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Having the Insula exhibition at VBG by Perdita Sinclair led to thoughts about seed dispersal by sea.

Of all the strategies evolved by flowering plants to disperse their seed, the method with potential to spread furthest yet most treacherously, must be by sea.  The salinity of sea makes it a poison to plants, greater still to vulnerable seeds, so seed that is sea dispersed almost always has a bulky, fibrous or impenetrable coat.  These large seed are released to the mercy of ocean currents and will travel the globe, eventually beaching in the unlikeliest of places. Easily spotted in the drift line some have developed folklore to explain their mysterious origins.

Lodoicea maldivica, the Coco de mer, is a giant fruit containing four seeds. Its suggestive shape, resembling a woman’s buttocks, held intrigue for centuries.  Found on beaches, these hard husks were thought to be from trees that grew under water, they commanded high prices and were held as enigmatic treasures by royalty and nobility across the globe.  It wasn’t until their discovery in the Seychelles in the 18th century that their true origin, growing from giant palm trees lining the island beaches, was found.

Another palm, the familiar coconut or Cocos nucifera, grows in a similar niche in tropical regions across the globe. So successful is the coconut in using the sea as a dispersal mechanism that it is now quite unknown where the coconut originally came from.  The habit of the coconut tree is strongly linked to its arrival and transport across sea; it germinates once deposited on the highest tide line after its fibrous coating as decayed.  From this start point it grows with a distinctive leaning of its trunk towards the sea, ensuring that at maturity the ripe fruits fall directly into the sea to be carried away.

Fringing the tropical shoreline are tall trees that will use the sea to disperse their seeds. Cerbera manghas, or sea mango, is a reasonably large tree with an extensive canopy that extends out over the tide, its egg shaped fruit contain a seed ready for a voyage to pastures new.

The trees that inhabit the fringes of estuarine river systems in the tropics are cloaked with climbers or lianas.  Many of these lianas use the ebbing and flowing sea beneath as the vector to spread its seed. The sea bean, Entada gigas and its related genus Mucuna share this habitat. Mucuna has many species, each with a very different shape; many are adapted to spending a long time floating in sea water.  When seen hanging from huge pods dangling beneath the canopy it is quite clear the intention of where the seeds are to be delivered to.  The legumes or beans are not the only climbing plant family to dangle their seed over water, the Merremia discoidesperma is related to our native convolvulus.  These distinctive seeds bear a cross on the underside, this shape combined with the mystery of their origin gave rise to many superstitions, they were given as charms to ward off evil spirits or taken as cures for numerous ills.

Within the sea itself grow trees we know as mangroves. Some mangroves have seed developed to avoid the dispersal of the sea, falling like spears into the soft mud. Heritiera littoralis, though, is inclined to drop its seed into the calm yet tidally dominated brackish water to be carried perhaps just a short distance or perhaps many miles.

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