In article , Tim Tyler writes
Kay wrote or quoted:
In article , Tim Tyler writes

Does anyone know why Nasturtium flowers have those strange horn-shaped
bits pointing backwards?

A pictu http://www.andysmall.com/Media/Jpegs/00006.jpg

My best guess so far: flower/bee counter-weight.

Think you know better than that? Please *do* let me know ;-)

Usually because the nectar is buried at the end of it to make sure the
pollinator has to go in a long way for it and gets well and truly
covered with pollen. Not sure if this is the case with nasturtiums -
haven't dissected one to see.

I believe this is not the right answer - since the "horn" contains no
flower stamens - and no pollen. The nasturtium's pollen is clearly
evident elsewhere.

I didn't say the pollen was in the spur! I wondered if the nectar was,
so that whatever pollinates the nasturtium in its natural habitat has to
get its body well into the flower and well coated with pollen. For that
to work, you would expect the pollen to be much nearer the entrance of
the flower, which is where it is.

The horn appears to be a simple hollow tube.

Heywood (ed) Flowering Plants of the World - Tropaeolaceae (Nasturtiums
and canary Creeper) "The perianth consists of a calyx with five distinct
sepals, one of them modified to form a long nectar-spur"

However, Proctor and Yeo "The Pollination of Flowers" seems to suggest
that bees take only pollen from nasturtiums, hence my wondering what it
is pollinates nasturtiums in their natural environment. A close
relative, Tropaeolum pentaphyllum, with a similar spur, is bird
pollinated, and that makes a lot more sense with the colour - birds are
attracted more to the bright colours, hence the bright reds and oranges
that we associate with tropical flowers.

You can see how it would work - humming bird feeds its long beak down
the spur to get at the nectar, in doing so getting its feathery body
liberally coated with pollen, which is rubbed off onto the next flower
it meets.

From the plants point of view, the adaptation means only humming birds
can get the nectar, so they are the most likely visitors, and since they
get this 'reward' from the plant, they have an added incentive to visit
- so the plant gets itself pollinated by something which isn't visiting
loads of different plants with incompatible pollen.


--
Kay
"Do not insult the crocodile until you have crossed the river"