Poaceae

Grasses are often challenging to identify because of the large number of species, their variability, and their small flowers with specialized parts and unique terminology. Non-woody plants with stems jointed, round to somewhat flattened in cross section, often hollow between the nodes; leaves alternate and basal in 2 vertical rows, leaf sheaths with overlapping margins that are usually not fused; a ligule composed of a fringe of hairs or membranous (paper-like) collar projecting above the top of the sheath; leaf blades usually linear with parallel veins; individual flowers (florets) and clusters of flowers (spikelets) each usually with two bracts; and fruit a grain (or less commonly an achene). Features of the spikelet, including its size, how it is compressed, presence or absence of glumes, number of flowers, presence of sterile or incomplete flowers, number of veins on glumes and lemmas, presence or absence of awns, and how spikelets are arranged, greatly help with accurate identification.

Flowers are arranged in spikelets consisting of a central jointed stalk that usually has 2 overlapping bracts (glumes) at its base and bears one or more small overlapping flowers in two rows above these. Each individual flower is arranged alternately and also has two bracts or scales at its base, the outer bract called a lemma and the inner called a palea. The palea is often translucent, smaller than the lemma, and partially enclosed by it. The glumes may or may not be equal in size). The glumes and/or lemmas may have straight or bent, needle-like projections (awns). The spikelets may be arranged in a spike, panicle, cyme or raceme. Each individual flower may produce both pollen and seeds, or may produce only pollen or seeds, either on the same plant or on separate plants. Sepals and petals modified into tiny swollen translucent scale-like structures at the base of stamens; these are not important for identification. Stamens usually 3 but may range from 1-6, or rarely are numerous; pollen-producing structures (anthers) often shaped like an arrowhead. Carpels 3, but often appearing as 2, fused; ovary situated above point of attachment of other flower parts; stigmas 2-3, feathery. Some flowers are sterile, and lack an ovary and sometimes stamens as well.

Fruit a solitary seed enclosed in a membrane, with 2 scales (lemma and palea) at its base. May be a grain (when the seed coat is fused to the seed) or an achene (when the seed coat is not attached to the seed). Fruits may fall as an entire spikelet, including the glumes, or as an individual flower with its lemma and palea.

Leaves alternate or basal, in 2 vertical rows on stem, usually consisting of sheath, ligule and blade. Leaf sheaths tightly encircling stem like a sleeve for some distance above the nodes, their edges usually separate but overlapping (fused in Bromus and Glyceria). At the junction of the sheath and the leaf blade, the sheath may project upward forming a collar or rim around the stem. This projection is termed a ligule, and may be membranous (thin and papery), represented by a row of hairs, or absent, and there may be 2 ear-like projections called auricles that tend to clasp the stem. Leaf blades are usually linear with smooth edges and flat with parallel veins, but may sometimes be absent or rolled into a tube. The blade may be continuous with the sheath, or there may be a stalk between the sheath and the blade.

Stems are usually round in cross section and jointed, with conspicuous and swollen nodes. The stem is often hollow between the nodes.

Rhizomes or stolons are often present. Roots fibrous, often developing from the stem or leaves except in young plants.