POACEAE (GRAMINEAE) (Grass Family)

Plants annual or perennial, herbaceous or sometimes woody, sometimes with rhizomes, stolons, corms, bulbs, and/or tuberous thickenings, monoecious, dioecious, or more commonly with perfect flowers. Aerial stems circular or less commonly somewhat flattened, jointed at the nodes and often hollow between them. Leaves basal and/or alternate, usually noticeably 2‑ranked, “grasslike” (usually linear), parallel‑veined, the bases strongly sheathing, the sheaths open or less commonly closed (fused around the stem all or most of the way to the summit), usually with a scalelike or hairy ligule on the inner (upper) surface at the tip (summit), the leaf blades sometimes lacking or highly reduced. Inflorescences with the basic units spikelets, these 1 to many, terminal and often also axillary, arranged into spikes, racemes, panicles, or variations of these types, usually composed of 2 basal, sterile bracts, the lower (first) and upper (second) glumes, and 1 to many individual florets (flowers) arranged in alternate, 2‑ranked fashion (rarely appearing spirally arranged elsewhere) along a short rachilla (spikelet axis). Florets usually consisting of an outer bract (with regard to the rachilla), the lemma; and an inner bract (with regard to the rachilla), the palea; these facing each other and subtending the staminate and/or pistillate portions of the floret. Perianth reduced to 2(3) lodicules (minute, fleshy scales) located around the base of the ovary and/or stamens, rarely absent. Stamens 1, 2, 3, or 6, but usually 3 (–120 elsewhere). Pistil 1, structurally with 3 fused carpels, but functionally with 1 carpel containing 1 ovule. Styles 2 or less commonly 1, 3, or absent. Stigmas 2 or less commonly 3. Fruits specialized achenes known as caryopses or grains (differing from standard achenes in that the fruit wall [pericarp] is fused to the seed coat), except in some species of the tribe Eragrostideae, which have achenes. Six hundred to 650 genera, about 10,000 species, worldwide.

The Poaceae are a large and complex family with many unusual floral and vegetative structures that require an extensive, specialized terminology. Many of these structures are illustrated in Plate 119. Readers wishing to learn more details of grass morphology and anatomy should consult any of several good references available on the family, notably Gould and Shaw (1983) and Clark and Pohl (1996).

Grasses are extremely important economically and have been cited as a critical factor in the development of human civilization. Grain crops, such as rice (Oryza sativa), wheat (Triticum aestivum), corn (Zea mays), barley (Hordeum vulgare), rye (Secale cereale), oats (Avena fatua var. sativa), sorghum (Sorghum bicolor), and various millets (several species of Echinochloa, Panicum, Pennisetum, and Setaria, among others) are major sources of food for both humans and livestock. Sugar cane (Saccharum officinarum) is a principal source of sugar. Other species, including several bamboos, also are eaten. Numerous native and introduced species are grazed by livestock in pastures or open ranges and as hay. The Poaceae also provide structural materials for buildings, furniture, fences, baskets, and numerous implements, ranging from chopsticks to fishing poles. Grasses are important in erosion control, soil conservation, land reclamation (following mining or other disasters), and as turf for lawns and golf courses. A variety of species are cultivated as ornamentals. On the negative side, a number of species are commercially important weeds. The family, which consists almost entirely of wind‑pollinated species, also contributes heavily to hay fever problems, although a relatively small number of genera produce most of the allergenic pollen.

Although only the fifth largest family of flowering plants in terms of number of species (ranking behind the Asteraceae, Orchidaceae, Fabaceae, and Rubiaceae worldwide, but second only to the Asteraceae in Missouri), the Poaceae probably are the most abundant angiosperm family in the world in terms of number of individuals and land coverage. Therefore, the family is extremely important ecologically. Grasses are major components of nearly all global vegetation types and contribute significantly to the total net photosynthesis and production of biomass worldwide. They interact ecologically and/or physiologically with a huge number of different plants, fungi, insects, birds, mammals, and other fauna. Herbivores and in particular seed eaters rely heavily on grasses for food. These include most North American game mammals, as well as songbirds and migratory waterfowl. Other animals, particularly some insects, require grasses as a substrate for laying eggs and a larval food source to complete their life cycles. A number of symbiotic and parasitic viruses, bacteria, fungi, and animals similarly have obligate relationships with grasses. Grasses also provide shelter and hiding places from predators for many small animals.

