Thomas' Plant-Related Blog

The first flowering plants evolved more than a hundred million years ago, while dinosaurs were still on the scene. Since then, they’ve come to dominate the world, largely outcompeting the plants that were there before, such as conifers, cycads, and ginkgoes. With some exceptions (particularly the taiga, the coniferous forests of Russia and Canada), the vast majority of the vegetation we now see are flowering plants, or angiosperms. That includes things that don’t obviously flower, like grass and oak trees.

There are a few different ideas as to why the new flowering plants did so well. Some modern ones focus on differences in the plants’ water transport: the slow seedling hypothesis says that baby angiosperms can get off to a sprint start, overshadowing competitors before they’d left the starting blocks. Making your food from sunlight might sound like an easy life, but plants have to fight for daylight, and getting on top can be the knock out blow.

Older theories tended to be more focussed on reproduction. It’s not simply a flower that officially distinguishes a flowering plant; unlike other plants, the unfertilised egg is completely enclosed, and the pollen (containing the male sex cells) has to drive a pollen tube through part of the parent plant. Their other name, angiosperms, comes from Greek words for ‘vessel’ and ’seed’.

Pines produce lots of pollen—inefficient, but it makes some nice abstract patterns on water. Image by finna dat on flickr (link on image).

Flowers are, however, important. The reason for showy flowers is to attract pollinators, most commonly insects. Today, the majority of flowering plants use insects to carry their pollen, whereas most gymnosperms (the older group of plants, including conifers) are pollinated by the wind. Insects have one clear advantage over the wind: they can track down another flower of the same species, so you don’t have to produce huge amounts of pollen. This was the basis for one theory about the rise of angiosperms: they simply did pollination more efficiently.

The new work throws up a big problem with that idea. From insect fossils, it looks like there were pollinators around in the Jurassic, which had evolved together with the gymnosperms that were around at the time. If angiosperms had tried to patent working with insects to transfer pollen, there would have been prior art.

To be precise, the scientists describe several species of scorpionfly, whose mouthparts look like they evolved to suck up some sort of fluid (but blood didn’t seem to have been on their menu). And the relationships of these groups suggest that the same way of feeding evolved several times. This would all fit in with some fossil gymnosperms, which don’t seem to have been terribly well adapted for wind pollination.

As the perspective (second reference below) puts it, however: “a key piece of evidence is missing: The authors failed to find any pollen associated with these fossils.” Pollen still stuck to fossilised insects would really clinch the deal, and is worth hoping for. But if you accept that insect pollination was going on, there are other groups of insects that could also have been involved, and the story’s certainly not far fetched.

Finally, I wonder if it was really gymnosperms which those Jurassic scorpionflies were pollinating. It’s not clear just when the first flowering plants did arise, but it has been suggested that their separate lineage goes back that far. The authors argue that, because several groups of insect evolved the same feature, they must have been using a widespread group of plants, which might rule out the ancestors of today’s flowering plants.

References

Ren, D., Labandeira, C., Santiago-Blay, J., Rasnitsyn, A., Shih, C., Bashkuev, A., Logan, M., Hotton, C., & Dilcher, D. (2009). A Probable Pollination Mode Before Angiosperms: Eurasian, Long-Proboscid Scorpionflies Science, 326 (5954), 840-847 DOI: 10.1126/science.1178338 (Technical)
Ollerton, J., & Coulthard, E. (2009). Evolution of Animal Pollination Science, 326 (5954), 808-809 DOI: 10.1126/science.1181154 (More readable)

Competition is a powerful force in biology. The image of ‘Nature red in tooth and claw’ calls to mind animals fighting over food or mates, and plants sucking up nutrients, or overshadowing their neighbours. But, particularly where the environment is harsh, it’s known that living things can actually give each other a boost. The most famous examples are symbioses, where both partners benefit from the interaction: ant plants, for instance, house a colony of ants, which defend their host. But in other cases, termed facilitation, help seems to be given more inadvertently.

The grass studied. Image credit: S.G. Reynolds, FAO.

In a hot, dry desert, the shade from one plant may be the best place for another to grow. One microbe breaking down a piece of dead wood may make it easier for another to live there. In the alpine meadow in Tibet that this study examined, the story is probably that plants slow down the evaporation of water from the soil, and possibly also shelter each other from particularly cold conditions.

Competition has long been known to promote inequality. If a group of plants start off about the same size, those that have a little advantage, for whatever reason, will grow quicker, and that gives them a bigger advantage. So their sizes spread out, as the successful competitors press home their advantage.

The new research finds that facilitation can also increase inequality. Is that a perverted act of charity, which gives most to those who’re already rich? Thankfully, it’s more a matter of chance: plants don’t grow evenly spaced, and those that happen to be near neighbours benefit more from facilitation than those that are isolated. The scientists do suggest, however, that larger plants may be more likely to get close enough to neighbours for the positive effect, which is more like ‘giving to the rich’.

Most of the paper concerns a computer model which they used to demonstrate this. But they did also an experiment with the grass pictured above, Elymus nutans (it doesn’t seem to have an English name). In line with the model, inequality increased at the lowest density (due to facilitation) as well as at higher densities (due to competition). Inequality is measured as the ‘coefficient of variation’, which is the standard deviation (how much measurements differ from the average) divided by the average (the mean).

Graph of inequality versus density

This was tested in experimental plots of 1m² each. That’s a good way to test for an effect at first, as it simplifies the situation. It will be interesting to see whether this sort of thing is important in natural communities, particularly as that involves more than one species interacting.

