The Monthly Fern: Northern Holly Fern, a spore shooter

This month's South Dakota fern is Polystichum lonchitis, Northern Holly Fern (Andre Zharkikh).

It was the first paragraph on the first page of Robbin Moran's Natural History of Ferns that hooked me—specifically the words of a thief in Shakespeare's Henry IV:

"We steal as in a castle, cock-sure; we have the receipt of fern-seed, we walk invisible."

So intriguing! Or maybe not. Maybe you doubt that fern-seed can bestow invisibility (1). But think about it ... have you ever seen fern seeds? 

In Shakespeare's time no one had seen fern seeds because they were invisible. In our time we haven't seen fern seeds because they don't exist. And for those who answered "yes" to the question above—spores are not seeds (2).

Northern Holly Fern, well-armed with clusters of sporangia, aka spore shooters (Andre Zharkikh, bar added).

Northern Holly Fern is widespread in the Northern Hemisphere, but often uncommon where it occurs. It's considered an arctic-alpine, boreal, and montane species, which may seem odd for a South Dakota plant. However it grows specifically in the Black Hills (3), which are famous for plants seemingly out of place. Northern Holly Fern is likely a relic of the last Ice Age, when western South Dakota was much cooler (but not glaciated).

Circumboreal Polystichum lonchitis (Cremastra); Black Hills added (location approximate).

Like most ferns, Polystichum lonchitis has little clusters on the underside of its leaves. These are sori (singular sorus), and deep inside are spores. Shape and position of sori are helpful in identification. Often each sorus has a little cover, the indusium, and its shape (or absence) also is useful in identification. Sori of holly ferns are round, lined up between the leaflet midrib and edges, and have peltate (umbrella-like) indusia—tiny round membranous covers on short stalks.

Northern Holly Fern's round sori with peltate indusia (Andre Zharkikh).

Now a short tour of sori diversity, providing a glimpse the level of detail needed for identification. It's handy to have a 10X magnifier.

Left: Maidenhair Fern, Adiantum pedatum, has linear sori partly covered by rolled leaf edges (false indusia). Right: Bracken, Pteridium aquilinum, has continuous sori with indusia hidden under rolled leaf edges (MWI & MWI).

Top: young Fragile Fern, Cystopteris fragilis, with round sori; indusia will wither as spores mature. Bottom: Wood Fern, Dryopteris carthusiana, with round sori covered by kidney-shaped indusia (MWI & MWI).

Common Polypody, Polypodium virginianum, has plump yellow sori with no indusia (MWI).

However we still haven't seen any spores. The sori's brown or yellow dots are not spores, they are sporangia. Inside the sporangia, finally, are the spores—in abundance!

 

Fern leaf with sori containing sporangia containing spores (and some scattered about); USDA Forest Service.

Fern spores are tiny, usually 30–50 micrometers across (1 µ = 0.001 mm), which is narrower than a human hair (source). Or think of it this way—a fern frond just 60 cm long will produce something like 7,000,000 spores! (source) As fine as dust, spores can fly in the lightest breeze, especially if the sporangium kicks them out of the house.

Holly Fern spores, scale bar = 10 µ. © Robbin Moran 2012.

In some ferns, the sporangia simply split open and let their spores fall to the ground. Last month's Sensitive Fern is an example. But in Holly Ferns, in fact in many ferns, sporangia hurl their spores. I like to call them spore shooters, as does Robbin Moran. Others call them launchers or catapults. In any case, their spores can reach speeds of 10 m/sec! (Llorens et al. 2015)

The diagram below shows a spore-shooting sporangium in action. The ring of blue and red cells is the annulus; the cells are filled with water. As the outside dries the annulus curves back, opening the sporangium and cocking the catapult or shooter. Elastic pressure increases until the annulus suddenly collapses, sending spores flying. They get quite a boost in speed, but probably just as important, they're dislodged from the interior of the sporangium. Apparently my "house" analogy above was appropriate. These clingy spores need encouragement to go out into the world.

Modified from Llorens et al. 2015.

Some readers may still be bothered by my earlier statement that spores are NOT seeds. After all, won't a spore give rise to a full-sized fern, like the pea that becomes a pea plant? Well ... yes, but not directly. A true seed has a full set of chromosomes (diploid), and can give birth to a seedling. But a spore has only half a set (haploid). Before a fernling can arise, fertilization must take place.

This leads us to the fern life cycle, which even pteridomaniacs call "the bugbear of botany students". Maybe "fern sex" would be less scary. In any case, stay tuned. I will squeeze it in amidst the beauty and intrigue of the next Monthly Fern.

Northern Holly Fern. Thanks to Andre Zharkikh, who kindly shares his many plant photos "to show other people the beauty of nature".

Notes

(1) To provide invisibility, fern seed must be collected precisely at midnight on Midsummer's Night Eve, while it's falling from the plant.

(2) If you thought spores were seeds, you're in venerable company. Carl Linnaeus, father of modern taxonomy, did also. He first thought the fine dust shed by ferns was pollen. But he then admitted he knew too little about primitive plants to conclude "whether what I see is seed, or dust of the anthers." However years later, in 1751, he announced that the dust was fern seed (Moran 2004).

(3) Flora North America and USDA Plants, sources usually considered reliable, do not include South Dakota in the known range of Polystichum lonchitis (the latter shows it as reported but without documentation). It was first documented in the Black Hills in 1977, and has been found elsewhere in the northern Hills since (see map from SEINet herbarium search, zoom in to South Dakota).

Sources, in addition to links in post

Llorens, C, et al. 2015. The fern cavitation catapult: mechanism and design principles. J. R. Soc. Interface 13: 20150930.

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

Pinson, Jerald. About Ferns. American Fern Society.

Summers, A. 2005 (Dec). Spore Launchers; Ferns and fungi that explosively reproduce. Natural History.

USDA Forest Service. Ferns. Helpful information, photos, and diagrams for aspiring pteridomaniacs. And there are coloring pages! (have a range of greens ready)