'The essence of spring.' Ferns unfurl in a curious seasonal dance

One of the more delightful things to watch for in spring is the annual unfurling of the ferns. They seem to suddenly appear, all coiled up and then, as the days lengthen and get warmer they gradually unfurl. I have a big patch of sensitive ferns in the floodplain behind my house that are just coming up. The chaotic mass of both tightly coiled and unfurling fronds are like dancers frozen in the midst of contortions for some complicated dance. I’ve always taken this for granted, it just happens. But this year I started thinking about why? And how? None of the “normal” plants start out like this. Why ferns?

The majority of fern species as they poke up through the soil from their underground rhizomes start out with the characteristic fiddlehead shape. I’ve noticed fiddleheads for sale along the road recently. These fiddleheads are a particular species - the ostrich fern - an iconic spring edible here in Maine. Most ferns are inedible, even poisonous so while all have a fiddlehead stage, most are not the fiddleheads you want to be eating.

Fiddleheads are said to be named after the headstock of a violin or fiddle, but I’ve often wondered where that design came from in the first place. Were ferns, with their unique coiled structure, the original inspiration?

Ferns are a group of primitive plants. They don’t produce flowers or seeds, but instead reproduce using spores. On some ferns around here (New York ferns, for example), you can find the spores housed in cup-like sporangia on the underside of the frond. In other ferns (cinnamon and sensitive ferns), you’ll find an entire frond dedicated to bearing spores (the fertile frond), while the other fronds have none. Those spores are not sex cells (sperm or egg), instead, when a spore lands in the right place it grows into a tiny, flat structure called a gametophyte. The gametophyte produces the sperm and eggs. The reason ferns are found in wet environments is the sperm has to be able to swim to get to the eggs. The flowering plants evolved more recently and overcame the need for motile sperm with the advent of pollen that doesn’t produce sperm until it has reached the egg (housed in the flower).

Fern fronds also differ from the leaves of flowering plants in the way they expand from a bud (a,k,a, vernation). In ferns, vernation is circinate, meaning the leaf unrolls from the tip, rather than expanding from accordion-like folds as most flowering plants do. Check out a young beech leaf right now. It's all folded up in the bud, ready to expand laterally, whereas the ferns are unrolling.

Each fern leaf (frond) is just one leaf that extends up from an underground rhizome. The leaf buds aren’t hanging out on the ends of branches waiting to expand as soon as it is warm and sunny enough. Instead they are buried. Immature fern fronds have a lot more work to do when spring comes. This might be the reason for their tightly coiled structure. The immature frond has to punch up through the soil and leaf litter above it to reach the sun. Being tightly coiled is thought to help the immature frond push upwards through the soil. The tight coil also protects the developing frond.

My second question - how does this happen - has a fairly straightforward answer. The furling and unfurling of a fern is controlled by unequal cell growth. To form the fiddlehead, the cells on the outer surface elongate more than those on the inner causing it to curl inward. When ready to unfurl, the cells along the inner side of the coil start to elongate more than those along the outer edge.

My third question - why are ferns the only plants to unfurl in this way - doesn’t appear to have an answer. Ferns are a lineage of terrestrial plants that emerged some 360 million years ago. Flowering plants are relative newcomers, not showing up until about 130 million years ago. The unique traits of the ferns are a reminder of their longevity on Earth. To me, they are the essence of spring, that most primeval of the seasons, the potential for life held in that tight coil, just waiting to unfurl.

Susan Pike, a researcher and an environmental sciences and biology teacher at Dover High School, welcomes your ideas for future column topics. She is looking for readers to send her the signs of spring they're noticing so she can document them on her website pikes-hikes.com. Send your photos and observations to spike3116@gmail.com. Read more of her Nature News columns online at Seacoastonline.com and pikes-hikes.com, and follow her on Instagram @pikeshikes.

This article originally appeared on Portsmouth Herald: Nature News: Ferns unfurl in wetlands in spring