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Fern Facts

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A fossil fern (© Peter Blake)

Ferns belong to a group of plants (gymnosperms) that propagate themselves by means of spores rather than seeds. Spores are produced on ferns’ leaves (fronds), whereas seeds are produced from flowers of seed-producing plants (angiosperms). Ferns are sometimes regarded as being rather primitive. However, studies of fossils of both ferns and their spores show that this method of reproduction has been successful for 350 million years, a lot longer than seed plants. Ferns also have proper roots and a vascular system, similar to seed plants, whereas other gymnosperms, such as mosses, do not. The vascular system connects the roots, which take up water and nutrients, with the body of the fern and with the leaves, which are called fronds, which is where photosynthesis takes place.


It is wrong to think that the ferns we see today are the same as 350 million years ago or even the same as when the dinosaurs roamed the Earth. Fossils show that most ferns have undergone considerable development, diversification and adaptation over this period to the point where there are now more than 10,500 species of fern adapted to very different conditions on every continent of the Earth except Antarctica. Even that continent may soon support ferns as, with global warming, there are now mosses growing in Antarctica and ferns are often the first vascular plants to inhabit new ground.

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Fern croziers (© Peter Blake)

Ferns also display a characteristic method of producing a new frond that is shared with only one other type of plant, the cycads. This is to produce a crozier that uncurls to produce the new frond. This curling may extend to both the main body of the frond and to the side leaves, the pinnae. Any wild plant found producing new leaves in this way is a fern, unless you are in an area where cycads grow!


Ferns inhabit a wide range of environments, and tropical ferns have the greatest diversity. The majority of ferns across the world are epiphytic and terrestrial ferns are less common. This may seem surprising to someone living in Northern Europe, where there are few native epiphytic ferns and most grow in the ground. However, a visit to a tropical rainforest will soon prove the point that growing in trees and on rocks provides an advantage in avoiding being eaten by herbivores or drowned in floods.


There are ferns that live underwater, in areas that have seasonal flooding, in damp pastures, monsoon forests, cloud forests, deserts, and in trees and cliffs. Growing these ferns presents a challenge in providing an environment which will support the needs of these plants. As with all horticulture, some plants are fussier than others. There are tropical ferns that will grow in a domestic living room with the minimum of care, whilst there are others that will require carefully controlled environments of light, heat, humidity and substrate. Learning how to cater for the needs of a fern is part of the fun of creating a fern collection, occasionally interspersed with tears!

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Fern sori (© Peter Blake)

In writing about ferns there are some words that are frequently used that are not common in everyday language. It may help to provide a brief glossary of some of these words, most apply to the anatomy of the plant.

 

The body of a fern is called the rhizome, which may grow upwards from the ground or creep over the ground, tree or rocks. It is from this that the leaves, called fronds, arise. If the fern lives on the ground it is referred to as terrestrial, if it lives in a tree or on rocks it is epiphytic. Those that live on rocks may be specifically referred to as lithophytic.


Fronds are often divided into smaller leaflets, pinnae, which may be further divided into pinnules. The surface of the fronds may be smooth or glabrous, or it may be hairy. The degree of hairiness can be indicated by words such as pubescent, hirsute, tomentose and villous, each term referring to a different length, density or stiffness of the hairs. The hairs themselves may be on the rhizome or on the frond and are usually referred to as scales. The stalk of the frond between the rhizome and the leafy blade is referred to as the stipe and when it enters the blade of the frond it is called the rachis. There are a few ferns that produce a waxy coating or powder on their stipes and the back of the fronds. This coating is referred to as an indumentum. This same term may be used for a dense coating of short hairs on the back of fronds.
 

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Specialised fronds adapted for carrying large numbers of spores (© Peter Blake)

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Fern gametophytes and a few small sporophytes (© Peter Blake)

The reproductive parts of a fern also have names not found elsewhere is horticulture. The most obvious parts are the sori which bear the spores which are contained in sporangia, tiny packets just visible to the naked eye. Sori may have a covering or indusium and this can be of different shapes depending on whether it is attached to the frond centrally, like an umbrella or peripherally like a clam shell. These are details that are needed for the accurate description of a fern but are not used in general discussion. However, it is important to note whether the sori of a fern have indusia or are naked. Most sori are carried on the back of a frond but some ferns produce an adapted frond that is designed just to carry large numbers of sori.

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In nature, most living creatures and plants have two sets of chromosomes in every cell. These cells are referred to as being diploid. Fern spores are single cells that contain only one set of chromosomes and are called haploid. These spores are released from the sporangia when the weather conditions are suitable. The spores eventually settle after travelling on the wind and, if that location is damp and shady, they will begin to grow into a gametophyte or prothallus. The gametophyte has both male and female sexual organs, known as the antheridia and archegonia respectively. Tiny motile ‘swimmers’ are produced in the antheridia and released into the moisture that surrounds the gametophyte. These swimmers are also haploid and try to navigate to an archegonia and fertilise a haploid ovum there. If that fertilisation is successful, the diploid status is restored. That ovum then divides and grows into a diploid adult fern, known as a sporophyte. This is known as the ‘alternation of generations’ and is a way of ensuring that there is genetic diversity.

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