INTRODUCTION TO FERNS
by Don Lubin
Ferns today in New England tend to be knee-high herbs of damp shady woods. They don't do much and have little economic value, but people often think they are pretty. I have seen ferns within half an hour of Boston that were seven feet tall. New Zealand ferns can grow to 80 feet. Some ferns grow in deserts. But the lacy undergrowth by a shaded stream or swamp is more typical hereabouts. Ferns are ancient, one of the first groups of plants to colonize the land, and even now they can help life on earth recover from a catastrophe.
For perspective, this chart shows where ferns fit in to the scheme of things.
Bacteria and single-celled organisms got life started and began to build a liveable atmosphere.
Plants mostly convert sunlight to living tissue.
Fungi were plants when I started school. They live on dead things, or even live ones.
Animals eat plants and fungi and each other.
Algae first lived in the sea.
Mosses grew a coating to keep them moist and moved onto the land. They created spores for long distance, and delayed, propagation. But most could never grow taller than about 4 inches because the 5th inch would run out of water and die.
Pteridophytes developed a vascular system to reach down to the water table and bring water to their upper parts, and transport oxygen and nutrients. They created lignin for strong woody stems. Then they then reached 120 feet in height.
And spermatophytes, most of the plants we have today, developed modern seeds and pollen and flowers.
Pteridophytes are three times as old as flowering plants.
Mosses and pteridophytes use spores to propagate, and a word for them is "cryptogams" because they hide their gametes.
Pteridophytes and spermatophytes are the vascular plants that have a circulatory system. A word for them is "tracheophytes".
So pteridophytes can be defined as the intersection of those two groups: either "vascular cryptogams" or "free-sporing tracheophytes".
Horsetails grow usually in wet habitats and have jointed stems, like bamboo. They may be branched or not. The stems themselves are photosynthetic and they don't have what we would consider leaves. These were the first group of vascular plants, and led the colonization of land.
Club-mosses are small plants nowadays, though they used to be tall like ancient ferns and horsetails and Isoetes. They have tiny needle-like leaves.
Selaginella in New England are very small plants, and not very common.
Isoetes or quillworts mostly grow at the bottom of ponds. Some can fix nitrogen.
Ferns were the first plants to have extended flat photosynthetic tissue we think of as leaves. There are now about 12,000 species worldwide, but most are tropical. In New England we have 68, or more if a new one is found, or a species is split, and fewer if a rare plant dies or its habitat is replaced by a mall.
There are some tree ferns planted in Golden Gate Park in San Francisco, and some 11 foot tall Giant ferns in south Florida. The tallest fern in New England is Lygodium, which sometimes reaches 25 feet by cheating: it is a vine that climbs up trees. I have seen Cinnamon and Ostrich ferns get to 7 feet in ideal habitats. Some of the Moonworts don't much exceed an inch.
Ferns comprise about 3% of the vascular plant species in the U.S. and Canada. In some ways they are unimportant nowadays among the dominant spermatophytes, just a pleasant adornment of our woodland paths. But they have had and continue to have an important role in life on earth.
Here is a chart of geologic time. The earth (the whole solar system, actually) cooled to a solid about 4.6 billion years ago. The atmosphere contained a lot of carbon dioxide and sulphur, and almost no oxygen. By three billiion years ago cyanobacteria had evolved and were converting carbon dioxide to oxygen, and by 2.3 billion years ago, oxygen was beginning to build up in the atmosphere. By 600 million years ago there had been enough oxygen not only to precipitate most of the metals out of the oceans, forming the the iron oxides we see in red rocks and red clay, but also to oxidize minerals on land, oxidize atmospheric methane, and create an ozone layer atop the atmosphere. Then oxygen levels really increased and life exploded. About 420 million years ago, the horsetails began colonizing the land, followed soon by club-mosses and ferns. Their vascular systems let them grow up into the air to spread their leaves, and down into the ground to spread their roots for structural support and for water, and to bring oxygen to those roots. They were able to grow away from the swamp edges. 360 to 300 million years ago is called the Carboniferous Age. Almost all the coal in the world was formed then, as tall ferns grew and fell and were washed into swamps and were buried in mud. As a teenager in northern Illinois I found quite a few fern pinnules fossilized in shale, like this one.
In this rather approximate computer simulation of atmospheric oxygen levels over the last 600 million years, the most striking feature is the peak of 35% at the end of the Carboniferous Age. It was never that high before nor since. It is 21% now. In those days dragonflies could have a two foot wingspan. Ferns provided the oxygen as well as the food that allowed animals to emerge from the oceans and colonize the land.
There have been five great mass extinctions, when more than 50% of life on earth died out.The biggest was 250 million years ago.
200 million years ago our still thriving Interrupted fern, Osmunda claytoniana, was growing in what has now become Antarctica, and left a fossil to be found under the ice.
140 million years ago, flowering plants began to appear.
65 million years ago was another mass extinction. An asteroid or whatever crashed into the Yucatan, and all the dinosaurs died out. For some time afterwards, pollen nearly disappeared from North America, replaced entirely by fern spores.
