Saturday, September 20, 2014

Two Common (and Commonly Confused) Cyperus

The graminoids (mostly the grasses, sedges, and rushes) have a reputation for being difficult to identify to species, but it really just takes some time and understanding of the parts that the keys are describing.  The families should be easy to distinguish.  Rushes (Juncaceae) have sepals and petals (or collectively tepals) surrounding the pistil and stamens, giving them, on a minute scale, the look of a "typical" regular flower such as a lily.  Grasses (Poaceae) and sedges (Cyperaceae) have flowers that are evolutionarily advanced and consist of just the most important parts for reproduction, the pistil and the stamens; the perianth is highly modified in these families and doesn't look like what you would think of as petals and sepals.  In the grasses, the flower is usually subtended by two bracts, known as the lemma and palea, and each floret (as these structures are known collectively) or group of florets is subtended by additional bracts known as glumes (usually two, sometimes one or absent).  In the sedges, the flower is subtended by a single bract, called a scale.  Grasses (nearly all of them, at least) have hollow stems with solid nodes.  Most sedges have solid or pith-filled stems without swollen nodes.  Most grasses have overlapping sheaths; sedges have fused sheaths.  Grass leaves are typically flat or rolled; sedge leaves are usually V- or W-shaped in cross-section.  There are other differences, but understanding these basic differences should at least get you in the right family.

As far as sedges go, Carex is the most diverse genus.  Being able to recognize groups or sections of the genus that have similar characteristics is often a very helpful tool because it allows you to narrow down your species options substantially.  Some of the other more species rich genera in the family Cyperaceae include Scirpus, Eleocharis, and Cyperus.  In the genus Cyperus, the flowers are perfect and there are no perigynia (paper-like sacs surrounding the female flowers) as there are in the genus Carex.  The scales subtending the flowers in Cyperus are folded in half, not flat or rounded as they are in many other genera.  Morphologically, the genus that looks most similar to Cyperus in the United States is Dulichium.  The flowers on plants in the genus Cyperus lack the long persistent styles (the tubercles) that are present in flowers on plants in the genus Dulichium, and Dulichium has axillary inflorescences whereas Cyperus has terminal inflorescences.

Within the genus Cyperus, as within the genus Carex, the species can be organized into several natural groups (subgenera) that share morphological characteristics.  Within subgenus Cyperus, which has flowers with three stigmas (as opposed to two), spikelets along a conspicuous rachis (as opposed to being in digitate or glomerulate heads), and rachilla (the axis to which the flowers are attached) that disconnect from the rachis (the axis to which the spikelets attach) only at the base (as opposed to having rachilla that disconnect beneath each scale), two of the most commonly confused species in the Great Lakes region are Cyperus esculentus and Cyperus strigosusCyperus odoratus is also often confused with these two species, but it is taxonomically placed into subgenus Diclidium because the rachilla disconnect beneath each scale, not at the base of the spikelet.  If you pull on a spikelet of a species in subgenus Diclidium from the end of the spikelet, the spikelet will break somewhere in the middle; pulling on a spikelet of a species in subgenus Cyperus from the end of the spikelet will result in the entire spikelet being removed from the rachis in an intact unit.

Cyperus esculentus inflorescence and leaves
With all of that background information, now we can look at Cyperus esculentus versus Cyperus strigosus to see the differences between these commonly confused sedges.  First take a look at the inflorescences.  Usually, Cyperus esculentus has spikelets that are orangish in color (see photo above), as compared to the yellowish or straw-colored spikelets of Cyperus strigosus (see photo below).  In addition, the spikelets of Cyperus esculentus aren't usually packed into the inflorescence as tightly as those of Cyperus strigosus.

Cyperus strigosus inflorescence
Next, let's look at the scales subtending each flower in the spikelets.  In the two photographs below, the structure in my hand is a  single spikelet, made up of several flowers, each subtended by a scale.

Cyperus esculentus spikelet
Without a ruler for scale, it's a bit difficult to see the difference in the length of the floral scales.  However, you should be able to tell that in the Cyperus esculentus spikelet photo (above), the scales are shorter (approximately 2-3 mm long) than those in Cyperus strigosus (3-4 mm long).  Although this difference isn't much, with some experience, you can begin to notice the short scales of Cyperus esculentus and the "long" scales of Cyperus strigosus on a quick inspection with a hand lens.

Cyperus strigosus spikelet
Yet another difference in the two species can be seen by looking at the base of the plants.

Base and roots of Cyperus esculentus
In Cyperus esculentus, the base of the plant (where the stem meets the roots) is soft and lacks a bulb (see photo above).  Often, at least later in the season, you can find tubers at the ends of some of the roots.  In contrast, Cyperus strigosus has a distinctly bulbous base (see photo below) and lacks any tuberous protrusions on the roots.

Base and roots of Cyperus strigosus
Another difference between the two species that can somewhat be seen in the first set of photos is that Cyperus esculentus is leafy at the base, whereas Cyperus strigosus is not leafy at the base.  In the first two photos, you can see a lot of leaves in the Cyperus esculentus photo, but the leaves are mostly absent in the Cyperus strigosus photo.  The foliage of Cyperus esculentus is also usually shiny and yellow-green, whereas that of Cyperus strigosus are not as shiny.

5 comments:

Ron Gamble said...

Thanks for the info! Maybe now I can get these two confidently straight!

ben said...

Great educational post

Scott Namestnik said...

No problem, Ron. These two are pretty easy to tell apart once you've seen them next to each other.

Thanks Ben. There'd been a lot of confusion on social media pages about these two recently so I thought it was worth a post.

Abby L. said...

Yes, thank you for this, Scott. Wow, what a well-written explanation!

Scott Namestnik said...

Glad you enjoyed it!