Saturday, August 27, 2011

Platycerium alcicorne: was born in Gondwana

About 150 million years ago in the Southern Hemisphere of the Earth had one large continent called Gondwana that was separated of the immense continent unique Pangaea. In the following millions of years during the Late Jurassic this great austral continent Gondwana went fragmenting with the movement of the tectonic plates and two subcontinents formed: moving towards the northwest the subcontinent formed by Africa and South America and moving towards the northeast the subcontinent formed by India, Madagascar, Australia and the Antarctic.

About 100 million years ago South America separated from Africa and went moving towards the west, whereas India and Madagascar separated from the block formed by Australia and the Antarctic and went moving towards the northeast at the vertiginous speed of 15 centimeters per year. About 90 million years ago during the Late Cretaceous Madagascar separated from India and in its displacement towards the north it ran aground when hitting the African plate and slowed down its displacement, whereas India continued ascending towards the northeast until hitting the Asian plate makes about 35 million years, initiating the formation of the mountain range of the Himalayas that has still not finished.

The fern Platycerium alcicorne formed in the great southern continent Gondwana some 150 million years ago during the Late Jurassic, just before they separated Africa and Madagascar, is therefore a very primitive fern that has changed very little over its long existence. At present this species lives in various regions of Africa, mainly in Mozambique and Zimbabwe and in Madagascar, Seychelles and Comoros. African specimens, formerly known by the scientific name Platycerium vassei are somewhat larger than the Malagasy and more resistant to drought. A recent genetic study confirmed the common origin of Malagasy and African subspecies, which, despite having separated more than 100 million years, are few differences in their genome.

Elkhorn fern, like all species of the genus Platycerium, lives as an epiphyte and always retains the antediluvian look 150 million years ago. Hill little to imagine it in ebullient tropical forests on the tops of the primitive coniferous and the arboreal ferns in a very warm and permanently humid atmosphere.

All Platycerium have two kinds of leaves or fronds. At the base of the plant on more o less horizontal position and a fan or kidney form is a basal sterile frond that covers and protects the rhizomes from drying out. The rhizome is short and is firmly anchored on the bark of a tree. From the center of the sterile frond they sprout fertile fronds growing upright at first and gradually becoming pendulous.

The basal sterile frond conserves the reniform typical form of prothallus or gametophyte of all ferns. It looks like a giant gametophyte. This detail reminds us of the great antiquity and primitiveness of the genus Platycerium ferns.

In this image of a sporophyte of Anogramma leptophylla fern springing from the fertilized oosphere of a gametophyte can appreciate the extraordinary resemblance to the sterile frond Platycerium. The gametophyte or prothallus of all ferns have kidney-shaped and are embedded and applied to the substrate, just as the basal frond of Platycerium. From fertilized oosphere of  gametophyte sprouts a tiny frond or sporophyte that in the case of Anogramma is shaped like a small elkhorn. The similarities are remarkable and surprising. Platycerium is possible that they are the ancestors of many modern fern that apparently have nothing to do with them. In the not too distant future the study of the genome of these primitive plants so will bring many surprises when compared with the genomes of other ferns.

Obverse of a fertile frond that is covered with a fine hairiness that confers a off-white green color to it, sometimes grayish. Its form remembers to a horn of elk. The botanist who described the Platycerium sort fixed indeed to this peculiar form and he baptized it for science combining two Greek words: platys that means plane and keras that means horn, that is to say, leaves in the form of equal flat horn to the graft and flattened horns of the elk. The name of the species " alcicorne" it comes from the combination of two Latin words and its translation is obvious: alci-corne = elk horn. 

On the underside of fertile fronds on the tips of the branches are the sori formed by milions of brown sporangia.

Undersides of other fertile frond of Platycerium alcicorne. The sori have the look and soft feel of velvet.

In this image nearest well appreciated the velvety appearance of the sori and the fine gray-white pilosity covering the fronds.

If we get closer we see the structure of a sorus which consists of closely spaced parallel rows of brown sporangia filled with mature spores.

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