Tuesday, 22 August 2017

Bat Plant a blood bank rather than necrotic nest


The Bat Plant doesn't have much to do with real bats. There are about 20 different species of Tacca, one in Australia but most in Asia through to tropical South America. All are pollinated it seems by tiny insects, and the flower structures are mostly quite somber in color. This one is a little different, with its white bracts (the two sail-like bits at the top).

I suppose you could say the flowers look like some kind of bat, particularly when these bracts are purplish to black. Then you might say the bracts were the bat's ears. But what about the extraordinary whiskers?  More like some alien cat, or rabbit, perhaps. Not all Tacca species have such striking facial features but the ones grown in horticulture are the showy ones.

When botanists, such as Gwynne Lim and Robert Raguso from Cornell University in the USA, look at this flower structure their minds turn to pollination, and pollinators. What would such flowers attract and how would this complex devise work to help a visiting animal take pollen from one flower to another (thus achieving 'out crossing' and opportunities for the plant to change and adapt)?

The fleshy flowers themselves are rather plain, protruding out from the ears and whiskers like a gum nut or plum of some kind.


Until recently (April 2017) most observers had assumed the flower colour and shape had evolved to mimic rotting meat or fungi. The kind of place a gnat or fly would seek out to lay its eggs, and in so doing transfer pollen. The flowers themselves, have nooks and crannies that are too small for your average carrion fly so it has been assumed gnats were the kind of insect that might be targeted by this floral design.

Alongside this conjecture was the uncomfortable fact that in the showy species - for example Tacca integrifolia and Tacca chantrieri - there seems to be a lot of self pollination. This is what happens when pollen isn't effectively moving from one flower to another, or isn't accepted for some reason by the receiving flower.

So why the gloomy coloured flowers, let alone the ears and whiskers? Lim and Raguso set out to answer this question by studying plants of Tacca cristata growing in two patches of remnant (but not pristine) vegetation in Singapore. They found female biting midges (or Ceratopogonids) the most common visitor to flowering plants. In the reported study of  Tacca chantrieri, where there was much more self-pollinatin, the researchers had found mostly trigona bees. The midges are smaller and can effectively collect pollen, because unlike the bees they can through the gaps in these flowers.


Lim and Raguso also concluded that the plant was not mimicking a lump of rotting meat or something similar, to act as a brood site for insects. Although both male and female midges like nectar, female midges have biting mouths to get blood from various animals, which they need for egg production. Colour and odours are likely to be the main attractants.

The researchers also noticed that the midges were more attracted to the vertical flowers, where pollination is likely to more likely than when the age and the male and female bits comes closer to direct contact, potentially leading to self-fertilisation.

Other visitors included rodents and macaques, although they ate the flowers rather than helped in pollination.

Images: Tacca integrifolia, the 'White Bat Flower' , in flower during March in the Royal Botanic Gardens Victoria Tropical Glasshouse.