Nettle, Where Is Thy Sting?

Our highlight taxon for this week is the Urticaceae, the so-called nettle family. I say "so-called" because, as with many plant families, the supposed representative member is not necessarily that representative at all, but just happens to be the best-known temperate member of a mostly tropical family. Urticaceae includes about 2500 species in a bit under 80 genera found world-wide (the image above, from here, shows the North American species Laportea canadensis). Members encompass all growth habits from herbs to lianas to trees, though tree species are relatively few (depending on whether or not the tropical tree genus Cecropia is included in Urticaceae or in a family of its own). They are wind-pollinated, and like other wind-pollinated plants the flowers are quite small and reduced. The stinging hairs for which the nettles are so well-known are actually restricted to a single tribe of Urticaceae, the Urticeae (also known as Urereae - Hadiah et al., 2003), most members of which belong to one of the two large genera Urtica and Urera.

Relationship-wise, the Angiosperm Phylogeny Group classification (Angiosperm Phylogeny Group, 2003) includes Urticaceae within the Rosales. However, phylogenetic analyses have established the monophyly of a smaller sub-group of the Rosales including Urticaceae, Moraceae (the mulberry and fig family), Ulmaceae (the elm family) and Cannabaceae (cannabis and hops) that corresponds to the Urticales of previous classifications (Hadiah et al., 2003). Relationships within the Urticaceae are a little more unsettled, a situation not helped by the fact that the family seems to have received relatively little monographic treatment since the work of Weddell in the mid-1800s. Monro (2006), in a molecular analysis centred on the largest genus of Urticaceae, Pilea, suggested that the five tribes originally established by Weddell fell into two clades, one containing the tribes Boehmerieae, Parietarieae and Forskohleae, and the other containing the Lecantheae and Urticeae, though the genus Myriocarpa (previously in Boehmerieae) fell into the Lecantheae-Urticeae clade. Monro (2006) also supported the inclusion of the genus Poikilospermum with the Urticaceae, specifically within the Urticeae (though Poikilospermum lacks stinging hairs). Other authors have included Poikilospermum in the Cecropiaceae, but Monro (2006) placed Cecropia distant from Poikilospermum in a trichotomy with the two Urticaceae clades (so unresolved as to whether or not it should be included within Urticaceae).



Economically, the Urticaceae are not a significant family. Species of Boehmeria, particularly B. nivea (shown above in an image from here), have been used for many years as a source of the fibre known as ramie. Ramie cloth was one of the textiles used for wrapping mummies in the Egyptian pre-dynastic period, but before mechanised methods became available extraction of fibres required a laborious process of repeated soaking and scraping. Industrial-scale production of ramie did not become practical until about the mid-1900s (Cook, 1984).

Some species of nettles of the genus Urtica are collected as edible herbs - the venom from the stinging hairs is destroyed by heat, so cooked nettles are quite harmless. An Indian species, Urtica tuberosa, also produces edible tubers. According Botanical.com, nettles can also be used to produce a fibre in a similar manner to ramie - however, the inferiority of nettle fibre to linen resulted in the decline in its use (though it did have something of a renaissance during the height of shortages in the Second World War). For the most part, nettle stings are more of an irritant than a significant medical issue, though some species are significantly more toxic than others. The New Zealand Urtica ferox (ongaonga or tree nettle, shown below in an image from Trek Nature) is a shrub of up to five metres in height that produces a strong enough sting to sometimes be fatal. At least one traditional chant of the Ngati Kahungunu iwi indicates that the ongaonga, along with other spined plants, was originally planted by Kupe, the discoverer of New Zealand, to protect the new land (Cowan, 1930):

Nau mai, e Tama,
Ki te tai ao nei.
Kia whakangungua koe
Ki te kahikatoa.
Ki te tumatakuru.
Ki te tara-ongaonga;
Na tairo rawa
Nahau e Kupe
I waiho i te ao nei.

Thou'lt be a powerful shield against
The weapons of the world;
The sharp and deadly spears,
The pricking darts and stings
That fill the foeman's armoury;
Thou'lt conquer e'en the barriers
Which Kupe the explorer raised
To guard this new-found land.

I haven't been able to find anything to support the rather more dramatic version recounted in Wikipedia that claims he placed these obstacles to evade the men whose wives he had stolen, so (unfortunately) I suspect this version to be a little dubious.

REFERENCES

Angiosperm Phylogeny Group (APG). 2003. An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II. Botanical Journal of the Linnean Society 141: 399-436.

Cook, J. G. 1984. Handbook of Textile Fibres. Woodhead Publishing.

Cowan, J. 1930. The Maori: Yesterday and To-day. Whitcombe and Tombs.

Hadiah, J. T., C. J. Quinn & B. J. Conn. 2003. Phylogeny of Elatostema (Urticaceae) using chloroplast DNA data. Telopea 10(1): 235-246.

Monro, A. K. 2006. The revision of species-rich genera: a phylogenetic framework for the strategic revision of Pilea (Urticaceae) based on cpDNA, nrDNA, and morphology. American Journal of Botany 93 (3): 426-441.