A close-up view of the Christmas Fern

The present article is one of four that take a close look at fern species. In order to reduce overlap as much as possible, each article focuses on only those characteristics that make the particular fern distinctive. One article of the four (Shield Fern – Dryopteris filix-mas ‘Crispa Cristata’) provides the most in-depth investigation including photomicrographs.

The evergreen fronds of Polystichum acrostichoides are glossy green, and persist throughout the winter. This accounts for the species’ common name Christmas fern, as it was available for use in Christmas decorations during earlier times. The genus name Polystichum is derived from the Greek polys, meaning many, and stix meaning row. This refers to the rows of spore containing sori that can be seen in the image above. The species name acrostichoides refers to the spore carrying structures being positioned on the topmost leaflets.

Although most ferns require shade, the Christmas fern can tolerate some sunlight. Moist, well drained, acidic soil supports the plant’s growth.

The leafy branch of a fern is called a frond. Fronds are composed of small leaflets called pinnae. On the left below, you can see the upper portion of the fern’s frond. Notice that the pinnae are positioned in an alternate arrangement. The Christmas Fern is referred to as being dimorphic (two forms) since some fronds are fertile, while others are infertile. The left image shows an infertile frond, which tends to be longer and narrower, with broader pinnae. On the right is a closer view of a fertile frond where the pinnae are thinner, and on one of which, sori can be seen as light coloured spots. One of the most distinctive characteristics of a Christmas Fern pinna (leaflet) can be seen in this same right-hand image. At the base of the leaflet there is an ear-lobe-like structure. Some describe the leaflet as having a Christmas-stocking shape which fits with the plant’s common name.

A closer view of the stem that holds the pinnae can be seen below. In a fern, this stem is called the blade, or more properly, the rachis. Notice that each serration on a pinna has a tiny bristle at its tip.

When a new fern emerges from the ground it has a distinctively curled top called a fiddlehead. This name was chosen to describe its resemblance to the end of a violin or fiddle.

In the remainder of the article we are going to take a look at the development of the fern’s distinctive reproductive structures. Unlike flowering plants which usually contain male stamens and female pistils, ferns do things very differently. They produce spores that grow into tiny plantlets called gametophytes, or prothalli, providing that the environmental conditions are suitable. The small circular structures that can be seen on the underside of pinnae (below) contain these spores. Note that at this relatively early stage, the structures are very pale green or white in colour.

Most fertile fronds have the spore containing structures located on the top third of their pinnae. In many pinnae the structures do not extend onto the upward pointing lobe. If you look carefully, you can see that each circular structure consists of a donut composed of very small spheres that is covered by a circular tissue with a dimple at its centre.

Notice that the image at left shows the difference in appearance between a pinna in the frond’s top third, and one in its bottom two thirds. Consider the image at right. Each clump made up of the tiny sphere-like structures is called a sorus. Each of the tiny spheres is called a sporangium. Each sporangium holds a very large number of dust-like spores. Finally, the circular, dimpled tissue that covers and protects each clump (sorus) is called an indusium.

Here is a still closer view of the reproductive components on the surface of a pinna. Remember that the indusium covers the sorus which is composed of many sporangia which, in turn, contain the plant’s spores.

At a higher magnification, details can be seen more clearly. If you look very carefully, you may be able to see that each sphere-like sporangium has a slightly raised, serrated rib ringing it. The ribs on sporangia become easier to see during a later period of development.

As each pinna matures, the sporangia enlarge and darken to a reddish-brown colour. The overall size of each sorus also increases, until the sori are in contact with one another. Since the indusium doesn’t increase in size, it covers much less of each sorus. Note the graduation in the colour of the sori from the top of each pinna to the bottom.

Notice how, in the image on the right, the sori have started to overlap with their neighbours. The location of sori on a leaflet is extremely variable. In the left image the lobe of the pinna is free of sori.

In the three images that follow, the indusia have been forced upward at their edges and folded by the proliferation of enlarging sporangia. The darkening colour has also made it easier to see the ridged ring around each sporangium. This ring is called the annulus.

Here is a view of the tip of a fertile frond at this stage. Notice that almost all of the sori are dark reddish-brown in colour.

Two views of the same pinna can be seen below. Notice that it is no longer possible to differentiate between individual sori. The upper portion of the pinna is almost completely covered by sporangia.

If you look carefully, you may be able to see that several sporangia have broken open, releasing their spores (which are so small that they are not visible at this magnification).

At the limit of the magnification possible with my macro-photographic equipment, it is easy to see the sporangia that have opened to release their spores. (Remember that each sporangium is approximately 0.5 mm or 1/50^th of an inch across!) For still closer views of sporangia and their spores, please see my article concerning the Shield Fern – Dryopteris filix-mas ‘Crispa Cristata’.

I suspect that many casual fern observers fail to lift a frond, and have a look at its underside. It’s on the undersides of pinnae however, that the real action takes place!

Photographic Equipment

The low magnification, (to 1:1), macro-photographs were taken using a 13 megapixel Canon 5D full frame DSLR, using a Canon EF 180 mm 1:3.5 L Macro lens.

An 8 megapixel Canon 20D DSLR, equipped with a specialized high magnification (1x to 5x) Canon macro lens, the MP-E 65 mm 1:2.8, was used to take the remainder of the images.

Further Reading

About Ferns                http://www.home.aone.net.au/~byzantium/ferns/about.html

Backyard Ferns        http://www.backyardnature.net/ferns.htm

Fern Reproduction     http://www.bbg.org/gar2/topics/botany/repro_ferns.html

Fern Sporangium      http://www2.auckland.ac.nz/info/schools/nzplants/fern_sporangium.htm

Gardening Ferns       http://www.hgtv.com/hgtv/gl_plants_ferns/article/0,1785,HGTV_3604_3450395,00.html

A Flower Garden of Macroscopic Delights

A complete graphical index of all of my flower articles can be found here.

The Colourful World of Chemical Crystals

A complete graphical index of all of my crystal articles can be found here.

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