All plants in the Pteridophyta Division are known as ferns and most have the easily recognisable fern-shape, with fronds that unfurl to form distinctive self-similar shaped leaves radiating from a central point. These ferns are often grown in gardens and like shady areas with moist soil. Their leaves have also been used to demonstrate fractals, as explained here.
However there are a smaller number of plants in the Pteridophyta Division that have very different forms, some do not even have leaves, others look more like clovers than ferns.
What makes pteridophytes different to other plants?
Pteridophytes differ from lycophytes (see previous blog about Lycopodiophyta Division) in that most have true leaves, called macrophylls. There are a few exceptions, such as Psilotales (see below) and horsetails (Equisetum).
Fern leaves grow by unfurling, starting off as tightly coiled balls. The manner of unfurling varies from species to species.
Like lycophytes, pteridophytes have no flowers, using spores to reproduce. The spores are produced by sporangia. All plants have sporangia in one form or another, but in ferns these can be seen in clusters, called sori (singular: sorus). Sori on ferns are yellow, brown or black and are usually found on the backs of leaves.
Also like lycophytes, the pteridophytes have a distinct sporophyte and gametophyte generation, with the sporophyte generation the dominant one, in ferns the sporophyte generation is the one with the leafy fronds. In lycophytes the sporophyte grows out of the gametophyte, the two are attached, however, with pteridophytes the gametophyte is a completely different plant. The fern gametophyte is only small, a few millimetres across, often growing under the ground and with primitive rhizoids instead of roots.
The Fern Family Tree
The Pteridophyta Division is made up of four classes and 10 orders (as always, these classifications vary from source to source, so I have chosen the most reliable system I can find). Of these, the Polypodiales Order is the largest, containing 80% of the world’s ferns, and also the most familiar, with all but a few of the ornamental ferns seen in gardens in Britain and Ireland.
When researching distribution in Britain and Ireland, I found this website very useful http://www.ferns.rogergolding.co.uk/index.html
The Pteridophyta Orders
These are tropical ferns and do not look fern-like at all. They do not have leaves, but small outgrowths called enations, – in leaves the xylem and phloem are inside the leaf, but in enations they are just beneath. Plants in this order do not have roots, but more primitive rhizoids (see previous blog: Mosses, Liverworts and Hornworts for more information). They consist of only two genera, Psilotum, aka Whisk ferns, with dark green stems and peanut-like sporangium, containing the spores; and Tmesipteris which has a more leafy, draping appearance.
Like the plants in Psilotales, these are not very fern-like to look at. They consist of moonworts, adder’s tongues and grape ferns. These plants contain a lot of chromosomes, with one species, Ophioglossum reticulatum (looks similar to the Ophioglossum vulgatum seen right), containing a total of 1,260 (humans have 46 and most animals and plants have between 20 and 60). Although Ophioglossum vulgatum looks a little like an arum lily, it is not the same at all, the central part is not a flower, but the sporophyte. Their gametophytes live below ground and use fungi to obtain their food rather than using sunlight.
Equisetum is the only surviving genus of the Equisetopsida Class, but it is a diverse genus growing all around the world, some are even aquatic. About twelve different species grow in the UK. These plants have a a distinctive appearance, single green stems that photosynethesize to compensate for the leaves that are reduced in size. Over three hundred million years ago horsetails grew up to 30 metres high and helped to form the first forests.
There is only one family in this order and all the ferns within it grow in the tropics. Some species have fern-like leaves, other have less divided leaves (see photo above). Some are giant with the fronds reaching up to 9m in length. Plants within this order can be recognised because their spore cases are fused together to form one long sorus. (see photo above)
Osmundales have been around for 210 million years, but only four genera are still living. They have fairly typical fern leaf shapes. Osmunda is the only fern that grows in the UK, it has photosynthetic fronds and non-photosynthetic, spore bearing fronds, which are brown, often referred to as flowers. Other than Osmunda, ferns in this order are tropical.
These are known as the filmy ferns, their leaves are only one cell thick between the veins, which gives them a delicate, gauzy appearance, some have a typical fern-shape to their leaves, but others not. The sori are on the edges of the leaves rather than the back. Most are tropical, but some can be found in temperate rainforests. Three species have been found in Britain and Ireland.
These are tropical ferns, some with forked leaves arranged in a circle, others with fern-like leaves. There are only three families.
This order contains three families, with most species found in the tropics and a few temperate, although none in the UK. Ferns in this order have delicate leaves, some with ferny appearance. In this order is Lygodium, a climbing fern that has become a problematic weed in America. It has two different types of leaf – divided leaves containing the sporangia and entire leaf just for photosynthesizing, (picture here.)
Ferns in this order are aquatic and mostly found in South America or Oceana. In this order are Azolla – the world’s smallest fern with leaves so tiny they look a little like duckweed; Marsilea that look like four leaved clovers (pictured) and pillwort that looks like quillwort, with thread-shaped leaves. Salviniales either float or grow in mud. Three species have been found in Britain and Ireland.
This order contains the tree ferns, one, Cyathea medullaris, can grow up to 20m. Tree ferns do not have wood and bark like other trees, instead they have modified roots growing above ground to form a mat that supports a slender stem, allowing the plant to grow tall. As shown below, tree ferns are often covered with hairs, and sometimes scales. All have typical fern-shaped leaves. The tree fern families are Cyatheaceae and Dicksoniaceae, Metaxyaceae and Cibotiaceae. Ferns in this order tend to be tropical, but a few are temperate. Dicksonia are sometimes grown in gardens in the UK but they need winter protection if the temperature drops below freezing. Cyathea have also been grown successfully in the UK.
This order contains 80% of worlds ferns, 250 genera and 9,000 species. They grow everywhere except Antarctica. Many have typical fern-shaped leaves, but not all (as seen in photo below). This is the order that contains most ornamental ferns found in gardens in the UK, for example – Adiantum, Blechnum, Woodsia, Polystichum, Onoclea, Matteuccia, Dryopteris, Asplenium (with entire leaves), Athyrium (Japanese Painted Fern) and some of these are native. Polypodiales also contains Pteridium, or bracken.
How old are ferns?
Ferns are often considered ancient plants, but while they do date back 350mya (first plants on land were 475mya, see previous blog about plant evolution for more information) all those families have died out now. There are a few ferns dating from 270mya that are still around now – Cyatheaceae and Dicksoniaceae, Gleicheniaceae and Hymenophyllaceae, but the vast majority of ferns only appeared about 75mya, around the time that orchids and lilies (both very recently evolved plants) appeared.
The Potato Fern
(Also written about in the Parasites Section of the Odds and Ends of Nature tab)
Solanopteris brunei is the potato fern in the Polypodiales Order, it grows in Central and South America in the branches of trees. Ants colonize the ferns, living inside the potato like tubers and providing protection for the ferns in return.
This is a great web page about Solanopteris and Lecanopteris crustacea, another fern that has a symbiotic relationship with ants, some good photos. http://www.cpukforum.com/forum/index.php?showtopic=48907