A close-up view of the Christmas Fern
View of the
by Brian Johnston (Canada)
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,
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.
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
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.
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
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
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 1/50th of an
inch across!) For still closer views of sporangia and their
spores, please see my article concerning the Shield Fern – Dryopteris filix-mas
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
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
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
February 2011 edition of Micscape.
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