This Fern Spike was caused by the more rapid reintroduction of ferns than of seed plants into the devastated areas. Spores may be more hardy than seeds, and are certainly lighter than most and can travel farther in the wind. Even after contemporary devastation such as the Mt. St. Helens eruption, ferns recolonized more rapidly than flowering plants. Interestingly, one of the ferns observed in the early repopulation was one native not to Washington State, but to Japan.
The last ice age peaked around 20,000 years ago, and since then the polar ice caps have been receding. At the peak, New England was covered with about a mile of ice. This heavily discouraged plant growth. Invasive plants are defined as having been introduced from outside the region, generally by deliberate or unwitting human intervention. But all of our plants* have migrated to this region during the last 14,000 years. The plants that survived the ice age found refuge in some more southerly location, and have slowly migrated back.
(*An exception is Trichomanes intricatum, a species in the same genus as the southern Filmy fern, which apparently rode out the ice age in gametophyte form in protected caves. It currently grows only as a gametophyte, incapable of genetic reproduction but propagating vegetatively via bits which break off and wash away.)
The sixth mass extinction is the one humans are currently causing. Apart from replacing habitats with agriculture and commercial development, and polluting air and water, and transporting invasive species to new areas, humans are burning much of the coal and oil that has sequestered carbon for millions of years, putting carbon dioxide back into the atmosphere, and warming the planet.
One way to tell ferns from other plants is that ferns emerge as "fiddleheads". They form the shape of a logarithmic spiral. This allows the tender growth tip to be protected inside tougher more developed tissue as the frond grows up. Ferns generally cover their fiddleheads with an armor of scales or hairs to discourage predation, and this indument persists on the stems of adult plants and affords a useful means of identifying species. This mode of growth is called "circinate vernation".
(The term "fiddlehead" is used in a second sense in the grocery trade to mean the young shoots of the Ostrich fern, the only fern species legal for sale as food in North America.)
The fern fronds we see are the leaves of the plant; the stems and branches are underground rhizomes. But unlike leaves of spermatophytes, fern fronds often bear their reproductive spores on the underside of the fronds.
A fern frond is not capable of secondary growth. If you pinch off the growth tip of a tomato plant, new growth is stimulated from the axils of the branches. But if the growth tip of a fern is damaged, its response is to grow a new frond. Part of why ferns seem beautiful is because they maintain the symmetry of their design.
In most ferns, some fronds have sori (fruit dots) on their undersides; these are called "fertile" fronds. Other fronds do not, and are called "sterile". The fertile fronds usually look a little different than the sterile fronds. Generally they are taller and more erect, as their main function is to release spores into the breeze for propagation at as great a distance as possible. The function of sterile fronds is primarily photosynthesis, so they often grow to face the available light, which is often the sky above. When the difference in morphology between the fertile and sterile fronds is noticeable, the plant is called "dimorphic". In some ferns this dimorphism is extreme, e.g. fertile Cinnamon fern fronds usually have no green photosynthetic pinnae at all, just an erect stipe topped with reduced and appressed cinnamon-colored pinnae bearing sporangia (spore cases).
Fertile fronds are very useful in identifying the taxon, but should not be relied on too heavily since sometimes no fertile fronds are present. Interrupted ferns are quite easily recognized by the gap formed when fertile pinnae shrivel and drop off. But often a large colony of Interrupted ferns will have not a single plant that has any fertile fronds.
This page shows the steps in genetic reproduction of ferns.
Here is a photograph of sori on the back of a Dryopteris frond, showing the sproangia and the receding indusia.
Autofertilization of gametophytes provides no advantage to the gene pool, though it can allow propagation in a pinch. Ferns, like seed plants, have various mechanisms, including timing, to assure that the ova of one gametophyte are usually fertilized by sperm from another. In some species the archegonia (ovaries) actually send out chemical signals to urge other nearby gametophytes to produce sperm rather than ova.
Most individual fern plants actually reproduce vegetatively. Those with horizontal rhizomes simply extend to new patches of ground (or rock surface). Some like Bulblet or Mother fern grow little "seeds" or plantlets that drop off and grow clones. Some like Walking fern grow new plants from profligate tips of their fronds.
Some grow clones by means of "spores" that do not undergo meiosis, a process called apogamous reproduction (Polypodium and Dryopteris hybrids). Pellaea atropurporea is a triploid plant, quite incapable of normal genetic reproduction, so like dandelions, it clones itself apogamously. This will not deepen its gene pool one bit, and perhaps in 30,000 years it will pay the price, but for now it is reproducing just fine.
Of all the millions of fern fronds in the world, probably no two are exactly identical, even from the same plant. The trick to identifying fern species is knowing which differences matter and which don't.
There are various approaches one can take to identifying plants.
It is tempting when e.g. writing a key to rely on the characteristics of fertile fronds, or on details invisible without magnification. It is more convenient in field identification to use such characteristics only for confirmation, and rely only on readily visible characteristics of sterile fronds.
