 3) and a single pistil (usually with 2 or 3, usually plumose styles). Each flower is enclosed in a floret composed of a subtending lemma (a rudimentary leaf sheath) and a palea (a two keeled, inner bract). The paleas is homologous with the prophyll that protects new vegetative buds, and probably the coleoptile, which is the first vegetative structure to emerge from a germinating seed. One or more florets are usually subtended by 2 sterile bracts (glumes), in a structure collectively called a spikelet. The glumes and florets are distichously arranged on opposite sides of a central axis (rachilla). The grass spikelet is actually a primary inflorescence. The collection of spikelets emerging from one flowering shoot (culm) is generally called an inflorescence, but technically is called a synflorescence. These secondary inflorescences are diverse in form, ranging from panicles to racemes and spikes, or combinations of these, such as a panicle of racemes, or digitate racemes. Spikelets break up at maturity in different ways. In many genera the whole spikelet falls intact, sometimes in sets of 2 or more spikelets. In other genera the floret(s) disarticulate above the glumes, either with the upper adjacent rachilla internode, or by falling free from the rachilla. In any case, each break points is referred to as a callus. Grass shoots are composed of nodes and internodes with leaves arising from the tops of the nodes. Grass leaves are composed of a sheath surrounding a culm, and a blade that diverges from the sheath at a juncture called a collar. Ligules are tongues or rims of tissue (sometimes all or apically hairy) that project upward from the inner surface of the sheath at the collar, and sometimes also on the outer side (abaxial ligules). Lateral extensions from the collar margins are call auricles. Sometimes the blade is quite narrow at the base, a condition called pseudopetiolate. The grass grass fruit or grain (a caryopsis – a single hard seed with a fused pericarp, rarely a utrical - with a loose pericarp) has its own terminology. The caryopsis is unique among plants in having an external, well-developed embryo external to a starchy (infrequently mostly lipid) endosperm. Opposite the embryo, on the ventral side of the fruit, which is often sulcate, is the hilum, which is the mark of the fusion of the nucellar tissue to the ovary wall. The hilum may be linear and extend up to the length of the grain, or be shorter, sometimes reduced to an elliptical, ovate, circular or punctiform outline.
Our herbarium has an extensive collection of bamboo specimens, primarily developed primarily by Floyd McClure and Tom Soderstrom. All of the bamboo specimens are databased, and images of all the pressed specimens are on-line. The bulky collection of woody bamboo culms and rhizomes will require special 3D imaging. Bamboos have C3 photosynthesis, but cross the warm season cool season divide. Tribe Arundinarieae, and tribe Bambuseae subtribe Chusqueinae, more than others, extend well into cool temperate habitats in high mountains and up to 40o N and S (America) or 50oN (in E Asia) in areas with monsoonal rains. The many herbaceous bamboos of tribe Olyreae, are confined to the American tropics, with the exception of Olyra latifolia reaching Africa, and the solitary species of subtribe Bergersiochloinae occurring in New Caledonia. Leaf impression in fossilized dinosaur coprolites found under the volcanic Deccan Plateau in India indicate these grasses were around in Cretaceous times.