The Carlisle Mosquito Online

Friday, December 17, 2004


Christmas Fern

Underside of the fertile leaflets showing the fruitdots in clearly defined rows, and also you can see the inward curving bristle-tipped teeth on some of the leaflets.(Photos by Kay Fairweather)

Name: Polystichum acrostichoides or Christmas Fern. The genus name comes from the Greek polys meaning "many" and stichos meaning "row". It refers to the rows of fruitdots or sori on the underside of the fertile leaflets. It is a large genus with over 225 species worldwide. Only two species occur in our area. The suffix "-oides" on the species name means "similar to" so this is a Polystichum fern that is similar to an Acrostichum fern. (Acrostichums are not in our field guides — they are ferns of the tropics.)

When and where seen: Christmas fern is evergreen so it can be seen throughout the winter. Steve and Roberta Spang have some clumps in the wild part of their back yard on Fiske Street and Steve has also seen it at Towle. I have seen a few clumps just over the Carlisle border in the Estabrook Woods. Although it is extremely hardy there doesn't seem to be a lot of it around.

Fern language: The entire frond of a fern is considered a single leaf; the subdivisions on the frond are called leaflets. The degree of laciness of the leaflets is one of the identifying characteristics for ferns. Christmas fern leaflets are "once-cut." Other more lacy ferns are twice-cut or even thrice-cut.

Distinguishing characteristics: Christmas fern is very easy to recognize. It is green at Christmas-time; the leaflets have an "ear" or a "thumb" close to the stem; the individual leaflets have incurving teeth ending in a bristle; the fruitdots on the spore-bearing fronds are only on the leaflets at the tip of the frond and they tend to grow in rows along the midrib; the fertile leaflets are markedly smaller than the sterile leaflets. Sometimes (presumably when the sori zoning board is on a break) the fruitdots go mad and completely cover the back of the leaflet with no space between them and no apparent organization. This is why the Christmas fern is "similar to the Acrostichum" which takes it a step further and has fruitdots on both the upper and lower surfaces of its fertile leaflets. In winter the fertile leaflets often wither, so at this time of year you may find Christmas fern with shortened fronds. In the growing season the fronds can reach three feet in length and tend to grow in bouquet-like clusters.

Habitat: In the language of Boughton Cobb, the Christmas fern "prefers but does not demand" a rich limy soil. Why don't we call it the polite fern? "Excuse me, would you please pass the lime?" The higher pH preference may account for its relative scarcity since much of our soil is naturally acidic. The Christmas fern is often found on sloping terrain like the banks of streams in the woods, or in ravines. Like most ferns, it prefers the shade but will also grow in semi-open areas.

Just above the ruler you can see the "thumbs" on the leaflets. (photo Kay Fairweather)
Other evergreen ferns: There are three very common evergreen ferns easy to find in any of the conservation land in Carlisle throughout the winter. The common Polypody is the smallest of these and often grows in shallow soil on rocks or in cracks in rocks; the Marginal Woodfern and the Spinulose Woodfern are more ferny-looking ferns, i.e. the leaflets are more finely divided. The Marginal Woodfern (twice-cut) is rather coarse textured and leathery; the Spinulose (thrice-cut) is more delicate and looks like it shouldn't be able to hold up in the frost and snow. And then there are the other winter-hardy ferns with a specialized habitat at the town center and recognizable by their yellow siding and green shutters.

Reference: Boughton Cobb, A Field Guide to Ferns, published by Houghton Mifflin.

"On the twelfth day of Christmas my true love showed to me..

Twelve bees a-humming, eleven otters sliding, ten frogs a-leaping, nine turtles snapping, eight skunks a-spraying (tough love), seven salamanders, six honey mushrooms, five bobolinks, four oven birds, three barred owls, two mourning doves, and a woodcock at the Foss Farm." K.F.

2004 The Carlisle Mosquito