Physiologically, grasses have been divided into two groups, based upon variations in photosynthesis that correlate with certain anatomical features (for reviews, see Brown, 1977; Campbell, 1985; Clayton and Renvoize, 1986; or most textbooks in plant anatomy, physiology, or taxonomy). The “normal” photosynthetic pathway is predominate in angiosperms and is known as the C₃ pathway. Globally, it characterizes species of the subfamilies Bambusoideae sensu lato, Centothecoideae, and Pooideae, as well as about 90 percent of the Arundinoideae, about 20 percent of the Panicoideae, and two genera of the Chloridoideae (for details of classification, see below). Such species are commonly classified as “cool‑season grasses.” They tend to be more common in cooler or wetter climates, and the Missouri species tend to flower in the spring and produce the majority of vegetative growth from the fall to the spring, with a period of dormancy during the hotter portion of the growing season and sometimes a short period of dormancy during the coldest portion of the winter. Many common forest species and some prairie grasses, particularly those found in the Glaciated Plains Division, are cool‑season grasses.

The C₄ pathway has developed independently in more than 20 angiosperm families, presumably in response to high‑light conditions and/or hotter, drier climates. It characterizes nearly all species of the subfamily Chloridoideae, as well as about 10 percent of the Arundinoideae and about 80 percent of the Panicoideae. Grasses with C₄ physiology have a series of anatomical features centered on details of the sheath surrounding the vascular bundles (veins) of the leaves that are known collectively as the Kranz syndrome (a discussion of these details is beyond the scope of the present volume). The physiological and anatomical specializations of such species allow them to photosynthesize more efficiently in conditions of high light and low soil‑moisture, and they have been classified as “warm‑season grasses.” In Missouri, warm‑season grasses usually produce vegetative growth throughout the growing season, tend to flower during the summer months, and have a period of dormancy during the winter months. Many common species of glades and prairies are warm‑season grasses.

Classification within the Poaceae is complex and relationships among some genera still are not completely understood. Traditionally, the family was split into two subfamilies, with varying numbers of tribes. Following a large number of morphological, anatomical, cytological, embryological, and phytochemical studies (and most recently molecular‑based research), most agrostologists (grass specialists) now divide the family into five to eight subfamilies, each of which can be divided further into a number of tribes. These subfamilies are the Arundinoideae, Bambusoideae (sometimes split into three subfamilies), Centothecoideae (sometimes considered part of the Arundinoideae), Chloridoideae, Panicoideae, and Pooideae. The classification adopted in the present volume is based upon the summary work of Clayton and Renvoize (1986), who recognized 6 subfamilies and 40 tribes worldwide, of which 17 tribes in all 6 subfamilies are represented by native and introduced species in Missouri. In the treatment below, the genera are arranged alphabetically in each tribe and the tribes are presented alphabetically, without regard to subfamily. See Campbell (1985) and Clayton and Renvoize (1986) for attempts to provide technical keys to subfamilies and tribes of grasses.

Determination of genus and species for a specimen in this family requires patience and detailed observations using a hand lens or dissecting microscope. Relatively subtle characters of spikelet or ligule morphology may be important for determinations. The components of mature spikelets need to be studied carefully, as sterile florets may exist and/or some structures, such as the lowermost glume or the palea of a sterile floret, may be absent or highly reduced in some species. Collectors should note that rootstocks and mature fruits often are necessary for accurate determinations. Although several keys have been published to distinguish grasses based exclusively on vegetative characters, in practice this can be very difficult, and vegetative specimens should be avoided. The determination of annual vs. perennial growth form is important for keying plants of most genera of grasses, but it can be difficult to establish for some plants. In general, annual species have “soft bases” that usually can be pulled from the ground with their roots relatively intact, and they show no evidence of rhizomes, tuberous root thickenings, or previous years’ stem or leaf bases. Perennials have “hard bases” that are frequently (but not always) difficult to pull up with intact rootstocks, often have rhizomes, tuberous roots, or otherwise thickened rootstocks, and usually have remnants of previous years’ stem and/or leaf bases.

The key to tribes below is adapted from the excellent key of Gould (1975) and also incorporates elements from Gleason and Cronquist (1991). The couplets are based on characters that traditionally have been found in keys to grass genera, and references to microscopic structures are minimized. As a result the key is quite lengthy, and several of the tribes, notably the morphologically diverse Aveneae, Eragrostideae, and Poeae, key out in a number of different places. This is unavoidable, as the technical characters that define the tribes are not amenable to inclusion in a practical key.