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Mysteries of Nardoo

Kerry Rathie

At a recent Fern Study Group meeting, an interesting talk on aquatic ferns by the Group's leader, Dr. Peter Bostock, mentioned some facts about nardoo (Marsilea spp.) that I did not know, and feel would be of more general interest than just to our fern group.

Nardoo is of course famous, as ferns go, for contributing largely to the unfortunate fate of explorers Burke and Wills. They saw the aborigines grinding up the dry sporocarps (the fern equivalent of fruit with seeds inside, but of course ferns are not flowering plants) to make a sort of flour, which they made into a kind of cake and ate without harm. The sporocarps contain high levels of thiaminase, a heat-resistant enzyme which breaks down Vitamin A, and the resultant vitamin deficiency weakened the explorers. The aborigines were unharmed, presumably because their diet contained many other components.

Peter has been looking at Australian and overseas Marsilea species, and now doubts whether the Australian species have been correctly classified. While they are growing in their 'normal' habitats of shallow water or wet mud, they are very long-lived, but do not spore. If under harsh conditions, the leaves (and the whole plant) can be very small, or can be tall and large under lush conditions.

Dendrobium beetle feeding
Marsilea mutica; Banded nardoo
Photo: Shortland Wetland Centre

M. drummondii leaves can be over 2 inches in diameter. Juvenile nardoo can have just a pinnate leaf, with 2 pinnae, not the '4-leaf-clover' arrangement, caused by folding, of adult leaves. Degrees of hairiness and other vegetative features all seem to vary mainly with the local environment.

The sporocarps do allow accurate identification, but are not produced until the ferns are slowly and steadily drying out, and most identifications have been made on unreliable leaf and stem (yes, I mean stipe plus rachis) features. Marsilea and its family are nearly unique among true ferns in producing two types of spore which germinate to give male or female gametophytes.

The megasporocarps tend to stay attached to the (asexual) fern, and they contain megaspores which produce prothalli (usually largely enclosed by the megaspore wall) with female gametes, while the mobile and smaller micro-sporocarps contain spores which germinate to give mobile male gametes. Brief wetting causes no germination, but a little under 24 hours of continuous moisture, in some species, can cause germination of a spore that can otherwise lie dormant for 100 years, maybe more.

Rapid prothallus growth, gamete production, and rapid asexual growth of what laymen see as the fern, can lead to Marsilea having a minimum generation length of only two weeks. Or, in a reliably wet spot, average generation length could be decades, maybe centuries.

M. hirsuta is the commonest species round Brisbane. M. cryptocarpa buries its sporocarps round the base of the rhizome. The African species, M. megalomanica has an erect 0.5 m 'stem' with lots of large sporocarps all up it, a bit like brussels sprouts. The sporocarp is derived from modified leaves.

In his 'Flora of Australia' article on the Marsiliaceae (in Vol. 48, pp. 166-173), and in his earlier book with Clements, David Jones talks of scales as well as hairs on Marsilea, but Peter can see only hairs; some long, some short, some lax.

We took a look and agreed.

From the newsletter of the Society for Growing Australian Plants (Queensland), December 2007.

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