Graminoids (Families Poaceae, Juncaceae and Cyperaceae)

Graminoids (Families Poaceae, Juncaceae and Cyperaceae) 

One of the most challenging things you will do at DWP is key-out graminoids (grass and grass-like plants). 

There are three families that you will need to be able to distinguish in order to indentify graminoids; the 

grasses (Poaceae), the sedges (Cyperaceae), and the rushes (Juncaceae). The following table lists the 

distinguishing characteristics of each. 

Family stems leaves flowers fruit 

Poaceae Round, hollow at Ligulate, sheaths open, 2-ranked* Modified into glumes, grain 

internodes 

lemmas, and paleas 

Cyperaceae 3-sided, solid Sheaths closed, 3-ranked* scales achene 

Juncaceae Round and solid Basal, forming tufts, sheaths closed Made of 6 tepals – 3 sepals, 

and 3 petals 

Many seeded capsule 

*Two ranked means the leaves come out on the two opposite sides of the stem. Three ranked means the leaves come out from three 

different ―sides‖ of the stem. 

Cross-section of a sedge, ―Sedges have edges‖. Grasses are round in 

cross section and generally have hollow internodes. 

Family Poaceae (grasses): 

A comparison of the typical flowers of the grass 

family (Poaceae), sedge family (Cyperaceae) & 

rush family (Juncaceae). In grasses, the 

individual flower is referred to as a floret. In the 

sedge family, each flower is subtended by a 

scalelike bract. Since the scalelike petals and 

sepals of the rush family are very similar in 

appearance, they are referred to as tepals. The 

ovary of a grass develops into a one-seeded grain 

(caryopsis). The ovary of a sedge develops into 

one-seeded achene, while the ovary of a rush 

develops into a many-seeded capsule. Sedge and 

rush stems often contain spongy aerenchyma 

tissue with abundant air spaces. This allows air 

to reach the root systems which are often 

submersed in water-logged mud. Rush stems 

(Juncaceae) are generally circular in cross 

section, while the stems of sedges (Cyperaceae) 

are typically three-sided (triangular).

Awn — a stiff bristle situated at the tip of a glume or lemma. 

Collar — the junction of the leaf sheath and blade. 

Culm — the stem of a grass. 

Floret — a single small flower, usually a member of a cluster, such as a spikelet or a head. 

Glume — the lowest two (sometimes one) empty scales subtending the usually fertile scales in grass 

spikelets. 

Lemma — the lowermost of the two scales forming the floret in a grass spikelet -- the uppermost, less 

easily seen, is called the palea. 

Ligule — an extension, often scarious (papery), of the summit of the leaf sheath. 

Nerve — same as a vein. The central vein running lengthwise on a scale 

Node — the point along a stem which gives rise to leaves, branches, or inflorescences. 

Palea — the uppermost of the two scales forming the floret in a grass spikelet (often obscure or hidden). 

Rachilla — a secondary rachis. The axis of a spikelet. 

Sheath — a tubular structure effected by the formation of leaf margins around the stem. The base of a grass 

leaf that runs from the node up to the blade. 

Spikelet — a secondary or small spike; specifically, in the Poaceae family, the unit composed or one or two 

glumes subtending one to several sets of lemma and palea combinations. 

Family Juncaceae (rushes): 

1

Note the perianth segments (sepals and petals) in these rush flowers. There are three sepals and three petals 

that look pretty much alike. Collectively we call them tepals – there are 6 tepals in a rush flower. You will 

not find this flower structure in the Poaceae or the Cyperaceae. 

At right is another picture of a rush. This photo 

shows the fruit of the rushes, the many seeded 

capsule has split open releasing the seeds. This 

fruit type is also unique among the three 

graminoid families. 

2

Family Cyperaceae (The sedges): 

There are numerous genera within the family Cyperaceae that you will encounter at DWP. We will discuss 

the family characteristics in general and three genera within the family. Examples of sedges, and their 

flower structure, are shown in the illustration below. 

The key to the most common genera of sedges you will encounter at DWP is provided. However, there are 

numerous genera not listed in this key that are present at DWP (e.g. Fimbristylis, Fuirena, Bulbostylis, 

Dulichium, Kyllinga and Scirpus), so use this key with that fact in mind. You will use this key during 

exercises designed to familiarize you with this group. A more detailed discussion of Cyperus, Carex and 

Rhynchospora follows. 

