View of the
filix-mas 'Crispa Cristata'
by Brian Johnston (Canada)
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
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.
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.
Shield Fern Dryopteris filix-mas
is therefore called a twice-divided,
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
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
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
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
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
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
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
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.
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).
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
The photomicrographs were taken
using a Leitz SM-Pol microscope (using dark ground and phase-contrast
condensers), and the Coolpix 4500.
A Flower Garden of
A complete graphical index of all
of my flower articles can be found here.
The Colourful World of
A complete graphical index of all
of my crystal articles can be found here.
Microscopy UK or their contributors.
Published in the
January 2011 edition of Micscape.
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