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FERNS

Ferns and allied plants, pteridophytes, appeared in the earth´s flora around 400 million years ago and the flowering plants 250 million years later. Although the do not have flowers and seeds, ferns are still highly evolved, not primitive.

There are 12.000 known species of fern in the world. Only 111 are in Spain. Under a 70-80% of the ferns are found in the tropical regions, while the rest are in temperate areas. In some places, like Polynesia, Madagascar, Southeast Asia and peripheral Andes areas are present more than 500 species of ferns.

They are distributed and replicated by a reproductive process shared with mosses and liverworts involving spores. Fern spores are very small and can be carried long distances on the wind. They can survive for decades in the atmosphere or the soil. Unlike a seed, a fern spore will not give rise to a new plant but instead a tiny scrap of green tissue known as prothalus upon which the male and female organs occur and where fertilisation takes place. It is this union that results in the new fern, the sporophyte, which has the spore-bearing leaves known as “fronds”.

Fronds are borne on a rhizome, a rooted terrestrial stem which takes two distinct forms, horizontal-creeping and vertical-compressed. Fronds which are regularly divided into pairs of leaflets are called pinnate. Where these paired leaflets are themselves subdivided along their central axis, then the frond is described as 2-pinnate. Further division can also occur creating 3-pinnate leaves. All are used in the identification process of ferns besides a molecular analysis to determine precisely the specie.

Ferns also spread vegetatively from underground runners, thus a hillside of bracken might have spread from just a single plant.

As an additional information, ferns are consider the forefather of lignin, a structural polymer that form cell walls in the support tissues, giving rigidity to vascular plants (Point 2 below). Initially, lignin was made as a result of the secretion of toxic excess of chemical compounds involved in defence against predators, like tannins.

 

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