Close-up View of the Shield Fern

The Shield Fern, or Male Fern, is a common plant in the Northern Hemisphere which grows throughout much of North America, Europe and Asia. In the ‘wild’ variety, fronds grow up to a metre in length, but in the dwarf cultivar studied here, ‘Crispa Cristata’, fronds tend to have a maximum length of about thirty centimetres.

Historically, the roots (rhizomes) were formed into amulets called “St. John’s hand” which were used as protection against evil spirits! The same rhizomes were ground up and ingested with an oily purgative in order to expel tapeworms from the body. Apparently, the root contains a compound that paralyzes the muscles of the worm causing it to release its hold.

Ferns are referred to as “Shield” ferns if they have a shield-shaped protective covering for their spore containing structures. Thus ferns belonging to the genera Dryopteris, Polystichum and Lastreopsis are sometimes called Shield Ferns. In the image above, these shield-shaped coverings are donut shaped, and silvery-gray in colour. The species name filix-mas is derived from the Latin filix, meaning fern, and mas, meaning male.

When a new plant emerges from the ground in spring, its top is curled into a spiral called a fiddlehead. The name was chosen because of its resemblance to the end of a violin, or fiddle. The two images that follow show a typical fiddlehead.

As the fiddlehead increases in length, it begins to uncurl. One stage in this process can be seen in the image on the left below. The image on the right shows a magnified view of the stalk, better referred to as the blade or rachis, that supports the fern’s leaflets. Notice the many orange-brown scales that cover the surface of the blade.

The fern leaf shown in the two images that follow, is called a frond. The frond is the entire fern plant, minus the root. Many leaflets called pinnae are attached to the rachis in alternate positions. Each pinna is composed of many smaller, sub-leaflets called pinnules. The Shield Fern Dryopteris filix-mas is therefore called a twice-divided, or twice-cut fern.

Notice that each of the pinnules is curled or ruffled at its rounded tip. This characteristic is referred to as a crisped pinnule. Now you know why the cultivar name is ‘Crispa Cristata’! (The Cristata term refers to cultivars without finger-like extensions on the pinnules.) Also note in the photographs that the rachis is longitudinally grooved.

The tip of a newly unfurled frond can be seen below in front (left), and back (right) views. Notice that the delicate vein detail seen in the earlier images of mature fronds has yet to appear. Both pinnae, and pinnules have scales, with the undersides displaying larger ones than the front sides. Also note that it is only the front of the rachis that possesses a groove.

In the remainder of the article we are going to take a look at the development of the fern’s distinctive reproductive structures. Ferns produce spores that grow into tiny plantlets. The small circular structures that can be seen on the undersides of pinnules (below) contain these spores. Note that at this very early stage, the structures are very pale green or white in colour.

The two images that follow show pale, membranous, “shield-like” structures, called indusia (singular – indusium), that cover the developing spore-containing organs of the fern.

Later in the growing season, the underside of a frond has rows of these spore-containing structures beside the stalks of its pinnae.

Closer views reveal that these structures are not arranged neatly.

The indusium is roughly donut shaped, and has a radial groove at one location on its surface.

The four images that follow show an indusium’s shape more clearly. In the highest magnification images, the translucency of its membrane allows us a fuzzy view of the spore-containing structures that it protects.

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 providing that the environmental conditions are suitable. The images below show that some of the indusia have shriveled, and turned orange in colour, revealing a cluster of yellowish-green, spherical structures beneath. The cluster of spheres is called a sorus (plural sori). Each spherical spore-containing chamber is called a sporangium (plural sporangia). Put simply, the indusium covers the sorus which is composed of many sporangia which, in turn, contain the plant’s spores.

As the magnification increases, the sporangia become easier to see.

Each indusium shrivels, and curls back differently. Some sporangia are almost transparent, while others are nearly opaque. The longer the sporangia are exposed to the air, the less transparent they appear.

Some time has passed since the last images were photographed. All of the reproductive structures on the frond’s underside have darkened in colour, increasing their contrast with the pinnae. Notice that some of the lower pinnae on the frond have no reproductive structures. This seems to be a common characteristic of many ferns.

The fern’s reproductive structures seldom develop at the same rate. A variety of stages can be seen below.

Notice that some of the sporangia appear almost black in colour.

In fact, these dark sporangia still contain their spores. It is these mature spores that give a sporangium its dark colouration. When a sporangium springs open to release the spores, it empties and looks much lighter in colour. The brown “dust particles” on the pinnule’s surface are actually the fern’s spores.

If you examine the image on the right, you may be able to discern that each sphere-like sporangium has a slightly raised, bumpy rib ringing it. This ring is called the annulus.

When a sorus is examined under the microscope, the bumpy ribs mentioned earlier are clearly visible. All of the sporangia have opened to release their spores. The image on the right shows a more highly magnified view of an empty sporangium.

Here are two photomicrographs showing a “full” sporangium. The dark brown colouration of the spherical structure is evidence of the spores within. (The fuzzy spots in the image are out-of-focus spores.)

Below is a very high magnification image showing the bumpy ridge, or annulus, that rings each sporangium.

In the view on the left below, the annulus looks almost like a spring. In fact, the sporangium does spring open with some force, and in doing so, it flings the tiny spores a considerable distance. The first time I examined a sorus under the microscope I was startled by the constant, random, springing-open of the sporangia within it! The image on the right shows a single spore clinging to the sporangium’s ridge.

Here you can see how a sporangium operates. The annulus breaks near the base of one side, arches backward, and breaks open the sporangium. Then, abruptly, the annulus snaps forward, catapulting the spores into the air.

Spores are irregular in shape. Only if the conditions are ideal will the tiny fern plantlet begin to grow.

High magnification photomicrographs reveal that each spore does have some irregular surface detail.

To conclude the article, I will attempt to describe, very briefly, what happens to a spore after it lodges in an “ideal” location. First, the spore grows into a tiny heart-shaped plantlet called a gametophyte. This gametophyte has only half of the genetic material of an adult fern. On its underside, a gametophyte has two sets of reproductive organs, one set composed of male parts, and the other of female parts. The male parts contain sperm cells, and the female parts, egg cells. Because the male and female parts are slightly separated from one another on the gametophyte’s surface, a thin film of liquid water is required in order that the sperm cells be able to swim over to the egg cells. This accounts for the fact that ferns are unable to reproduce in sunny dry environments. If the trip is successful, sperm and egg fuse to make a cell that contains a full adult fern set of genes. The cell then undergoes division, and eventually develops into an adult fern. (For a more detailed, complete description of the entire process, please see the reference About Ferns below).

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.

The photomicrographs were taken using a Leitz SM-Pol microscope (using dark ground and phase-contrast condensers), and the Coolpix 4500.

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

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