Friday, 4 November 2011

Nettles without stings

Every plant family name is typified by a genus, which means the name of the family is based on the name of a genus.  For instance, the name Brassicaceae, for the mustard family, is based on the name Brassica, which is the so-called type genus of the family.  But it would be a mistake to think the type genus is necessarily typical of the family in any other way. Viola, with its bilaterally symmetrical flowers and capsular fruits, is not a typical member of Violaceae, nor is wind-pollinated Plantago typical of Plantaginaceae.
The same applies to the nettle family, Urticaceae.  Most people have been stung by nettles, and I used to think, wrongly, that this is a family of stinging plants.  Heywood et al. (2007, but see Hadiah et al. 2008 for an update) refer to five tribes in the family and mention stinging hairs for only one, the Urticeae.  Interestingly, all five tribes are represented in New Zealand, suggesting the members of this family may be easily dispersed.
Tribe Urticeae includes the widespread temperate genus Urtica, which has five native and four naturalized or casual species in New Zealand.  These nettles all have stinging hairs, but U. australis on the subantarctic islands has very few of them. 
Urtica australis on Enderby Island, Auckland Islands.
Nettles often grow in nitrate-rich environments and they can accumulate rich nutrient reserves in their leaves.  This makes them a target for herbivores and the stings are an effective protection from some grazers.  Nevertheless nettle soup is a traditional dish in Europe.  Also nettles are readily eaten by caterpillars and thus they provide environmental support for butterflies.  I wonder if U. australis has few stinging hairs because it has few natural predators in its subantarctic home.
Urtica australis, flowers and a few hairs
New Zealand has one of the most spectacular nettles, Urtica ferox (the name means fierce) or ongaonga.  Ongaonga stings can be fatal, especially to horses and dogs, but humans have also been affected.  One botanist colleague has had a lucky escape, only just managing to stumble out of the bush in time.  Friends called an ambulance when they found him numb, partly paralysed, and disoriented.
Ongaonga, Urtica ferox.
The stiff stinging hairs are like little (not so little in ongaonga) hypodermic needles that inject the toxins under the skin.  The brittle tip of the hair breaks off on contact, and as the hair is pushed into the sac of toxins at its base the toxins are forced up its hollow shaft.  These toxins include formic acid, serotonin, and histamine according to Wikipedia, while some species contain oxalic acid or tartaric acid.  Connor and Fountain (2009) mention a toxin called triffidin that may also be involved.
With such active chemicals, it's no surprise that Urtica is widely used in herbal medicine and homeopathy.  Homeopaths believe that a substance that causes a symptom in a healthy person can be used to treat that same symptom in a sick patient. They also believe the more dilute a solution is, the more powerful it is as a treatment, even when so dilute that none of the original substance remains. They combine those two principles and believe highly diluted nettle extract is a remedy for skin rashes.
Nettles in the tropical genus Dendrocnide grow as small trees with very dangerous stings, and another genus Laportea is also notorious.
Parietaria judaica
The rest of the family is not so scary.  Parietaria judaica (pellitory, Tribe Parietarieae) is a common weed around Wellington city and many other parts of the country.  There is a native species, P. debilis.  Pellitory doesn’t have stinging hairs, but it is implicated in asthma and skin irritations.
Elatostema rugosum
Elatostema rugosum (parataniwha, Tribe Elatostemeae) is a large herb from the north of New Zealand, where it grows on stream sides and wet cliffs.  There’s also a good patch of it in the Wellington Botanic Gardens, just downstream from the duckpond.  No stinging hairs here either, so it’s an attractive plant for the garden.
Pouzolzia australis (Tribe Boehmerieae) is a small tree found on the Kermadec Islands as well as Norfolk and Lord Howe Islands.
Finally, Australina pusilla (Tribe Forsskaoleae) is a little creeping herb that’s said to be common in coastal and lowland forests.  I think I must often have overlooked it, so now I intend to keep an eye out for it.
So all these New Zealand Urticaceae are stingless nettles except for Urtica.  They shouldn't be considered to have evolved from ancestral plants that had stings, since stings seem to have evolved only in Tribe Urticeae.  Stinglessness doesn’t mean they’re entirely benign though.  The flowers in this family are small and wind pollinated, and the copious pollen production associated with wind pollination makes them a minor cause of hayfever.
Connor HE, Fountain J 2009.  Plants that poison—a New Zealand guide. Manaaki Whenua Press.
Hadiah JT, Conn BJ, Quinn CJ 2008. Infra-familial phylogeny of Urticaceae, using chloroplast sequence data. Australian Systematic Botany 21: 375–385.
Heywood VH, Brummitt RK, Culham A, Seberg O 2007.  Flowering plant families of the world. Firefly.

No comments:

Post a Comment