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The first vascular plants

The vascular system was one of plant-kind’s greatest breakthroughs. It is the plant’s equivalent of animals’ blood circulation. The first vascular plants appeared around 400 million years ago during the Silurian period.

[For phloem, scroll down 5 paragraphs]

Like blood, the vascular system transports nutrients around the plant and removes waste products. But it doesn’t work the same way at all. In particular it uses expendable water instead of precious blood.

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.

Non-vascular plants were the original pioneers that colonised the land – although I have read of signs of ‘algal mats’ having been found dating back thousands of millions of years earlier.

The first plants to colonise the land were things like mosses and liverworts. The liverworts (more) were the first. I’m not quite clear when mosses first appeared, but they look pretty ancient too. For a time these pioneers had the place to themselves. But the pressure was on, both to extract more nutrients from the soil, and to spread their spores further.

For this they needed height. And to grow high a plant needed a proper system for transporting water and nutrients about. It was at this point that plants developed the vascular system. The vascular system has two components. First there is the ‘xylem’ which is a system of tubes made of dead cells. There were no leaves. But whatever passed for them allowed evaporation to suck water and nutrients up through these tubes to the upper parts of the plant. That gets the minerals sucked in through the roots upwards. But it doesn’t get the chemical nutrients produced in the ‘leaves’ downwards. So there’s another system called the phloem, which uses living cells to push nutrient-laden water down again. I’ve read that this still isn’t fully understood.

The vascular system gave the plants that had it a huge advantage, and they quickly took over the planet. This picture is of Cooksonia, dated to 410 million years ago. It grew to a few centimetres high, and is one of the earliest true vascular plants. It comes from the website of Hans Steur. Cooksonia didn’t have leaves. As we’ve said, leaves didn’t appear until much later.

Incidentally, in his ‘History of Life’, Richard Cowen.Cowen depicts a non-vascular plant (Aglaophyton) which grew to some 20 cm. As we keep saying, don’t expect things to be simple in this game!

A complete ecosystem has been found, petrified by silica-rich waters from a volcanic spring, around the village of Rhynie in Scotland. It was 410 million years ago, shortly after vascular plants first appeared. The fossil plants still stand upright, though never above knee-height. Even the individual cells remain visible. And the creatures that were living among them still grip their stems.

There’s still a niche for the ancient non-vascular pioneers even today. But they can’t store water and they can’t suck it up from beneath the surface. So they are confined to damp places, where at least some of the time there’s water in abundance.

[Click for the next chapter in the story, trees.]