Ferns

Ferns are one of the most primitive land plants, their fossil record going back hundreds of millions of years. They were once the dominant land flora, but with the emergence of the flowering plants and the earth’s changing climate led them to be out evolved, and the majority of fern species became extinct.

Ferns are not flowering plants instead reproduction is via spores rather than seeds. They differ from flowering plants by reproducing through the alternation of two generations. With the first generation small sporelings are produced, with the second, ferns.

Ferns die back each winter, the dead fronds acting as protective insulation to the dormant crown. In spring the fronds start to appear as curled young fronds called crosiers (resembles a bishop's crosier) which later uncoil into the fern frond that we all know.

The best places to find ferns are in areas of dampness or high humidity such as woodland or close to streams. This is primarily due to the fern’s lifecycle which requires these conditions for successful fertilization. However some species have adapted, like the invasive bracken which has taken over large swathes of dry heath land in Cornwall.

Physiology

In the majority of fern species, the leaves are pinnate, i.e. are featherlike with pinnae arranged on either side of the rachis. This shape can be repeated in the pinnae themselves.

The rachis grow out of the rhizome an underground stem. The rhizome is covered by a protective layer of scales or hairs. The extension of the rhizome can be a major mode of propagation as is the case with bracken.

Fern roots tend to be thin and wiry. The roots grow out of the rhizome into the soil. Here they collect mineral and nutrients which are transported to the leaves via a vascular network of tubes.

Life Cycle of Ferns

During the spring on the underside of the fern fronds minute sacs i.e. sporangia can be seen. The sporangia are arranged in groups known as sori.

The arrangement, colouring and shape of sori differ between fern species and are an important factor in the identification of species.

Depending on the species, the sporangia are covered and protected by a membrane the indusium.

Sporangia Arranged in Sori

Sporangia have a row of thicker U shaped cells known as the annulus.

These cells as they lose water lead to stresses in the sporangia, at first distorting it and finally splitting it leading to the ejection of dust like spores.

The dust like spores can be carried by air currents high up into the atmosphere and transported large distances.

The spores find their way to soil and require warm damp conditions to germinate. The spore germinates and grows not into a fern but instead a small flat prothallus.

The prothallus is a self sufficient plant, that can photosynthesise and absorb minerals and water.

The prothallus produces both male and female organs, the antheridia and archegonia.

The antheridia produce sperms, which when released can only move in a film of water. Their objective being to fertilize oospheres, located in the archegonia. There is only one oosphere per archegonia.

A new fern develops from the fertilized oosphere, at first relying on the prothallus for nutrient, but eventually the fern can stand alone and the prothallus breaks down.

Identification Factors

There are three levels of idenfication dependant on the scale of the characteristics under review. The first is by eye, the shape, size, and colour of the fronds. The rhizome is covered by a protective layer of scales or hairs which vary in size shape and colour. The second can be carried out via a good magnifying glass, the arrangement, colouring and shape of sori and indusium. The third is by using the microscope, the differing shape and surface pattern of spores.

There can be problems with identification as within one species there can be a large range of varieties. Additionally it is not uncommon for species to hybridise.

Bracken Pteridium aquilinum

The invasive bracken must be the best known or more appropriate most notorious fern. It has left behind the need to live in moist conditions and propagating by sexual reproduction. It instead has successfully spread, by the underground extension of its rhizome. Bracken does not favour strong shade and will thrive in open conditions. In Cornwall large tracts of heath land has been colonised by bracken. Bracken can reached a height of six feet.

Hart’s Tongue Phyllitis scolopendrium

Hart’s tongue fern is easy to identify. This common fern unlike almost all other species found in Cornwall, as its light green leaves do not have separate pinnae.

The distinctive sori are long and straight. The hart's tongue fern is an indicator of alkaline soils which are not the norm in Cornwall. However it does thrive in walls maybe benefiting from the affect of the alkaline mortar.

Long and Straight Sori of Hart's Tongue Fern