Reference:

Chu, C., Weiner, J., Maestre, F., Xiao, S., Wang, Y., Li, Q., Yuan, J., Zhao, L., Ren, Z., & Wang, G. (2009). Positive interactions can increase size inequality in plant populations Journal of Ecology, 97 (6), 1401-1407 DOI: 10.1111/j.1365-2745.2009.01562.x

A number of mass extinctions punctuate the fossil record, dealing a sharp blow to life on Earth. The best known (although not the biggest) is the one that did for the dinosaurs, some 65 million years ago. Unlike some mass extinctions, there’s at least one smoking gun: a damn great rock crashed into the planet, somewhere near Mexico. It’s not a closed case, though; there are various theories, including the possibility of an even bigger rock landing near India.

If a massive rock did indeed hit the Earth, it would have made the world’s nuclear weapon stockpile look like a few party poppers. There would then have been earthquakes, volcanic eruptions, fires, colossal tsunamis, acid rain, and darkened skies from the dust thrown up. In short, all the things you get in Horizon, or the more exciting bits of the Bible.

This study looked at the effect of the disaster on algae, by measuring various chemicals, produced by the algae and stored in rocks. Different groups produce different complex chemicals, and the more algae there were, the higher the concentration of the relevant chemical. When the mass extinction hit, the rock laid down records a sharp drop in algal populations, as you’d expect. The dust blocking out the sunlight was probably the key cause—like plants, algae get their energy from sunlight by photosynthesis.

The key question, though, is how quickly they recovered afterwards. From the chemicals, it looks like they bounced back almost at once—within a century, the study’s authors claim, algae were back to more or less the previous density. Algae are at the bottom of the marine foodweb, so that would have set the stage for other species to recover, and for new ones to evolve into the niches left by extinctions.

Distinguishing a mere century after 65 million years is no mean feat. That’s on the same scale as ten seconds seen from a distance of three months. Are we sure that the chemicals are well enough fixed in place in the rock to record that difference? At the layer representing the mass extinction, there is a sharp change in the chemical signal. If the chemicals had ‘run’, we’d expect it to be more smeared out. At that level, however, there is a change in the type of rock (from a marl to a clay) which might affect how well the chemicals were fixed. Finally, the scientists note that this record is only from one place (in Denmark). If the same thing was going on over the whole ocean, we should be able to confirm it by studying sediments elsewhere.

Reference:

Sepulveda, J., Wendler, J., Summons, R., & Hinrichs, K. (2009). Rapid Resurgence of Marine Productivity After the Cretaceous-Paleogene Mass Extinction Science, 326 (5949), 129-132 DOI: 10.1126/science.1176233

This one’s an old bit of research, but a favourite of mine. It’s not groundbreaking science, but when I first heard about it, I just went ‘oh, wow’, in amazement at what natural selection can come up with! In short, it’s a flower shaped to reflect sonar so that bats can find it.

Mucuna holtonii flowers (image credit: jmlynn, webshots.com)

Flowers can be pollinated by all sorts of animals. Bees and butterflies are familiar pollinators, but there are also flowers that stink of rotting meat, to attract flies. And, in the tropics, where nectar is available year round, larger animals (which live longer than a year) can become specialist nectar feeders. Bat pollinated flowers tend to be pale (colours are wasted at night), strongly scented, and produce very large amounts of nectar (larger creatures need more energy to fly). One species, however, has a further trick up it’s sleeve.

Mucuna holtonii doesn’t have an English name. It grows in Central America as a vine, and it’s a legume (a relative of beans). When its flowers are ready to release pollen, they raise the top petal (the ‘vexillum’), which you can see in the photo above. A bat turns up to drink the nectar, and when it pushes into the flower, the anthers explode, dousing it in pollen. Because the flower releases its pollen in one go, it doesn’t need to attract a second bat, so it doesn’t refill with nectar.

Like insect-hunting bats in Britain, Mucuna’s pollinators use echolocation: they make a brief squeak, too high for us to hear, and use the echoes to work out where things are. The scientists had noticed that, using a loudspeaker to do the same sort of thing, the top petal reflected much more sound than something its size ought to. How was it doing that, and was it advertising to the bat?

Rays being reflected towards their source by mirrors at a right angle

The shape of the petal suggested the how. Bike reflectors and cats eyes (the ones on the road) have mirrors at right angles. As the diagram on the right shows, these work to bounce back light in the direction it came from—so when your headlights shine on it, you see the light. Sound and light are both waves, and Mucuna’s top petal is working in the same way for the bats’ sonar pulses. The German scientists tested this using a loudspeaker and a microphone: sure enough, the echo was loudest when the two were in line, both when the loudspeaker was in front of the flower, and when it was 30° off.

To show that this really was a signal to the bats, they got two more lines of evidence:

1. When some of the flowers had the top petal cut off, or filled with cotton wool, they were far less likely to be visited by a bat. 88% of unmodified flowers were visited, but only 21% of those with the top petal cut off, and 17% of those where it was filled with cotton wool.
2. Some closely related species are pollinated by bats which don’t use echolocation. The top petal in their flowers is not shaped in the same way to reflect sound.

Case closed, if you ask me. In 2003, the same scientists claimed that the bats could, by echolocation, distinguish newly opened flowers from those that a bat had already visited (so would have no nectar). That’s not implausible, but I’m not convinced: the bats could just be remembering which flowers opened when.

References:

von Helversen, D., & von Helversen, O. (1999). Acoustic guide in bat-pollinated flower Nature, 398 (6730), 759-760 DOI: 10.1038/19648

von Helversen, D., & von Helversen, O. (2003). Object recognition by echolocation: a nectar-feeding bat exploiting the flowers of a rain forest vine Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology, 189 (5), 327-336 : 10.1007/s00359-003-0405-3