Sedge Biology

Dispersal

As with grasses, the apparent seeds of sedges (Cyperaceae) are achenes (AY-keens), which are small seedlike fruits containing just one seed each.  They never open.  Rushes (Juncaceae), by contrast, have small capsular fruits having three chambers, opening along three splits to release multiple seeds. 

Sedge achene

In this Rush you can see the fruits opening by three slits.

Additionally, Rushes have what look like six petals (technically they are perianths of six papery tepals) surrounding the flower, resembling a tiny Easter Lily.  Sedges have either no perianth, or a perianth of mere bristles, or a perianth of bristles plus two or three tiny paddle-shaped tepals, or---outside of our range and rarely---perianths resembling those of Rushes. 

Rush fruits surrounded by "petals":

Vocabulary help:

The perianth (PEAR-ee-anth) is the combination of sepals and petals---the non-sexual parts of the flower.  Tepals (TEA-pulls) are perianth parts where sepals and petals are not differentiated, much like having petals with no sepals.  Think of lilies, or amaryllis, agaves,  yuccas, or onions.

Achenes disperse by flotation, by adhering to passing creatures, by ant activity, by wind, and by traveling within the digestive tracts of seed-eating animals. 

A documented example of internal transport is in Scirpus where waterfowl eat the achenes of some species. The achenes surviving the digestive passage have a tremendous boost in germination---in one study 94% germination reported vs. 2-9% germination following other treatments. 

Achenes of Rhynchospora megalocarpa are particularly large and sometimes blue-colored, no doubt providing the extra toughness and food reserves required for establishment in the harsh scrub habitat of this species, and perhaps appealing to rodent seed-eaters or perhaps birds. More berrylike and likely to enlist bird dispersal are the achenes of Sawgrass, Cladium jamaicense, where the outer layers soften to resemble a fleshy fruit.

The achenes of Scleria become glossy white BBs, perhaps catching the eyes of birds.  Ant dispersal is documented for Scleria.  In Scleria triglomerata ants like the fleshy hypogynium (the achene base in some species of Scleria), dragging the achenes back to their fertile and pre-tilled nests.   Scleria georgiana has "glands" interpretable as ant bait at the bases of the achenes.

Achene from Scleria triglomerata:

Many sedge achenes (e.g., Eleocharis, Rhynchospora, Scirpus) have barbed bristles, apparently to help the achenes cling to feathers or fur, or probably in the cases of long thin bristles, to ride the wind.   In a few species, such as Rhunchospora plumosa and R. megaplumosa, the bristles resemble feathers.  Many species of Rhynchospora, and other sedges, have elaborate achene surfaces with ridges, bumps, and networks.   Interestingly, this is true also in other non-related achene-bearing plants, such as members of the Alismataceae.   Perhaps the sculpturing helps the achene cling to wet muddy bird legs.

Achene with feathery bristles (Rhynchspora plumosa):

In Bulbostylis, Eleocharis, and Rhynchospora the base of the style remains on the achene as a beak or "tubercle" (see the photo above).  This accounts for the names Rhynchospora (rhynchos = nose, Greek), and Bulbo-stylis is self-explanatory.  Roles of the snoutlike, buttonlike, or conical style bases are puzzling.  The structures are diverse, ranging from nearly flat-shaped to dagger-shaped. 

Of course maybe sometimes a style base is merely a style base, but usually this structure is well defined and well developed, looking more adaptive than stumpy.  In some species, such as Rhynchospora inundata, it is longer than the achene body.  Completely guessing, maybe the beak has something to do with orienting the achene for germination. And/or it may help with flotation or, in some cases, catching on the wind, water, or passing creatures.  Another possibility is that in some species an elongate beak may help with breaking the spikelet apart in wind and rain.   We've not found much written on this subject.

Rhynchospora inundata achene with bristles and very long beak:

Floating is important.  Many sedges inhabit swamps, marshes, and shores.  The most obvious floaters are the saltwater Bolboschoenus maritma with buoyant achenes, the seaside Beach Stars (Remirea maritima) with corky rachillas, and  the achenes of Cyperus odoratus where the rachilla breaks apart with a barrel-shaped spongy piece of the rachilla accompanying each achene.

The flat perigynium (foliar sheath around the achene) units of Carex flutter on the wind.

Carex perigynium (bottom image):

One of the strangest adaptations is the white bony case covering the achenes in Oxycaryum (name from Greek for sharp nut). A hard case around anything suggests protection, perhaps from the elements or from the digestive process of the ducks known to eat these achenes.   In our opinion, this is the most likely "main" function of the hard coat.

Oxycaryum cubense achene:

The Oxycaryum achene-case could also help with flotation.  Or for some wild speculation, could the casing inhibit germination, thus allowing the achenes to disperse widely before sprouting?  Oxycaryum forms dense monospecific stands where the apple falling close to the tree would be useless.  Better to float far away to a new mudbank.  Interestingly, the species can form unrooted free-floating mats. 

Vivipary (baby plantlets produced asexually) is scattered among sedges.  In our area this can be observed with Eleocharis baldwinii, E. vivipara, probably in Scirpus, and occasionally in other species.  The inflorescences bend to the ground and sprout new plantlets without benefit of flowers, pollen, fruits, or seeds.

Sedges can be prolific rhizome formers.  Within a genus it is common for some species to be rhizomatous and others to be free of rhizomes.

Pollination

Sedges are mostly wind-pollinated plants with long feathery stigmas to capture pollen from the breeze, and with anthers shedding oodles of pollen onto the wind.  In Fimbristylis the long style is fringed below the stigmas.

Showy perianths (sepals and petals) are absent, although some species have tiny perianths (see above for vocabulary help).  These are too small and too hidden to serve as visual attractants as petals usually function, but they could protect the delicate flower.  Paddle-shaped tepals are seen easily by dissecting Fuirena spikes.

Fuirena achene surrounded by perianth "paddles."

Some observers (who have encountered thrips covered with pollen) suggest thrips as pollination vectors for Fuirena.  This would require energized, mobilized thrips!   Kyllinga odorata has compact near-white flower clusters interpreted by some biologists as probably insect-pollinated.  The flower clusters are a little fragrant.

Kyllinga odorata:

Rhynchospora colorata and R. latifolia have bright white eye-catching petal-like bracts subtending the inflorescences, and are generally believed to be mostly insect-pollinated.   Sedges in other regions have yellow bracts. As discussed in Tucker (see our Intro page), insect pollination is probably a rare sedge adaptation to life in comparatively enclosed forested habitats where wind is not effective in carrying pollen.  We suspect the same for the grass Amphicarpum muhlenbergianum which has underground self-pollinated flowers in addition to having what may be insect-pollinated above-ground flowers.  (See our Grass Biology page.)

Rhynchospora colorata with white bracts:

Self-pollination occurs in sedges.  Cyperus iria reportedly pollinates itself while the flowers are held within the unopened scales.  Bulbostylis species sometimes form spikelet clusters almost underground---at the base of the plant.  These invite investigation.

Meiosis and pollen formation in sedges (and in rushes) is unusual.   In both families, unlike other plants, the centromeres are not well defined, and are dispersed along the chromosomes.   The pollen grains remain in tetrads (the four cells resulting from meiosis), with only one of the grains remaining viable in sedges.