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Cardiomanes reniforme
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This article was published in The Fringe, August 2016

Kidney Fern or Kopakopa

Kidney fern is an instantly recognisable endemic fern thanks to the 10cm rounded fronds scalloped in the shape of a heart or a kidney. This is reflected in the botanical names: 'Cardiomanes' means 'transparent heart', while 'reniforme' means 'kidney shaped'. The Maori name, kopakopa, means to clasp or grasp. To some degree the fern is scented, and Maori used it to indicate they were in mourning. The fern is able to grow without interference from other plants on wide expanses of the forest floor as the rhizome emits a chemical which inhibits germination of other seedlings within the area it covers.

This seemingly fragile frond will shrivel in hot dry weather, and you might think it has died when you look at the crisp remains, particularly on Rangitoto Island's scorching scoria rock fields. But shortly after the application of a light misting, the fronds recover their usual translucent beauty again.

The group of plants known as 'filmy ferns' have fronds appearing to be one cell thick and so are nearly 'see-through'. While most of them belong to the Hymenophyllum genus, kidney fern has been classed separately in the genus Cardiomanes, and it is both distinctive and widespread throughout New Zealand. At various times it has been called Trichomanes reniforme and Hymenophyllum nephrophyllum.

The fern forms wide spreading clumps that can be on the ground, or climbing up tree trunks. Nikau is a particular favourite where the fern spore lodges amongst a clump of moss and gradually spreads to encircle the trunk, or it may be clustered along a branch high above the forest floor, usually nestled amongst other epiphytes. It loves the humidity of the sheltered forest interior and normally dies off if exposed to wind or sun, but occasionally you can find it surviving in an open situation despite the constant drying it must suffer. The thin string-like rhizome grips the branch or rock and creeps through the protective surrounding moss pushing up a new frond every few centimetres.

The wondrous structure of the frond is clearly discernible if held against the light.

The winged stalk or stipe is attached in the notch of the kidney-shaped frond and then divides into two at the beginning of the vein network. It then divides again and again up to seven or eight times. Consequently there are from about 64 to 128 vein endings on the outer edge. Some fronds evolve into fertile ones with the spore enclosed in a capsule half sunk into the frond and half protruding, from which the spore will be released when ripe. The perfect frond is a masterpiece of exquisitely delicate construction which always causes observers to exclaim how beautiful it is.

Next time you are in the bush, take the time to search for kopakopa and enjoy the sense of awe that such delicacy can survive in the harshness of our forests.



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