The Monthly Fern: Water Clover & its odd spores

"It is worth clarifying that these plants are not clovers." (Photo by Bill Dodd).

For July, South Dakota's fern-of-the-month is the Hairy Water Clover, Marsilea vestita. Among our ferns it's quite the oddball—in habitat, behavior, leaves, and especially the spores. They suggest that one of its ancestors was the evolutionary start of seed plants!

Water Clovers grow nearly worldwide. On the order of 45 to 65 species are recognized (experts disagree on number), of which 5 are native to North America. M. vestita, the Hairy Water Clover, is the only species in South Dakota (so far). It's known from many sites across the state, in shallow water and on mud. Plants are rooted (not free-floating) and tolerate seasonably dry conditions, for example persisting after a pond has dried up. In fact, wet followed by dry can aid in reproduction and dispersal (Montana Field Guide).

Marsilea vestita is rhizomatous and colonial; in wet habitat, leaflets usually are horizontal (MWI).

M. vestita in a field; in drier habitat, leaflets often are ascending (Mary Ellen (Mel) Harte photo).

Hairy Water Clover can grow as tall as 20 cm on moist soil, or to 40 cm in water (in order to reach the surface). Leaves are dimorphic—sterile and fertile—but neither is fern-like. Sterile leaves have blades with four rounded triangular leaflets, and look a lot like four-leaf clovers! No other plant can be confused with Marsilea (1). 

Sterile leaves of Hairy Water Clover have divided blades on long slender stalks (MWI).

The sterile leaves also are unusual in behavior. Water Clovers are the only ferns known to be nyctinastic—moving with the onset of darkness. During the day leaf segments are nearly horizontal. Then as the sun sets they bend upward, forming a packet of sorts (Montana Field Guide has an account of this and other interesting features of Water Clover).

Fertile "leaves" are located near the base of sterile leaf stalks. In shape and size they resemble beans or peppercorns (source of another common name, Pepperwort). Being unusual they of course have a special name—sporocarp (= spore body); oldtimers like Linnaeus called them capsules.

Marsilea vestita. The hairy sporocarps contain 2 kinds of spores (lower right). Britton & Brown 1913.

Young sporocarps are greenish, hairy and slightly soft. With maturity they dry out, darken and become very hard. In this state they can survive for many years; the record is said to be 100. The Marsilea sporocarp is an effective unit of dispersal, often by way of waterfowl digestive tracts.

Like almost all ferns, Water Clovers have sori—clusters of sporangia which contain the spores. But the arrangement is quite different. In a typical fern, sori are located on the underside of leaves. In Marsilea, they're neatly arranged inside the sporocarp.

A typical fern; sori are clusters of sporangia, which contain dust-sized spores (2). USDA Forest Service.

Marsilea sporocarp with sori; below it, a sorus with sporangia, which release spores (no source given).

A cross-section through a Marsilea sporocarp (above) reveals an orderly but complicated interior. Inside the container-like sori are 2 kinds of sporangia. This is where things get exciting. They produce 2 kinds of spores—male and female!

Most ferns, 99% in fact, release a single type of spore—tiny, 1-celled, asexual. But not Water Clovers. They're among the 60 fern species (out of c. 10,500 total) with male microspores and female megaspores. These are heterosporous ferns; interestingly, all are aquatic (more here).

When a Marsilea microspore bursts open, many sperm (aka spermatozoids) swim off in search of an egg to fertilize—not unlike sperm of typical ferns. It's the megaspores that are so unusual. Not only are they 10 times the size of a typical fern spore, they're complex, with specialized parts.

Marsilea megaspore, c. 0.8 mm long, with 2 cells (no source given).

Shortly after leaving its sporangium, the megaspore divides to become two joined cells. The upper cell will produce an egg, which gives off chemicals to attract sperm. If a sperm successfully wriggles through the opening and reaches the egg, fertilization takes place, leading to development of an embryo and then a baby fern.

In comparison, the megaspore's basal cell is huge, and rich in carbohydrates and fats. These will sustain the developing fernling in its first days, before it can photosynthesize. In this way, a megaspore is like a seed, which supplies nutrients for its young seedling. Perhaps an ancestor of Marsilea was the evolutionary beginning of seed plants (more here).

Some readers may be wondering where the gametophytes are—those tiny independent plantlets that are the sexual stage of ferns. Good question! Water Clovers do have gametophytes, but they are minute and NOT independent (another similarity to seed plants). For more about the life cycle of heterosporous ferns, see (3) in Notes.

Simplified?

Now we finally arrive at the long-promised answer to the burning question, "How many spores would fit in a typical [empty] can of soda?"

Fern spores are truly tiny. A handful looks like a pile of dust. To show just how small they are, Robbin Moran (2021) calculated the number of average-sized spores that would fit in a typical can of soda, which has a capacity of 355 milliliters. For spore volume, he used 125,000 µm3, assuming for simplicity that a spore is a cube. What do you think? How about a ballpark estimate?

Hmmm ... 777,000?

Maybe 10 million?

According to Robbin's calculations the answer is 4,440,000,000 (4.4 billion). Yikes, that's a lot! Yes, spores are tiny indeed (4).

Notes

(1) While Water Clovers are easily recognized, distinguishing the Hairy Water Clover from others in the genus is not easy, requiring sporocarps. If you intend to document an occurrence, be sure to collect both types of leaves. See Marsilea in Flora of North America for a species key and descriptions.

(2) It seems sporangia are readily mistaken for spores, as a search for images of "fern spores" suggests. Many of the images actually are sporangia clustered in sori.

(3) In the previous Monthly Fern, I made a big deal out of the 2-stage life cycle of ferns, which involves separate tiny green sexual plantlets—gametophytes—that give birth to baby ferns. Heterosporous ferns have gametophytes, but they are minute and not independent. They develop inside the persistent spore wall, where they give rise to either sperm-producing antheridia or egg-producing archegonia (gametophytes of typical ferns usually have both). For the female gametophyte, developing inside the spore wall provides additional protection for the embryo, but there's no access to sunlight to photosynthesize food for the fernling. That's why the nutrient-rich basal cell is so important.

Life cycle of Marsilea, a heterosporous fern (labels added). See Milne Publishing for details.

(4) I couldn't find Robbin Moran's article online. If you'd like to read more about fern spores, and all of Robbin's calculations and conclusions (e.g. 4.44 billion spores taken together has a surface area nearly equal to 8.5 ping-pong tables), send me an email address and I'll send you a PDF file.

Sources (in addition to links in post)

Hooker, WJ, and Greville, RK. 1831. Figures and descriptions of ferns, principally of such as have been altogether unnoticed by botanists, or as have not yet been correctly figured. Vol. 2. BHL

Milne Publishing. Marsilea. Accessed June 2025.

Montana Field Guide. Montana Natural Heritage Program. Hairy Water Fern—Marsilea vestita.

Moran, RC. 2004. The Natural History of Ferns. Timber Press.

Moran, R. 2021. Fern Spores, Soda Cans, and Ping-Pong Tables. Fiddlehead Forum (May–Dec).

Pinson, J. About Ferns, American Fern Society.

PremaBotany (Prema Iswary). December 2018. Marsilea.