Adiantum reniforme, a living fossil fern

This strange fern of rounded fronds belongs to the family of Adiantaceae and  it is an antediluvian relic, a small living fossil. Its global dispersed distribution with widely separated populations that have evolved separately and have given rise to local varieties, suggests the great antiquity of Adiantum reniforme that several million years ago, in a cold period with the level of Oceans lower than currently, populate a vast region covering much of Asia, Africa, Madagascar and Macaronesia. Later, as a result of a global climate warming, the ocean water level rose and separated the Asian populations from those African and those African from those Macaronesian and Malagasy.

 

Adiantum reniforme in the Bosque de Los Tiles on the island of La Palma in early May. (Double click on the photo to enlarge.)

Botanists are six varieties within the species:

-Adiantum reniforme var. reniforme, growing in Madeira, the Canary Archipelago and the islands of Cape Verde;
-Adiantum reniforme var. pusillum, a dwarf variety endemic to the Canaries, who lives in all the islands except Lanzarote and Fuerteventura;
 -Adiantum reniforme var. asarifolium, a rare hairy version with thick black margins and large sori, found in Madagascar, Mauritius, Réunion, South Africa, Kenya, Malawi, Gambia and Senegal;
-Adiantum reniforme var. hydrocotyloides, exclusive of the island of Réunion;
-Adiantum reniforme var. crenatum, which lives only in Madagascar;
-Adiantum reniforme var. sinense, which grows only in China and is in serious danger of extinction, with only four small populations known in the region of the Three Gorges of the Yangtze River, between the province of Sichuan and the province of Hubei, a place regarded as a refuge of plants that survived to Quaternary glaciations. 

Group of Adiantum reniforme in the Bosque de Los Tiles with the copy of the previous picture in a very damp wall facing north. 

In the Macaronesian Islands this fern prefers to live in habitats on rupicole rocks and walls oozing bright but not direct sun in the laurel forest clearings, with a subtropical climate and frost-free. In South Africa, Madagascar, Mauritius and Reunion Island it grows in rainforests with a similar climate to Macaronesia. In Kenya, Gambia, Malawi and Senegal, have a tropical climate too warm for this fern, is confined to the mountains where the climate is cooler and more humid. The four small Chinese populations living in the Three Gorges of Yangtze River also prefer cool and moist habitats, free of frost, located between 80 and 480 meters. Given the increasing human exploitation of this region of China, the number of copies of Adiantum reniforme var. sinense is decreasing at an alarming rate and now stands at a dramatic extinction. 

Adiantum reniforme growing between the stones of a wall facing north in the town of Los Sauces in the island of La Palma. This fern is highly resistant to long periods of drought. In the picture looks like moss growing on the fern is completely dry.

Beautiful specimen of Adiantum reniforme growing on a bed of moss in the cleft of a rock oriented northeast in the Portuguese island of Madeira in mid-May. Long petioles are appreciated black or dark brown on each frond with small rounded blade on the end. (Double click on the photo to enlarge) 

Several  fronds of Adiantum reniforme with its beautiful design in the form of Chinese pai-pai. Shows the radial flabellate arrangement of nerves of blade, which usually measure between 3 and 5 inches in diameter, has a leathery texture as plastic and presents a vivid green. In the Canary Islands this fern is called " tostonera" for its resemblance to an ancient coin called "toston", which was used in Spain and the American colonies during the sixteenth century.

Underside of a frond of Adiantum reniforme with the beautiful distribution of sori in the blade edge. Particularly striking is its reniform or kidney-shaped  form that gives the species name, design radial nerves and the fine pilosity covering the underside of the blade.

Detail of sori still immature and the fine light brown pilosity covering the underside of the blade. (Double click on the photo to enlarge.)

Adiantum reniforme sori watched side with his edged pseudoindusium. When the sporangia are ripe, the pseudoindusium be lifted to allow dispersal of spores. 
 

Microphotograph of an sporangium of Adiantum reniforme with mature spores within the transparent bag, before being dispersed.

Adiantum reniforme spores, very large and golden brown.