"a_plutonium" wrote in message
ups.com...
Last week I encountered a patch of stinging nettle and had my hand
irritated for some hours.

Ouch!

For those
unaware of stinging nettle, it is a plant with needles on its stem and
tipped with acid so as the
hand or legs brush against this nettle, the skin is irritated.

Not exactly. The hairs are actually like stiff little hypodermic
needles with glassy walls and a tip that neatly snaps off on contact
so that the hair's contents are injected into the tiny wound the
hair makes.

I have a question on stinging-nettle. What animal caused the evolution
of this stinging plant? Is it grazing animal
such as rabbits or some hoofed grazer?

Why not both, or potentially plant-damaging animals in general? The
tiny needles with painful nerve-stimulating juice inside might work
just as well on almost any animals. This article suggests that the
stinging hairs defend against mammals but not insects:

http://www.jstor.org/view/00301299/ap060198/06a00120/0

I tend to think stinging nettle
evolved because of the prescence
of humans? I wonder if stinging nettle is due solely to the prescence
of humans?

Why humans? There seems to be no reason to think the nettle's stinging
hairs have evolved in any specific relationship with humans. There are
plenty of other animals out there to be stung.

That planet Earth would
not have stinging nettle if not for humanity. And whether there are
biological firsts for stinging-nettle and
Homo sapiens.

Question: is that true?

Again, why think it has anything specific to do with humans?

Second question: If true then how old is this
co-evolving relationship?

The stinging-hairs type of defense has evolved independently in
several different groups of plants. Even in true nettles [family
Urticaceae] there are numerous stinging species in several genera
as well as many non-stinging genera.

http://en.wikipedia.org/wiki/Urtica
http://en.wikipedia.org/wiki/Stinging_nettles
http://en.wikipedia.org/wiki/Dendrocnide
http://en.wikipedia.org/wiki/Dendrocnide_moroides

Besides "true" nettles, there are similarly very nettle-like stinging
plants in the Euphorbiaceae [e.g., Tragia, Cnidosculus] the Loasaceae,
and perhaps some other families.
e.g:
http://waynesword.palomar.edu/wigandia.htm

Many other plants have other sorts of irritating hairs that likely
also play a defensive role.

Some of these adaptations are undoubtedly fairly old [in true nettles,
cladistic analyses may tell us whether stinging hairs would have been
present in the last common ancestor of all modern stinging Urticaceae].

One neat thing worth pointing out is that nettles and similar plants
are genuine examples of _venomous_, not merely poisonous, plants.

cheers