The growth of vegetation
Sunday 08 February 2009
For millions of years heavy rain fell on the Earth's barren land masses, wearing down the ground into a lifeless muddy silt.
High levels of carbon dioxide in the atmosphere at this time meant that the rain was acidic, increasing rock erosion and weathering. The first plants were just squidgy things that looked like small seaweeds and green mosses. They were descendants of the ancient blue-green algae – the oxygen-producing Cyanobacteria – and they clung close to beaches, rivers and streams. The transformation of these small, soggy clumps of moss into tall, graceful trees that could live thousands of miles from the water's edge demonstrates some of nature's most spectacular feats of biological engineering...
The vascular revolution: How plants eventually evolved into tall trees and forests
Think how hard it would be to design a tall tree that could flourish in the wild. For a start, there is the business of staying upright. Ideally, a 40m-high tree should be able to withstand a hurricane without toppling over. Next, a steady supply of water and nutrients are needed to sustain the whole tree. The bits of the tree that make food – the leaves at the top – must be as near to the Sun as possible, which in a thick, dark forest means being tall enough to make sure that all the other trees don't block out the sunlight. But being tall means being further away from the main source of water, which is stored somewhere in the ground. Finally, if the tree's family is to flourish in future generations, it must be able to reproduce successfully. That means just dropping seeds willy-nilly on the ground below simply won't do, because young trees can't thrive if they have to compete with their parents for sun, food and water. Seeds have to be spread further afield. How is this to be done, when trees can't walk or swim?
Designing a tree is no simple thing which is probably why the earliest plants – mosses, liverworts and hornworts (the family is called the Bryophytes) – stayed exactly where they liked it best: near to the water's edge, thriving only in inlets and bays in river mouths and beside streams. They completely ducked the idea of being tall. Their strategy was to hide from the wind by staying small and keeping close to a good source of easy-to-find water, so as to avoid drying out.
These plants had no proper roots, leaves or internal plumbing system to deliver water and nutrients. But if trees and plants were to colonise the vast tracts of barren land a half-hearted attempt at escaping from the sea like this was no long-term solution. About 420 million years ago, the first signs of a new approach began to emerge in the form of "vascular" plants. Ultimately, all the world's trees and forests are derived from them.
Small beginnings: the first vascular plants
The first vascular plants weren't anything spectacular to look at. They comprised smallish shoots, only about 50cm high, with thick stems and firm, spiny leaves. These vascular plants had ingeniously invented a chemical called lignin, that toughens the walls of plant cells. Plants that have little or no lignin stay small and floppy – like herbs or garden flowers. Although the stems of plants like these can feel rigid, they are held up only by the force of water within them. If the water supply dwindles, the herb or flower wilts.
Plants with lignin in them can stay upright even when a drought sets in. With great precision, lignin-toughened cells are stacked and interwoven in carefully constructed layers to make wood – the magical stuff of trees. Lignin also provides tubes through which minerals and water are transported up and around the tree.
The first evidence of lignin came from plants called Rhyniophytes (named after their place of discovery in Scotland – see "The Miracle of Rhynie Chert", below). These are now extinct, but their descendants are all round us – indeed, everything woody ultimately comes from these early pioneers of the land. Mind you, it took a while – at least 40 million years – for these small plants with toughened stems to become tall, graceful trees.
By the time we get to the Carboniferous Period (360 million years ago), trees were growing in huge numbers. The earliest, called Lycophytes, were simple structures. They had roots and branches which divided into a "Y" shape. But they could also be very big, with some specimens, such as the lepidodendron tree, as wide as 2m and as tall as a 12-storey building.
Growing up: life in the world's first forests
Except for the wind, and maybe a scratching sound inside a hollow log or a faint buzzing in the branches, it was eerily quiet in this prehistoric world. There were few animals, and no birds – it was still far too soon for them. And the landscape looked pretty much the same in all directions – an endless thick, dark greenish-brown, a blur of identical-looking trees. Very few varieties existed at this time. There were also no flowers. The Earth would have to wait at least another 150 million years before it could witness a first bloom. Compared with trees, flowers are a modern fad.
The lycophyte trees that dominated ancient forests lacked one crucial ingredient, and this would ultimately lead to their graceful decline into extinction some 270 million years ago. They lacked true leaves. They mostly used scales on their trunks and thin green blades on their branches for photosynthesis instead. It was left to a relative of the vascular plants found at Rhynie to come up with the concept of creating little green solar panels attached to the tips of branches. These were the euphyllophytes – literally "good leaf plants". Most trees that are alive today are descended from them. Euphyllophytes quickly grew into several varieties, including ferns and horsetails.
If it weren't for lycophytes, ferns and horsetails, the whole history of modern civilisation – and our lives today – would have been very different indeed. These early trees colonised the land in their millions. When they died, most of them sank into swampy marshes, where, over millions of years, they were compacted, hardened, chemically altered and metamorphosed by heat and pressure, ultimately becoming coal. This source of chemical energy would eventually fuel the Industrial Revolution, millions of years later.
While lignin helped trees become strong and leaves trapped the energy of the Sun to make food, trees still faced the difficulty of finding a steady and reliable supply of water that somehow had to be channelled all the way to the top of their canopies – water that was often dispersed many metres below the ground. Trees rose to that challenge in two ways. The first relied on cultivating a good crop of friends to help. The second was down to ingenious design.
Tree roots grow downwards to find water. But to help them, they often enlist the support of another group of highly versatile living things. Fungi, neither plant nor animal (although for a long time they were grouped with plants), form their own separate, almost invisible underground kingdom.
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