The most generally useful clues to the identity of a fern are:
Degree of cutting
Size, color, habitat, sori placement, teeth, blade indument, and even spore size can also be useful.
Most of our ferns are twice-cut. This means dissection in the range of pinnate-pinnatifid to bipinnate lobed. Other groups of ferns are cut three times, or once, and a very few are not cut at all (entire). The approximate degree of cutting is generally fairly easy to see. This refers to the part of the blade where the dissection is greatest, generally the inner portion of the basal pinna.
[entire] . .llllll . . .lllll .... . . . . . . [once-cut] . . . . .lllllll.llllllll.... ..llllllllll... . . . [twice-cut] . . . . .... ........... . . .... lllllllllllllllllllllll.llll . . . [thrice-cut]...............
If the rhizome grows vertically, the fronds emerge in clusters around the emerging rhizome end, forming a vase shape, with fronds facing the center. From a horizontal rhizome fronds will grow up singly, and face the light. This difference is quite useful in identifying species.
[clustered] . . . . v. . . . . [scattered]
The "armor" used to protect the emerging fiddlehead often persists on the stipe and sometimes the rachis and even blade of the fully unfurled frond, though it may shrivel or drop off late in the growing season. Hairs are one cell and round in cross-section: scales are one cell thick but two or more cells wide, and hence flat in cross-section.
............[glabrous] . . . .. ................... .. . . . [hairs] . . . . .. . . ......... . . . . [scales] ........... . .. . . . . . . ["wool"]
Most fern blades are pointed at the top and widest either in the middle or near the base. Ostrich ferns are widest near the top. Maidenhair ferns have quite another shape. Bulblet and Walking ferns are very long and narrow. A number of ferns have a tripartite shape in which each basal pinna is nearly equal to the remaining upper portion of the blade.
The study of fern hybrids should be delayed until one has a reasonably thorough knowledge of the identification of the parent species.
Sometimes when the parents have distinct morphologies, the hybrid can be identified reliably by its frond morpholgy alone, intermediate between the parents. But often (Polypody, thrice-cut Dryopteris, Cystopteris) the parent species are so similiar that morpholgies of the three taxa (two parents, one hybrid) would overlap. In these cases it is necessary to have a "fertile" sori-bearing frond of the hybrid and find either aborted spores or at least aborted sporangia. I doubt it is possible to identify with certainty any New England Polypody until the spores have matured, which may not occur until October.
Ferns (and living things generally) have two sets of extremely similar chromosomes. In genetic reproduction these two sets separate in a process called meiosis. The offspring then receive one set from this parent, and a similar set from the other parent, and once again have two sets.
In a hybrid, the two sets of chromosomes received from the parents differ significantly. Only closely related species can form viable hybrid offspring, as the dissimilar sets of chromosomes must work together to form a functional organism. Sometimes, as with Dryopteris X slossonae, they do this with difficulty, and a variety of genetic errors are often visible, such as twinning of the frond or a pinna, a pinna growing in place of a pinnule, or pinnae or pinnules missing.
Hybrids are usually sterile, as the chromosomes in even a viable plant are so mismatched that meiosis can not occur.
Sometimes there is a mutation in which all the chromosomes are doubled. When this happens in a species the resulting genome is called autopolyploid. The new genome can result in different characteristics. For example, Asplenium trichomanes trichomanes is the diploid form and grows fairly erect from crevices in a variety of rocks. The tetraploid Asplenium trichomanes quadrivalens grows supine on the surface of limestone.
The genome resulting from chromosome doubling in a hybrid is called allopolyploid. The effect on a sterile hybrid is to make it a fertile species, since the new genome does have a pair of each chromosome and can undergo meiosis. Quite a few fern species have originated in this way, including Polypodium virginianum, Cystopteris fragilis and C. tenuis, and Dryopteris campyloptera, carthusiana, and cristata.
Interestingly, one parent of both Dryopteris carthusiana and D. cristata has never been found, but from chromosome pairing it is clear that the parent was the same. Apparently the same situation occurs with respect to Cystopteris fragilis and C. tenuis.
In Dryopteris clintoniana and Cystopteris laurentiana the chromosome doubling mutation happened twice, resulting in a hexaploid genome.
We are fortunate in terms of pteridophyte diversity to be north of the terminal moraine, the southern limit of progress of the glacial ice sheet. (The last one ran through Long Island and southern Rhode Island.) The glaciers pushed out almost all plant life, but they also scoured the landscape, scraped and crushed bedrock, and provided an enriched mineral complement to our soils.
In 2004 Ray Abair & I travelled to mountains in North Carolina as part of a New England Botanical Club field trip. As always when travelling, we looked for ferns on the side of the road. A surprising number can be identified even at high speed. But for most of the way there were none to be seen. Coming home we saw essentially none until we returned to New York state, and no interesting diversity until we had reached Massachusetts. Yay glaciers.
Vermont has 62+/- fern species, the most of any state in New England, which has 68 altogether. These numbers may be higher than those for Michigan or California or any other U.S. state except Florida, which has semitropical habitat in the south.