Morphology of Grasses in Illinois

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Learn more about the history of different types of grasses in Illinois and how the grasses have evolved over time.
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“Grasses” by Robert H. Mohlenbrock, is an interesting read for anyone looking to learn more about the grasses of Illinois.

Grasses: Bromus to Paspalum (Southern Illinois University Press, 2002) by Robert H. Mohlenbrock, is an insight into the variety of grasses growing in the Illinois area. Find out which grasses grow best and how they have evolved. If you live in the Illinois area, see if any of the grasses grow anywhere near you. This excerpt can be found in the “Introduction.”

Grasses belong to the family Poaceae (also  called  Gramineae). Until recently, most botanists grouped  grasses and sedges ( Cy­ peraceae) in the order Graminales (or Poales). Anatomical, morphological, and other more recent evidence show that, in addition to grasses and sedges, other families such as the Xyridaceae, Commelinaceae, Pontederiaceae, and Juncaceae share some of the same characters and  should be grouped together. This view is followed here so that these six families are considered to com- prise the Commelinales. The Xyridaceae, Commelinaceae, Ponte­ deriaceae, and Juncaceae are treated in Flowering Plants: Flowering Rush to Rushes ( 1970); the Cyperaceae will be forth­ coming in two subsequent volumes.

The nature of grass structures generally  is so different  from that of other flowering plants that a special terminology is applied to grasses. A thorough understanding of these terms will enable one to identitify more readily an unknown specimen.

Grasses are annuals, biennials, or perennials. Annuals have tufts of fibrous roots and live for a single growing season. Peren­nials may be tufted, or they may have rhizomes (horizon­tal, root-producing stems below ground, or they may have stolons (horizontal,  root-producing  stems  above ground, or a short, thick, subterranean crown.

The stem which bears the leaves and inflorescence is called the culm. While the culm may be hollow or solid, the nodes (where the leaves arise) are nearly always solid. The culms may be simple or branched. Often they are jointed (geniculate) near the base. Culms may be erect, divergent (spreading), or prostrate and matted.

Grass leaves are borne at the nodes in two planes along the culm. This condition is referred to as 2-ranked. Some­times, because of a twisting of the culm, the 2-ranked condition is not apparent. The leaf is composed of a blade and a sheath. The sheath wraps around and encloses a portion of the culm. If the margins of the sheath are united, forming a cylinder, the sheath is closed; if the margins are not united, the sheath is open. The blade is the free  portion of the leaf. It is parallel­ veined and generally elongated, although some grasses with rather short, broad  blades occur. The blades normally are flat, but they may be folded or inrolled into a slender tube (involute). Along the inner face of the leaf, where the blade adjoins the sheath, there is often a ciliate, membranous, or cartilaginous structure of varying size and shape known as a ligule. In some grasses, some of the leaves are not blade­ bearing, therefore consisting merely of sheaths.

The inflorescence is the aggregation of a group of spikelets (the  basic unit  of the  grass inflorescence). An elongated, simple axis with pedicellate spikelets borne along it is called a raceme; if the spikelets are sessile along the simple axis, the in­ florescence is a spikeShort-pedicellate spikelets crowded on an elongated, simple axis make up the spike-like raceme. If the inflorescence is branched, and the spike- lets are pedicellate, the term used is panicle. The pan­icle may be very wide-spreading and open (diffuse), or it may be contracted so much as to resemble a spike. This latter situation gives rise to the term spike-like panicle.

The tip of each branch of the panicle normally bears a spike­ let, although in Setaria and Cenchrus, some of the branch tips are sterile and modified into bristles.

The spikelet is composed of an axis, called the rachilla, along which are borne bracts in two ranksThe lowest two bracts bear no flowers in their axils. These “empty” bracts are the glumes. They  frequently are unequal in size although rarely un­like in texture. Both glumes are essentially lacking in Leersia and Zizania, while the first (lower) glume is usually absent in Paspa­ lum, Digitaria, Eriochloa, and Lolium. Elymus hystrix usually has its glumes reduced to awns. A sharp ridge down the back of a compressed glume is called  the keel. Sometimes the entire spikelet falls at maturity, while in other species the glumes remain behind. In the first case, the spikelet is said to disarticu­late below the glumes, while in the latter case, it is said to dis­articulate above the glumes.

Above the glumes are one or more  bracts which usually bear a flower within. These fertile bracts are the lemmas. Facing each lemma is a usually somewhat smaller palea. Between the lemma and the palea is the flower. In Chasman­ thium and Panicum, the lowest lemma does not produce a flower, while in Melica and Chloris, the uppermost lemma is sterile. In Phalaris, the two lowest lemmas are reduced to scales. Lemmas generally are of the same texture as the glumes, although the fertile lemma in Panicum is indurated. The callus of a lemma may refer to a swollen, hardened area at its base (as in Stipa and Aristida )  or a tuft  of hairs  (as  in Calamagrostis ). Lemmas  also may be keeled. Spikelets with a single fertile lemma are said to be 1-flowered, while those with two or more fertile lemmas are several-flowered.

The palea is smaller than the lemma and usually of more deli­ cate  texture. In Panicum hians, the palea becomes indurated at maturity. The palea is often absent in Agrostis. Most paleas have two keels down the back.

The grass flower is much reduced from the flower of Liliaceae and  other more showy flowering plants. It consists of three sta­mens (occasionally 1-6) and one pistil. Each stamen bears a 2- celled anther. Each pistil is 1-celled, with but one ovule, but there usually are 2-3 styles. At the base of the flower usually are found 2-3 small scales thought to represent the perianth. These scales are the lodicules.

Most grasses have a fruit known as a caryopsis, or grain. The seedcoat of the single seed is united directly to the matured ovary wall (pericarp). (The pericarp is free from the seed in Eleusine, Crypsis, and Sporobolus). At maturity, the grain drops free from the lemma and palea, or it may fall while enclosed by the lemma and  palea.

The lemma, palea, and enclosed flower comprise the floret.


Reprinted with Permission fromThe Illustrated Flora of Illinois Grasses Bromus to Paspalum by Robert H. Mohlenbrock and Published by Southern Illinois University Press.

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