When leaves aspire to be flowers

It’s well known that dress and scent is all about sex-appeal. Okay, this may just be an ordinary botanical bias, but this is something about which many plants excel, to the point that one can quite safely say they are probably the most evolved organisms in this regard. I mean, flowers are attractive enough to convince other species to contribute to their own reproduction (more or less successfully, and sometimes it is somewhat hot), and dupery is not even limited to insect species… That said, flower scents are not always decent. But still.

The main evolutionary trigger behind this simple and marvellous plant feature is to have pollinators pollinating efficiently. That is, transfering pollen to some other flower, best is usually to some other plant. Else they are just selfing, something they can manage themselves alone (and some actually do very well). But in most cases, the more you attract pollinators, the more you reproduce successfully. Since any mutation directly affect reproductive success, this evolutionary game is riding an easy train.

[Photo © from Esta_ahi] -Most of the time, flowers, as perfect reproductive organs, and because it is easier to concentrate the reproductive functions altogether, produce everything needed to be visited: colourful petals, wonderfull scents, eventually nectar and pollen ready to go. But there is no reason why they would do most of it, apart from the very obvious truth that pollinators really have to visit the reproductive organs at the end. That is, with regard to long distance attraction, anything the plant can do is good. Whether from the flowers or… from anywhere else (but roots are at a basic disadvantage here). That’s what happens in Chamaerops humilis, the dwarf palm, a Mediterranean species. Here is its short story…

[Photo © Eric Hunt, www.plantworld.org ]
-This palm species is dioecious (oops, see the plants sex short story), with male and female individuals. It is therefore really in need of an efficient pollen transport and pollination service, for it cannot self.

It also has a highly specialist pollinator: a weevil called Derelomus chamaeropsis.  Just like in any good mutualism (that is, when two species become evolutionarily so intertwined that both need and benefit from each other), these two species have a very intimate relationship: the weevil seems to be the most efficient pollinator of the palm.  In return, the palm provide the weevil with food, shelter and even some place to lay eggs and have larvae to develop. Larvae grow up in the rachises of the male plam inflorescences. Of course, they leave the plant after emergence, without forgetting a welcome load of pollen grains. Some of the newly emerged weevils will visit a female plant next. So far, so good.

But let’s go back to the palm again. Flowers and inflorescences have some scent. But they are protected under firm bracts, and hidden around the leaves. Not the best way to have any delicious and attractive smell sent far away. Even if the inflorescence produce hundreds of flowers (see picture above). This is indeed a serious pittfall to long distance attraction of weevils, but what do you want?

[Photo © from Cyberfug] -Well, this is where a rather ingenious compromise was adopted (oops, I mean selected for, of course): leaves can be sexy too! The palmate leaves of the dwarf palm have a nice little structure at the base of the “digits”, which produce a marvellous and strong scent at flowering season. This indeed allows for a greater dispersion of the attractive scent than flowers would ever do. So we have here an interesting situation where leaves play the flower-game for long distance attraction of pollinators, and flowers only play the game once the pollinating weevils are really next to the palm.

To date, this example is almost unique in the litterature, though this is probably due to the strong bias in favour of flowers and lesser interest in the potential role of other parts of the plants when it comes to reproduction. Sure you know, leaves are the place for photosynthesis! (Or would it be stems? ;-)

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For those that have access to scientific articles, I strongly recommand the following paper:

Dufaÿ, M., Hossaert-McKey, M., and M. C. Anstett. 2003. When leaves act like flowers: how dwarf palms attract their pollinators. Ecology Letters 6:28-34.

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