Fern Seeds?

Woodsia (left) and Polystichum; from "A popular history of the British ferns" 1862, BHL.

Today we start with a quiz with one question:

TRUE or FALSE? Spores are the seeds of ferns. Just as a pea gives rise to a seedling, which grows into a full-sized pea plant, a spore gives rise to a baby fern, which grows into a full-sized fern like its parents.

And the answer is ...

Prothallus of Aspidium (wood fern; probably today's Thelypteris). Source.

... FALSE! A spore germinates to produce a prothallus, which can reach full size (ca. 1–2 cm across) in a few months.

Maybe some readers are surprised. Probably some knew the correct answer, or answered correctly because one choice seemed too obvious. Or maybe you were as I was last month—scratching my head trying to remember the fern life cycle, "the bugbear of many introductory botany students" (Moran 2004). This is unfortunate. Off-putting diagrams and terminology shouldn't keep us from the fascinating story of fern sex!

In an earlier post, I explained that I'm part of a group of botanists revising Vascular Plants of South Dakota by Theodore Van Bruggen. Currently I'm immersed in the world pteridophytes—ferns and their allies—which makes me a pteridologist, or at least an aspiring one.

In the fern section of the University library, I found A Natural History of Ferns by Robbin Moran of the New York Botanical Garden. His combination of science and stories has made my fernventures much more enjoyable. For example, the fern life cycle now makes sense. A light went on when I found a good starting point—the fact that fern spores, unlike seeds, do NOT produce miniature versions of their parents when they germinate.

Maybe you remember, maybe vaguely, that a typical fern has two life stages ... two independent free-living plants. The familiar one is the leafy fern plant, which produces spores. This makes it a sporophyte. The spores develop in tiny structures called sporangia, which form clusters on the underside of fern leaves.

Clusters of sporangia lined up on the underside of a fern leaf. Each sporangium is filled with spores. There may be several million spores in this view. Source.

Sporangia are much more exciting than their name would suggest. They don't just open, they don't even split suddenly. They cock themselves and shoot spores at speeds reaching 10 m/sec! (Llorens et al. 2015). Sporangia are catapults!!

The diagram below shows a sporangium catapult in action. Note the row of blue water-filled cells (a). When the outside surface of this row dries, it curves backward, opening the sporangium and cocking the catapult (b). This builds up elastic pressure until the row collapses suddenly, in just 30 µsec or so, sending the spores flying (c).

Modified from Llorens et al. 2015.

Fern spores are as fine as dust and can travel far. One lucky enough to land with its brethren in just the right place will germinate to produce not another familiar fern plant, but rather a stemless, rootless, leafless, tiny plant called a prothallus. What's the point? Sexual reproduction! The prothallus is a gametophyte, a producer of gametes (like pollen and ovules, sperm and eggs). The gametophyte is where fern sex happens.

Fern gametophytes are often heart-shaped or ~bilobed. Richard Droker photo.

A fern gametophyte is a simple structure, like a tiny piece of green lettuce when viewed from above. But on the underside are hairlike rhizoids that anchor it in place, and the very important antheridia and archegonia, which produce sperm and eggs respectively.

In the presence of water, sperm are released and swim off in search of sex. With luck, a sperm will reach an archegonium, either on the same gametophyte or often on an adjacent one. There it winds its way down through the cells of the "neck" and fertilizes the egg cell at the base. The result is a zygote, which produces the first leaves of the sporophyte as the gametophyte disintegrates. It continues to grow to become a familiar spore-producing fern. And so the cycle continues.

Young fern leaves growing from zygotes on disintegrating gametophytes. Richard Droker photo.

There is no seed anywhere in the life of a fern! Modified from source.

It took a long time for botanists to agree that ferns do not produce seeds. Even the great botanist Carl Linnaeus was sure fern seeds existed, though he wasn't sure where. "[I] must confess my ignorance whether what I see [today's spores] is seed, or dust of the anthers [pollen]" he wrote in 1737. By 1751, he had concluded the dust was indeed the miniscule seeds of ferns.

It wasn't until the mid 1800s that fern seeds finally vanished, as did their power. For there was a time when humans used fern seeds to great advantage. "We have the receipt of fern-seed, we walk invisible" cried one of Falstaff's thieving henchmen. Robbin Moran's book begins with this Shakespearean reference to fern seeds. But no spoilers here—you will have to read it yourself.

For more about the lives of ferns ...

American Fern Society. "About Ferns"

Brooklyn Botanic Garden. "How to Grow Ferns from Spores" 

Llorens, C, et al. 2015. The fern cavitation catapult: mechanism and design principles. J. R. Soc. Interface 13: 20150930. https://royalsocietypublishing.org/doi/10.1098/rsif.2015.0930

Moran, Robbin. 2004. A Natural History of Ferns. Timber Press.

USDA Forest Service. "Fern Reproduction"