Ferns are a group of well-known plants that first developed during the carboniferous period about 350 million years ago when the climate was stable, warm, and wet. Ferns were globally abundant in woodlands and marshlands providing suitable habitat for them.
Ferns were the first group of plants with vascular systems. Vascular plants utilize tube-like structures, called xylem tissue and phloem tissue. Water and minerals are moved from roots to fronds (leaves) through the xylem tissues and are used in photosynthesis to create food that is distributed throughout the rest of the plant in phloem tissues. Vascular systems also provide internal support so these plants can stand upright on their own.
Early scientists noted that ferns came back every year, but produced no flowers or seeds. In 1669, it was discovered that spores grew on the surface of the fronds, but it was not until the mid-18th century that the entire reproductive process was understood. Spores are one-celled organisms that start to develop in springtime. They appear by the thousands as small green bumps on the undersides of fronds. As summer progresses, the sporangia, a capsule that contains the spore, turns brown. Clumps of these are called sori, and can easily be observed on fern fronds. In late summer, when the sporangia mature, they open and release their spores. Some plants will forcefully shoot their spores away from the fronds and some will open and let the spores drift away, caught by any small breeze.
Most spores will not land in a spot with favorable growing conditions (fertile soil and water), but the spore may remain viable for up to a year, should conditions change. Spores do not contain a small plant like a seed does. A spore starts as a one-celled organism that grows by dividing itself in two during the first phase of reproduction called the gametophyte generation. As this division continues, a small structure called a prothallium grows, getting its nutrients directly from contact with water. Two more structures develop. The archegonium contains an egg, and the antheridium contains sperm. When water is present, the sperm will swim to the egg, fertilize it, and eventually a new, self-supporting plant grows.
The fern develops underground over the winter. In spring, when ground temperatures have risen, fernlets will push tightly coiled fiddleheads through the soil into the open air. A fiddlehead, also called a crozier, is a group of young, coiled fronds (leaves) of a fern. As the fern grows, the upper and lower surfaces of the fronds grow at different rates, and the fiddlehead uncoils, straightening out into several fronds. The plant is entering the second phase of reproduction, known as the sporophyte generation, where spores will be grown on the undersides of the fronds.
Ferns can also reproduce through cloning. The walking fern, Asplenium rhizophyllum, grows long fronds that eventually bend over and touch the ground. Wherever a tip touches moist soil, new roots are sent out, and a cloned plant develops. The Boston fern, Nephrolepsis exaltata, grows runners – leafless stems that branch out horizontally. Wherever they come in contact with moist soil, roots are sent out to clone another Boston fern. The staghorn fern, Platycerium sp, grows buds on its roots. As the soil around the plant is moistened by water containing nutrients, a new plant pushes upward through the soil from each bud.
Ferns grow well in a variety of habitats. Many species prefer damp woodlands, but some grow on cliff faces or in rocky, dry locations. Be sure to watch for various species of ferns when you are out walking. A good field guide can help with fern identification and provide distinguishing information on this amazing plant group.
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