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The first trees

Trees are in a sense the pinnacle of achievement of plant-kind. And they first appeared near the end of the Devonian period, some 370 million years ago. By being tall, trees are able to spread their seeds over long distances. And with their broad canopy they are able to shade out other plants.

[click for previous chapter in story, vascular plants, for sex, for seeds]

Where conditions are suitable modern trees take over a landscape completely, leaving all other plants to do what they can with the leftovers. We in Britain tend to think of our natural landscape as mixed. But in fact, as soon as the ice retreated, the land quickly became entirely tree covered. I’m told that even places like Dartmoor and the Scottish highlands were thickly forested. It was only when humans returned, and started to ‘manage’ the landscape with fire and flint axes, that our ‘open’ landscape appeared.

But it was not always like that.

The first vascular plants appeared towards the end of the Silurian period, about 420 million years ago. This picture of ‘Cooksonia’ comes from Hans Steur’s website. A vital part of the vascular system is the ‘xylem’, which is a system of tubes made of dead cells. The xylem has to be stiff, because it’s used to suck water up from the roots.

From the beginning, plants needed to grow as high as possible, so that their spores could spread further. The xylem reinforced cells turned out to be just the job.

Incidentally, the university textbook I’m using says that the xylem system relies on suction to transport nutrient-laden water upwards. This may be fine for ordinary plants and bushes. But anybody who did science at school should be worried now we’ve started talking about trees. It is fundamentally impossible to suck water upwards more than about 10 metres (the exact height depends on the weather). Even a powerful suction pump won’t do better then that. So trees must have invented some additional wrinkle to enable them to grow high. My textbook admits that the phloem system (see above link) isn’t fully understood. So maybe this aspect of the xylem system isn’t either.

The key to getting the stiffness that xylem needed was lignin. I was once told that lignin was originally a waste product, that cells deposited outside their walls to get it out of the way. When the need for this stiffness became apparent, the lignin was drafted in for this additional duty. As the push came for plants to grow larger, taller and stronger, the amount of lignin incorporated increased. A treetrunk is nearly all lignin.

However the road to full ‘treedom’ was not straightforward. The first tall structures were already around 410 million years ago. They were featureless columns standing up to 6 metres high. And they turned out to be fungi!

The next development was around 385 million years ago. In many ways these structures were trees. They grew to 8 metres (25 feet) high and more. They had no leaves, but they were clearly efficient photosynthesisers. They had genuine roots, though only shallow ones. But they were not made of wood. They were not related to modern tree ferns, though they like them. This modern tree fern is growing in the Savil Garden?? near London. Their primitive spore systems confined them to damp places. But despite these limitations, they were a successful species and have been found worldwide.

However, growing in their understorey, like mammals during the age of the dinosaurs, were the ‘archaeopterids’. The archaeopterids made some key advances which enabled them to move out of the shadow of the cladoxylopsids; and to form their own dynasty.

These included wood, deep roots and large leaves. It also included a more advanced spore system, in which the female spores provided food for the embryo. This made them less dependent on an instant supply of nutrients from the soil than the cladoxylopsids, and permitted dryer places to be colonised.

By 370 million years ago the first true tree, Archaeopteris (not to be confused with Archaeopteryx the bird) had appeared. It could grow to a height of 20 metres or more, and it’s probably a predecessor of modern conifers. This reconstruction comes from Dennis Murphy’s Devonian Times (www.devoniantimes.org). Dennis’s site is well worth a visit if you want to know more about the Devonian.

Archaeopteris gradually took over many of the drier parts of the planet.

But in the swampier areas, they and even the cladoxylopsids were overshadowed by giant horsetail and clubmoss trees. These fell into the swamps when they died, and were quickly buried. This deprived the carbon-loving bacteria the oxygen they needed to recycle the dead remains.

The result was today’s coal seams. Much of today’s coal supply was deposited during a 20 million year period at the end of the Carboniferous. Why so much? Well Pangaea was beginning to build, and apparently this gave rise to a gradual rise in sea levels. So these already low-lying and swampy lands sunk, and new layers of forest grew and died on top.

The archaeopterids’ deep roots had a dramatic effect on the climate. It is part of the job of roots to break up the bare rock, and to dissolve out nutrients from it. In fact of course they get bacteria to do the difficult chemistry for them – just as plants and we animals do today. And of course the deeper the roots the more effective the bacteria can be.

The result was a huge increase in erosion, with vast amounts of organic carbon being swept into the oceans – in addition of course to the huge amounts being buried in the coal seams. Over the period of the Devonian, carbon dioxide levels dropped by about 95%, and greenhouse conditions were replaced by a severe ice age.

Sex

Next we must discuss sex. I don’t know when sex was invented. Maybe that’s because the scientists don’t. As we’ve seen, the archaeopterids used the female spores to help them conquer the drier regions of the planet.

There’s been a huge amount written on sex, much of it conflicting and possibly not thought through too well. On balance it seems that sex ‘costs’ too much to be a sensible way of adapting to changing environments. This isn’t to sayu of course that, once it’s there, it won’t be made use of in all sorts of different ways.

But sex must be very important because nearly all animals and plants rely on it heavily.

The best explanation that I’ve seen is that sex is a response to disease. An ‘organism’ has to respond much more quickly to a new disease than it does to most changes in the environment. But pathogens can respond much more quickly still. What sex gives you is a variety of minor variations within a population. Asexual propagation doesn’t do this. Each copy is a perfect clone. This is why plant growers like it so much.

So when a new disease strikes, however deadly, hopefully there are a few within the population who are already lucky enough to have a certain resistance to it. They may get terribly sick, but as long as they recover, they can form the nucleus for rebuilding the population.

Seeds

This brings us on to seeds. Before seeds, a plant wishing to engage in sex had to have a film of water in which sperm could swim to fertilise the ovum. And plants did wish to engage in sex. They had already invented spores, which enabled them to spread their kind on the wind. Next the spores diversified into male and female spores. As we’ve seen the early trees were already into this.

Then the male spores transformed into pollen. Pollen is a packet of sperm, very light, and encapsulated in a waterproof case. Initially, and until flowering plants appeared a couple of hundred million years later, plants relied on the wind to blow the pollen from one plant to another.

The female spore became the ovum, and tended to stay with the plant until a pollen grain lighted on it. Thus fertilised it became a seed. Once seeds were invented, then trees in particular could spread far and wide. Winged seeds have been found in late Devonian rocks. Richard Cowen, in his ‘History of Life’, reckons that trees, true leaves and seeds all appeared during the late Devonian, say around 360 million years ago. I’ve also read that seeds came somewhat earlier, but in the light of the article from which I got much of this information (New Scientist 24.11.07) I’m inclined to go with Cowen.