1 Achene enclosed in a sac (perigynium), with style and stigmas protruding through a terminal opening; 

flowers unisexual, the staminate flowers either above or below the pistillate in the same or separate 

spikelet, leaf blades with a midrib…………………………………………………………………….Carex 

1 Achene not enclosed in a perigynum; flowers perfect (except in Scleria, which has exposed and bony 

white achenes) 

2 Florets unisexual; achene exposed and with a white, bony-crusty covering……………………….Scleria 

2 Florets bisexual; achene enclosed within scales and lacking a whitish covering. 

3 Scales of spikelets 2-ranked, obviously in 2 rows along the axis, bristles absent, achene 2-3 sided….. 

…………………………………………………………………………………………………..Cyperus 

3 Scales of spikelets spirally arranged on axis 

4 Spikelet solitary, stems leafless except for bladeless sheaths……………………………...Eleocharis 

4 Spikelet many, stems leafy……………………………………………………………..Rhynchospora 

The genus Carex: As mentioned previously 

male (staminate) and female (pistillate) flowers 

are separate in Carex. They are not always in 

entirely different spikes, however. In the photo at 

right the stigmas of the female flowers and 

anthers of the male flowers are clearly 

distinguishable from one another. 

3

This series of photographs clearly shows the papery perigynium that surrounds the achene in this Carex sp. 

Note the long protuberance on the achene, which is the remnant of the style (review flower structure if you 

need to). The fruit characteristics shown in these photos are unique to this group. 

The Genus Cyperus: Besides the solid triangular stem the thing you might notice first about plants in 

the genus Cyperus is the distinctly 2-ranked arrangement of the spikes evident in the photographs below. 

Note how the individual floral units (spikelets) are clearly arranged along opposite sides of the rachis. 

These also tend to be flattened in appearance. Unlike Carex, plants in the genus Cyperus do not have a 

perigynium. Within each scale is an achene, shown in the right photo. We will practice identifying these in 

the field, so you will become comfortable with distinguishing a scale from a perigynium. 

4

The Genus Rhynchospora: Plants in the genus Rhynchospora lack a perigynium, and have scales 

that are arranged spirally on the axis (A) rather than in the flattened 2-ranked pattern seen in Cyperus. The 

fruit type in this group (like that in Carex and Cyperus) is the achene (B). The unique appearance of 

Rhynchospora achenes will help you recognize these plants in the field. However, these are considered by 

some to be one of the more difficult groups of plants to ―field identify‖ to species due to the small size of 

the achenes of many species, and due to the fact that in order to accurately identify many species the 

achenes need to be fully ripe. But like the other challenging groups discussed thus far, with practice one 

can achieve some measure of confidence with in identifying these plants. 

A B C 

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The achenes in photos B and C clearly show some of the characters important for the identification of 

Rhynchospora; the beak, perianth bristles, and surface features of the achene body. In order to make a 

species determination in this group it is essential that these features be clearly visible. This may require 

collection of specimens for observation under the dissecting scope. We will spend some time identifying 

these in the next few exercises. 

Conclusions 

Well that’s it. Now you’re an expert! Like anything worthwhile it takes time, patience, and practice to 

become proficient at plant identification. I think you will be surprised at how many different plant species 

you will be able to recognize and a plus is that you will probably gain some additional appreciation for the 

diversity, and intricate beauty of graminoid plants. 

References 

Websites: 

http://www.accessexcellence.org/RC/Ethnobotany/page3.html - good little site on plant classification 

http://www.csdl.tamu.edu/FLORA/Wilson/tfp/hdwtfpbs01.htm - link to a plant taxonomy course website. 

Good overall taxonomy site, good descriptions of plant families. 

http://flora.huh.harvard.edu:8080/actkey/image_key.jsp?setId=100 – excellent plant family key 

http://www.geobotany.uaf.edu/teaching/biol474/plant_family_charact.html - guide to plant families 

http://arnica.csustan.edu/key/key.html - fantastic online fruit key 

http://herba.msu.ru/mirrors/www.helsinki.fi/kmus/botvasc.html - online guide with assorted botanical links 

http://www.plantatlas.usf.edu/ - fantastic source on Florida plants from the University of South Florida’s 

Institute for Systematic Botany. 

http://www.virtualherbarium.org/vh/db/main.php - A virtual herbarium, search for and view herbarium 

specimens on this site 

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Graminoids (Families Poaceae, Juncaceae and Cyperaceae)

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