Ferns are very special plants, because when we see a fern we are looking at a type of plant, which has been around an unimaginably long time, as ferns have made up a large part of the earth's flora since at least the Carboniferous era, and plants very much like ferns may well have existed even in the late Devonian, over three hundred and fifty million years ago. Ferns were probably the most advanced or complex of plants throughout much of that time, and eventually gave rise to the Seed Ferns, which in turn, before they went extinct, gave us, perhaps via the Cycads, the Conifers and Flowering Plants which make up most of the modern flora. Yet still the ferns are here today, and still making a major contribution to our plant life. Ferns are generally regarded as being among the most complex of the none flowering plants. For unlike the mosses and liverworts, the ferns have true leaves, stems and roots, yet they still have one character in common with these, so called, primitive plants. Which is that they reproduce themselves by means of spores, rather than flowers or seeds.
If you look on the underside of a fern frond, (posh name for its leaves) on a summer’s day you will quite likely, especially if it is quite a large mature frond, see some things that look very much like warts or blisters. These are know to botanists as Sori and usually have cover on them called an indusium, and when these are completely ripe, the indusia will split open, to reveal some smaller structures that usually look like bobbles on stalks, these are the spore shedding organs called sporangia. Each sporangia will contain 64 or more spores, which are shaken out by the flexing of the sporangia’s stalk, which takes place as the air temperature and humidity change. When the spore, which is just a single cell, lands on the forest floor or some other suitable place, it begins to grow, but not into a fern, instead it grows into an entirely different plant called a prothallus.
The prothalus does not look in the least like a fern, it is just a single nearly flat leaf stuck to the floor by little white root hairs, it is never much more than a centimetre across, and that is basically all there is to it. But the prothallus is none the less the sexually active stage, in the ferns life cycle, for ferns are plants with two different stages in their growth. When the prothallus is mature it produces male and female sex organs on its underside, (Antheridium and Archegonium), and soon releases free-swimming sperm into the dew and rain of the forest floor. Which hopefully fertilize the egg cells of another prothallus, then, usually in a year or two from the spore landing, a new fern will begin to grow. When this happens the prothallus has done its job and it then withers away and dies completely, and eventually, usually within two to three years, you will have a mature fern plant.
The Diversity of Ferns.
Ferns however are by no means all leafy plants of the forest floor, rather being a group of plants which has been around in the world for such a long time the ferns have adopted almost every habitat and form of growth possible for green plants. Many of them would not easily be recognized as ferns. For example Pillularia globulifera is a a small fern which looks very much like a patch of green turf and is only found growing on the bottom of muddy ponds, or you may like to look at the photo of Ophioglossum a low growing fern of grasslands.