Science - August 26, 2010

Stinking for sex

Wilbert Hetterscheid has discovered about sixty new Amorphophalli in the course of his career, making him the undisputed leading world expert in the weird and wonderful world of these ‘shapeless penises’. About time for a thesis.

Willem Hetterscheid in the cellar of the Herbarium with an Amorphophallus inflorescence in alcohol.
Yes, it's quite an icebreaker at parties, laughs Hetterscheid. His passion for the plant species Amorphophallus never fails to raise a laugh. So be it; he didn't come up with the name. That was done by the Dutch botanist Carl Ludwig Blume in 1825, when he came across a sample of the plant species in Indonesia. But it is certainly an appropriate name: you do not need a lot of imagination to see that.
Hetterscheid, until recently curator of the Wageningen botanical gardens and the tropical greenhouse, and now curator of the Von Gimborn Arboretum in Doorn, has been devoting his spare time to the Amorphophallus for twenty years. Why? 'Pure emotion really. As a biology student at Utrecht I was already interested in the weird aspects of the plant world. Like bulb plants with their strange growth and flowering patterns. At some point I came across the Amorphophallus, the ultimately weird plant.'
Amorphophalli belong to the Arum lily family. About two hundred species have already been identified, at least sixty of them by Hetterscheid. 'And it just goes on. I think we will end up with three hundred, and certainly 250.' That more and more Amorphophalli keep being discovered has to do with the plant's characteristic cycle, says Hetterscheid. 'It blooms for a very short time, a week at the most. So you just have to come across it in that week. Because you can't tell much from the leaves. You have to have seen the flower too, otherwise you will just walk past it.'
The thesis that Hetterscheid has been working on for twenty years is essentially a taxonomical revision. In other words, a description of a plant family. 'That was last done in 1924 and has not been done since.' On the insistence of the Herbarium in Leiden - 'we need someone to take a good look at this' - Hetterscheid got involved in a long-term PhD project. There is a reason for it taking so long. Hetterscheid has had to do all his research in his spare time. 'An added problem is that these plant families are spread over an enormous area. That usually means a lot of fieldwork, because I cannot be dependent on herbariums. All you find there are flattened Amorphophalli that do not tell you much.'
Hetterscheid decided to go about things differently, and not to go looking for the plants himself. 'I arranged for the plants to come to me.' Through an extensive network of contacts, he built up a big collection of 'shapeless penises' over the years. 'That saved me three years of travelling', he feels sure. Over the past seven years, the collection has been housed in the tropical greenhouse in Wageningen: about 1,500 samples of more than 150 difference species, ranging from a couple of centimeters to several metres in height.
But Hetterscheid did much more than just collect, describe and inventory the plants. He signed up researchers right, left and centre to scrutinize all aspects of the lives of these plants. Their smell, for example. Amorphophalli are known to stink. Throughout their short flowering period they give off odours reminiscent of rotting garbage, mould, dead bodies, sewage and manure. Although funnily enough there are also species that smell good to us, reminding us of aniseed, apples or boiled sweets.
The plants do not just stink for the sake of it. They take the trouble to smell like that for a clear purpose: sex. 'As a rule, plants go about their business quite slowly', explains Hetterscheid. 'But Amorphophalli are in a big hurry about some things. For example, scent development. The pollination chamber opens on the first day of flowering and the plant must attract pollinators then. So it needs a very penetrating scent.'
At that point the plant turns up the heat. 'Cells in the appendix [see illustration] are brimful of starch and mitochondria which convert the starch into heat. That goes very fast and in a couple of hours the cells are completely burnt up. Literally kaput. The party's over.' The effect of this fast digestion is that parts of the plant become 15 degrees warmer, warm enough to release aromas and make the plant irresistible. For certain pollinators, that is. The chosen insects are drawn to the stench as to a magnet. Either because they think there is something there for them, or to lay their eggs. Once they've been caught in the pollination chamber there is no escape for a while. Amorphophalli have their ways and means of ensuring that: like slippery walls. But some species provide a warmer welcome, according to Hetterscheid. They have a few sterile flowers scattered among them which provide a protein-rich meal that tempts the insects to stay around.' After pollination several things change and the insect is able to escape again.

Amorphophallus cut open
It is the endless diversity and adaptability of the Amorphophalli that has fascinating Hetterscheid all his life. And then he has not even got onto the topic of their camouflage colours. 'The pattern of patches on the leaves. Some species imitate woody plants, for example. I have shown photos to experts who were convinced they were twigs with lichen on them. But they were Amorphophallus leaves. It is the chameleon of the plant kingdom.'
Hetterscheid has some idea how that diversity came about, too. 'Amorphophalli are pioneers. You see them at the edges of forests and at disturbed spots in the forest. Places that can be colonized quickly and forcefully. Their reproductive drive is very strong. That gives them scope for experimentation and variation, a chance to react quickly to changing circumstances. Amorphophalli are tremendous adapters and survivors. The question is of course: how does the genome do that? I haven't got round to that yet.'
But the thesis is coming out soon: probably early next year. The chapters are ready in draft. 'It has got to happen now', thinks Hetterscheid. And there are even plans for a version for a wider public.
If you wanted to see flowering Amorphophalli in Wageningen, you are too late. Now that the tropical greenhouse has been closed, Hetterscheid's collection has gone. He has brought it to the University of Hamburg. But there is still plenty of dead material left: about 1,500 jars of preserved inflorescences. 'My juice collection', jokes Hetterscheid.
Sweaty feet
The Amorphophallus family has a wide range of scents at its disposal. Some of them smell like boiled sweets or aniseed, while others smell more like sweaty feet or rotting flesh. And otherwise like citrus, banana, apple, frying fish or rotting vegetable matter.
Hetterscheid distinguishes twelve groups of scents. He divides them into two groups on the basis of chemical analysis: complex scents made up of a rich mixture of organic alcohols, and simpler scents dominated by one substance. Under the latter category, come the pleasanter aromas (apple, boiled sweets, aniseed) known in chemical terms as acetates. As for the disgusting decomposition-like stenches, they are composed largely of organic alcohols.
Hetterschied discovered a correlation between the scent and the appearance of Amorphophalli. 'The rotting flesh smells often seem to go together with a darker-coloured inflorescence, as if the plant were imitating a corpse. And that attracts a particular sort of pollinator.' The form of the pollen plays a role here too. 'That entire complex of correlations between the species of Amorphophallus, the scent of the flowering plant and the shape of the pollen was unknown. At least, not at that level of detail and variation. This really is a very strange bunch of plants, I tell you.'