What are ferns?
Ferns form the second largest group of vascular (i.e. having specialised water and sugar conducting tissues, seen as veins) land plants after the flowering plants (the angiosperms). There are about 12,000 fern species worldwide, most of them tropical. They are the modern descendants of an ancient group that originated about 350 million years ago, more than 200 million years before angiosperms first arose. Ferns are distinguishable from angiosperms by their lack of flowers, and from angiosperms and gymnosperms (such as conifers) by their lack of seeds. Instead, ferns, like all other so-called “lower plants”, colonise new sites by the dispersal of unicellular spores produced in sporangia. Ferns, together with horsetails, the ferns’ closest relatives, and clubmosses and quillworts, which are only distantly related, are distinguished from all other land plants by their life cycle, which consists of two alternating phases capable of independent existence. However, only ferns aggregate the sporangia into sori, usually on the backs of fronds (leaves), whereas in horsetails and clubmosses the sporangia are grouped in terminal cones and in quillworts they are embedded in leaf bases. Ferns are distinguishable from mosses and liverworts (the bryophytes) which have much simpler conducting tissue, sporangia borne singly on the end of short stalks, and a life-cycle with one independent phase.
So how do I know my specimen is a fern?
If there are spore-forming structures on the undersurface of fertile fronds (leaves), it is a fern. In only 5 British species (Royal Fern, the three Adder’s Tongues and Moonwort) are the sporangia aggregated at the end of fronds. Pillwort (Pilularia globulifera), the least ‘fernlike’ of the British species, belongs to the small group of ferns that are ‘heterosporous’ (i.e. produce two types of spore, large megaspores and small microspores, unlike the other, ‘homosporous’, ferns that produce one spore type). In Pillwort, the sporangia are contained within small seed-like capsules called ‘sporocarps’.
If young fronds unfurl from a crozier or fiddlehead, it is a fern. Only four related species (the three Adder’s Tongues and the Moonwort), belonging to an ancient group that has evolved independently from the other ferns for about 300 million years, do not show this feature. (Young flower stalks of the insectivorous plant Drosera appear as croziers but are unlikely to be confused with a fern!)
Only ferns with fully expanded but sterile fronds could be sometimes confused with plants of other groups. Most have distinctive frond morphology which is soon learned but observers new to fern identification might confuse non-flowering individuals of some herbaceous (non-woody) angiosperms with non-sporing individuals of some ferns found in similar habitats. The basal leaves of some members of the Carrot family resemble fronds of some fern species like the Broad Buckler Fern, but they differ in having a sheath at the base of the leaf stalk which envelops adjacent leaves or stems. The undivided leaves of the Wild Garlic (Allium ursinum) are superficially similar to the fronds of the Hart’s Tongue Fern but differ in having no conspicuous mid-vein and being deciduous, whereas the fern is wintergreen. The similarly undivided leaves of Plantains (Plantago species) and Sorrels (Rumex species) can be confused at a distance with sterile fronds of Adder’s Tongue, but they are less fleshy and have conspicuous veins. The much divided leaves of the Lousewort (Pedicularis sylvatica) are remarkably fern-like, reminiscent of small Spleenworts but their habitats could hardly be more distinct with Louseworts restricted to marshy heaths and the Spleenworts limited to rock faces and walls. The only ferns that could be confused with bryophytes are the very small Filmy Ferns that also grow on very damp rocks and tree trunks, but the ferns have, in addition to quite different spore-forming structures, a more conspicuous mid-rib in fronds which are larger than most mosses.
The life cycle of a homosporous fern
The familiar fern plant is just one phase of a two-phase life-cycle. This spore-forming phase, or sporophyte (“spore-plant”), alternates with an egg- and sperm-forming phase, the gametophyte (“gamete-plant”), to form the complete sexual life cycle.
A diagram of the life cycle of a fern can be found here.
The gametophyte is morphologically very different from the sporophyte and completely independent of it, living as a separate individual plant. When beginners are first introduced to the fern life-cycle, they sometimes find it hard to understand that the gametophyte of, for example, Bracken is as much a plant of that species as is the well-known and conspicuous sporophyte. Perhaps the closest parallel, familiar to many, is the butterfly life-cycle, where the larva or caterpillar of the Large White is an individual of that species just as is the imago or flying adult.
The gametophyte develops from a single-cell spore. Although in the Filmy Ferns the gametophyte is a perennial mat-forming filament of green cells, in most homosporous ferns, the gametophyte is small (rarely more than 1cm across), short-lived (rarely surviving for more than a year or two), and very simple in structure. It is not differentiated into leaves, stems and roots, but develops into a simple flat plate of green cells, referred to as the prothallus (plural prothalli), anchored to the surface of the substrate by single-celled colourless rhizoids. It is frequently approximately heart-shaped as a consequence of the formation of “wings” of cells either side of a growing region (meristem) where the cells are formed. The wings are only one cell thick. On the undersurface of the central “cushion”, several cells thick, develop the archegonia (singular archegonium), each containing one egg, and the antheridia (singular antheridium), each containing many antherozoids or sperm. When released, a sperm is capable of swimming several centimetres to an archegonium. Having entered the archegonium, a sperm can penetrate the egg. Fusion of the two nuclei results in a zygote. The zygote then develops into a new juvenile sporophyte, or sporeling, which is initially dependent on the prothallus but soon develops leaves and roots and becomes independent as the gametophyte dies. The sporophyte becomes fertile within a year or two under favourable conditions, and can then live many decades or even centuries. Mature sporophytes produce single-cell spores in sporangia (singular sporangium) aggregated into sori (singular sorus) on the undersides or at the ends of fronds.
The spores are produced in enormous numbers, up to 10 billion each year on a large plant of Male Fern, and are dispersed by wind. Many fall close to the parent plant, but there is evidence that some spores can travel hundreds or even thousands of miles. If they alight at a suitable site, they germinate and within a few weeks form a new prothallus. They require light, moisture and an equable temperature to develop and they need an open site free from competition by faster growing plants like mosses and seedlings. Some spores become buried in the soil and remain dormant there in the dark unless brought to the surface. Buried spores may remain viable for many years, forming a soil spore bank resembling the seed banks of flowering plant weeds.
Most spores fail to develop or the gametophytes succumb to competition, grazing or disease. Consequently, it is not easy to find gametophytes in the wild. They are most easily found on bare soil, porous rock or rotting wood in damp conditions shaded from direct sunlight. They seem to prefer sloping surfaces, perhaps for better drainage or because they are less likely to be covered by fallen leaves, which appear to be a major cause of restricting the sites available for gametophyte development in a deciduous wood. Once found, gametophytes are usually very difficult or impossible to identify to species.
The life cycle of a heterosporous fern
Native Pilularia globulifera and naturalised alien Azolla filiculoides are the only heterosporous ferns growing wild in Britain. They form hard, seed-like sporocarps (modified fronds in Pilularia near the base of a normal fronds, modified indusium in Azolla as an outgrowth of a submerged leaf lobe) which contain the sporangia. Heterosporous ferns produce large megaspores and small microspores which develop into unisexual megagametophytes and microgametophytes respectively. Both types of gametophyte are much reduced compared with those of homosporous ferns and they mature very rapidly, within a few days or even hours. The mature microgametophyte is little more than an antheridium which eventually releases antherozoids, while the megagametophyte develops a small cushion of cells which emerges from the protective spore wall just enough to expose one or more archegonia. After fertilization, the embryo is initially dependent on the megagametophyte but rapidly becomes an independent sporophyte.