Inflorescences are described by many different characteristics including how the flowers are arranged on the peduncle, the blooming order of the flowers and how different clusters of flowers are grouped within it. These terms are general representations as plants in nature can have a combination of types.
That portion of a flowering culm upward from the node at the base of the uppermost leaf is the inflorescence. Since true flower parts in grasses are inconsequential in identification, spikelet arrangement will be used for description of the inflorescence type. The inflorescence of grasses has been classified variously. Nonetheless, there are three basic types: the panicle, the raceme, and the spike.
Spike – the spikelets are attached directly (sessile) to the central axis of the inflorescence (rachis), without a subtending stalk. Raceme the spikelets are attached directly to the rachis by a single stalk (pedicel). Panicle – the spikelets are born on branches from the central axis of the inflorescence.
Certain problems arise when using only these three inflorescence types to describe a number of genera. Allred (1982) suggested that descriptive adjectives be used to qualify the pattern variation. For example, green spangletop (Leptochloa dubia) would be a panicle of racemose primary branches, blue grama (Bouteloua gracilis) is described as a panicle of spicate primary branches, and windmillgrass (Chloris verticellata) a panicle of digitate spicate branches.
The genera, Andropogon and Hordeum, usually have both sessile and pedicellate spikelets on the main axis and are spicate racemes. If the main axis is branched, with the branches having pedicellate spikelets borne singly along the branch, it is a panicle of racemose branches.
Pohl (1968) termed the inflorescence of Andropogon a rame, which is an inflorescence branch with repeating pairs of sessile and pedicellate spikelets. Therefore, the genus Andropogon, with certain exceptions is a ramose panicle. Some genera such as Phleum have very short pedicellate branches, and are called spikelike panicles.
Within an inflorescence the term rachis refers to the central axis. For branched inflorescences, the branches from the rachis are called the primary branch. Further branching leads to secondary branches, tertiary branches, and so forth. Another term often used in inflorescence description is peduncle, defined as the stalk of a cluster of spikelets (Gould, 1978). Spikelets. The basic unit of the grass inflorescence is the spikelet.
The central axis of the spikelet is the rachilla to which spikelet parts are attached. Attached to the lower most portion of the spikelet are two (sometimes one or none) sterile bracts called glumes . Above the glumes on the rachilla may be one or more florets. A floret consists of two bracts, the lemma and the palea, which enclose the grass flower. The grass flower consists of a vestigial perianth called lodicules and the stamen and pistil. Lodicules increase in size at flowering and force the lemma and palea open such that stigma and anther exertion can occur.
Characteristics of the glumes, lemma, and palea are usually the mechanistic basis for separation of grass species. They, like the leaf from which they were derived, have midribs which vary from obscure to prominent and nerves (veins) which vary in number, location, and prominence. Often the midrib of the lemma is extended into an awn. Awns may be fused at the base as in Aristida, twisted as in Stipa, or bent or geniculate as in Danthonia. Most often the palea fits within the enrolled edges of the lemma and does not often offer distinguishing characteristics.
Illustration of a bristlegrass (Setaria) spikelet.
Grasses shed their seeds as naked caryopsis, florets, spikelets, or entire inflorescences. Shedding of their seeds is accomplished by disarticulation. Certain points of attachment form abscission layers (points of weakness) which readily break at maturity thus disseminating the seeds or fruits. Members of the genus, Poa and related genera, typically disarticulate above the glumes and within the spikelet, while Panicum and related genera disarticulate below the glumes shedding the entire spikelet.
Some species shed naked caryopses such as Sporobolus. Others shed the entire inflorescence such as Schedonnardus. Compression: Spikelets are seldom round, but usually are flattened or compressed.
Lateral compression occurs when the bracts of the spikelet are flattened from the sides, while dorsal compression occurs by flattening from the back of the bracts. Poa and related genera generally exhibit lateral compression. Dorsal compression is most often found in spikelets of Panicum and related genera where a sterile floret is borne below the perfect one.