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Field Notes

Covering the natural wonders of the Pacific Northwest

June 1, 2011 at 12:00 PM

When the word is as pretty as the fern

cercinate vernation.jpg

So it is in nature sometimes, such as with cercinate vernation, the process by which a fern unfurls from its tight, coiled bud.

This photo of ferns unfurling is by Seattle Times photographer Steve Ringman, shot on the Sammamish Plateau this spring:

From vernal, meaning spring, and circinate, meaning circular, the annual slo-mo unfurling of ferns from their coiled buds is one of the delights of spring.

The tucked growing tip of the fern is protected within the coil, only exposed as the fern stands tall at last.

In The Natural History of Ferns by Robbin C. Moran, published by Timber Press, Moran explores the shape, spira mirabilis, which occurs over and over in nature, in a variety of living things, including fern fiddleheads. As she notes, this beautiful shape evokes harmony, structure and a satisfying regularity: “Reflecting on this orderliness, the English botanist Nehemiah Grew concluded in his Anatomy of Plants (1682), ‘Nature doth everywhere geometrize.'”

This lovely, pleasing shape can be seen in a nautilus shell, a ram’s horn, a plant’s tendril. Nature doesn’t seem to care what the tissue is, it makes this shape, over and over. What causes it?

Quite simply, unequal rate of growth of inner and outer surfaces. When one surface grows more than the other, coiling occurs automatically, regardless of the material involved, Moran explains. Bone, shell, hair, flesh or plant tissue. In fern fiddleheads, unequal growth is caused by cells on the outer surface elongating more than those on the inner. The spiral shape persists as long as the unequal growth is maintained. The fiddlehead uncoils only when the cells on its inner surface start to elongate, and only straightens when they are in equilibrium.

The visually striking — and pleasing — aspect of the curve is its unchanging shape as it grows, the larger spirals are just expanded versions of smaller spirals. These interrelated mathematical proportions led Swiss mathematician Jakob Bernoulli (1645-1705) to refer to this spiral shape as the spira mirabilis, or wonderful spiral. Wonderful, indeed. Ferns, take a bow.

Not all ferns are equally tidy, though, notes Judith Jones of Fancy Fronds nursery in Gold Bar. “Sword fern is not a typical spiral, it will droop forward and be elongated, and look like it needs water, than right itself. All ferns have a distinctive manner of unfurling, you can cut all the fronds off and look at the buds and know what they are. I love watching them come up, they are just, oh my gosh, so beautiful.”

Robert Cleland, professor emeritus of plant physiology at the University of Washington explains the unfurling process is really no different that that of a cucumber seed sprout, unfurling its crooked neck as the light hits it, and it stretches and grows.

The same forces are at work with ferns. Light hits the coiled buds, and the plant takes up water. By a cue — perhaps hormones? — that softens the inner cells of the coil, they begin to expand, bringing the sizes of the cells on the inner and outer side of the fern into equilibrium. That allows the frond to straighten as it rises toward the light, with the whole operation powered by the pressurization of water uptake, and fueled by photosynthesis. Quite an operation, all taking places a quietly and gracefully as a ballet.

A good place to appreciate ferns unfurling just now is the Bullitt Fireplace Trail on Squak Mountain in Issaquah. A veritable fern garden, with sword ferns lining both sides of the trail, it’s also a gentle spring training hike, with a steady uphill grade, and a picnic table at the top. You can even take your dog.



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