Terms to know for grass anatomy:

vegetative structures,

above and below ground

The most obvious parts of a grass are those we see above the ground surface, so I will start there. Grass stems are called culms and they are normally hollow with solid joints called nodes.

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Sheaths and nodes can be variously hairy or smooth.

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blade

sheath

Leaves are made up of two parts: the sheath and the blade. The sheath wraps around the culm, while the blade angles away from the stem. The junction of the blade and sheath is the collar region. Think of a collar wrapping around your neck with an opening at the throat. At the front of the sheath-blade juncture is the ligule, or where a ligule would be, which is the case for the few grasses that lack a ligule. More about ligules later.

Sheaths can be open or closed for various parts of their length. This is a diagnostic character for some genera and species. A closed sheath is like a pullover sweater, which has no opening in the front. It might have a V-neck, which some grass sheaths also have. The V-neck can be plunging (deep and long) or more modest (short).

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Closed sheath of California brome, showing a V-shaped opening.

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The plunging V-shaped opening of the closed sheath of meadow brome.

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The sheath can hug the culm tightly or be inflated like this one of annual wheatgrass.

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Open sheaths have a space between the edges near the top. The edges overlap at some point along the culm, like a wraparound robe.

Sheaths can tear easily, but you can run a needle or fingernail carefully down one edge and feel a slight resistance when you reach the closed part.

Features of the collar region include auricles and hairs, which come in a variety of forms.

Auricles are appendages that are attached where the blade meets the sheath. They range from a small bump or bulge in Festuca to long "ears" that clasp the culm in Hordeum and Elymus.

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Festuca

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Hordeum

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Pseudoroegneria

Elymus

Hairs on the collar can be a diagnostic feature. For example, hairs form a ring around the back and sides of the collar of pinegrass (Calamagrostis rubescens), making identification of non-flowering plants easier. 

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Danthonia species often sport long hairs on the collar, as shown here for California oatgrass.

 

Note the overlapping edges of the open sheath in both Calamagrostis and Danthonia.

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Ligules

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Ligules are the little flap on the top side of the leaf where it joins the sheath. I've been told that the purpose is to keep water from flowing down the leaf blade and collecting between the sheath and the culm, where mold can grow. In central Oregon, where it rarely rains, the ligule could prevent abrasive sand from blowing in.

Whether they have a function or not, ligules are a feature we can use to identify grasses because they come in a variety of shapes and sizes.

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Only one grass genus in our region has no ligule at all: barnyard grass (Echinochloa).

 

This is what it looks like when the ligule is not there. 

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Membranous ligules may be long, short, or in-between. The upper edge can be blunt (truncate), pointed (sharply or in a long tapered tip), lacerate (raggedly torn, cut or cleft) or tipped with a fringe of hairs. The ligule can be attached in a straight line or in an angle up the sides to the center of the back of the ligule (decurrent).

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The decurrent membranous ligule of Cynosurus echinatus.

The raggedly torn membranous ligule of Hierochloe occidentalis.

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The long membranous ligule of Deschampsia cespitosa has a long-tapered tip. 

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The short membranous ligule of Elymus glaucus is truncate.

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In Eragrostis pectinacea, the ligule is a membrane topped with a fringe of hairs.

In Danthonia unispicata, a line of hairs form the entire ligule.

Leaf blades also have distinctive shapes and adornments. Some blades are flat, others are rolled. Orchardgrass leaves are folded in the bud and retain a crease down the center of the leaf. Among the fescues, some have flat leaves while others are rolled in a way that you have to tear them to get them to get them open.

Blades

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Leaf blades of Festuca elmeri and F. subuliflora are flat.

Leaf blades of the fine-leaved fescues, like Festuca trachyphylla and F. idahoensis, are rolled.

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Leaf blades can be smooth, hairy, scabrous or glaucous (with a bluish waxy coating).

The scabers along the edges of rice cutgrass leaves can leave you bleeding. 

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Small glands that appear as bumps on the leaves can be alone or at the base of hairs.

Stolons and Rhizomes

Stolons and rhizomes are stems that stray from the vertical plane and explore above ground (stolons) and below ground (rhizomes) territories. You can tell that rhizomes are not roots because they have nodes. Sometimes a horizontal stem will grow along the ground for a ways, then go underground. Is that a stolon or a rhizome? Maybe it is just proof that grasses don't read our botany books.

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Bermuda grass (Cynodon dactylon) spreads by stolons, which most of the time sprawl along the ground. But stolons are equally useful for climbing over fences and up into trees and shrubs.

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Rhizomatous grasses can be equally aggressive, as anyone who has weeded quackgrass (Elymus repens) out of their garden can attest. The rhizomes can grow right through potatoes and most other obstacles.

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Rhizomes help stabilize grasses that grow in habitats of shifting sand, such as Leymus flavescens.

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Rhizomatous grasses often form a dense sod, which makes them suitable for lawns, ball fields, and pastures.