Kingdom of Plants: A Journey Through Their Evolution

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With the evolution of flowers, the face of the Earth was transformed forever.

© Rob Hollingworth

All of the early plants up until the Middle Devonian possessed male and female gametes which required water, in some form, for their fertilisation. As the original aquatic plants had a totally submerged existence, their sperm cells could freely move through the water to fertilise their ovum cells. This method of fertilisation put some obvious limitations on where they could survive, and out of water their reproductive strategy would have been impossible. Although bryophytes lived on land, they still relied on a partially wet environment to transfer their sperm cells and spores to their female gametes. We know from bryophytes living today, such as mosses, that when their surroundings are saturated some species store up several times their own weight in water as a reserve, and they are also able to stop their metabolism if their habitat dries out for long periods. These water-dependent land plants were therefore best suited to colonise the damp tidal shores of lowland streams of the Devonian forests, and mosses and ferns can still be found to thrive in these environments today. The need of these amphibious plants to be linked to a moist external environment for their reproduction would have been very limiting in all but the dampest of habitats, and so any plant that was able to break this reliance on water would have had an immense advantage. In the drier terrain further from the shoreline there would have been an abundance of space, light and nutrients. Natural selection soon favoured plants with the ability to grow and reproduce in the dry air of these new habitats. Their trick to surviving in dry air was to package up their reproductive cells in desiccation-proof capsules that could carry them through the air. Capsules we know today as pollen.

Equisetopsida

For over 100 million years, horsetails dominated the understorey of the Devonian, Carboniferous and Permian forests, growing up to 30 metres high.

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The first pollen structures that evolved were tiny packages of genetic material, light enough to be carried on the wind to the female cells of a neighbouring plant. On reaching their destination they put out a little tunnel through which their sperm cells could swim down to achieve fertilisation. For the first time, male and female plant structures were able to swap their genetic information over large distances in the dry air. To maximise their dispersing ability many pollen-bearing plants grew taller, and in time the skies filled with airborne DNA from a multitude of pollen-spewing Devonian flora. Although plants would still require water for photosynthesis, it was now possible for them to colonise new, drier regions of the land. From the coastal forests, plants began to push further into the empty expanses of the ancient world.

As pollen plants began to spread their domain further inland, and it became necessary for their gametes to travel over even greater distances to achieve pollination, a further major shake-up occurred in the way in which plants reproduced. This was one of the most dramatic innovations in the evolution of plants on land – the evolution of the seed. The earliest plants which exhibited seed-like structures are known as the progymnosperms, dating back to around 385 million years ago. They included trees like Protopteridium, and the leafy, 10- metre-tall Archaeopteris. The fossils of the trees from this period indicate that some, but not all, possessed structures resembling primitive seeds, suggesting that this was a time when the future of the seed hung in the balance. Like all plants before them, progymnosperms produced spores, but uniquely they were able to produce two separate types – micro-spores and mega-spores. This trait, called heterospory, suggests that progymnosperms were the most likely antecedents of all seed plants. Their ability to create variable spores is thought to have been the crucial intermediate evolutionary stage between plants with free-floating single spores and those with true seeds containing a spore-borne embryo.

The first true seed plants, which descended from the progymnosperms over 350 million years ago, were a group of tree-like ferns called pteridosperms, belonging to the major division of plants called gymnosperms. The word gymnosperm literally means ‘naked seed’, as they produce seeds which are not fully enclosed in an ovary. In earlier seed-less plants, the gametophytes were released outside the parent plant, but in the pteridosperms the gametophytes were microscopic in size and retained inside the reproductive parts of the plant. This created a moist ovule in which fertilisation could take place, in essence creating a plant within the parent plant. Coupled with this, these embryonic packages were encased with some starting-off food, meaning that they could be transported, ready to germinate as soon as they found themselves in the right conditions. The protective packaging of these seeds also enabled them to remain dormant after dispersal, and wait until conditions were perfect to grow. This prevented the precious genetic material contained within from being wasted in times of flooding or drought.

Today seed-bearing plants are the most diverse group of all vascular plants. The evolution of the seed enabled the proliferation of land plants on the wind, in the water, along the ground and in the stomachs of animals. During the Carboniferous and Permian periods, the gymnosperms evolved prolifically, with their extant relatives today including conifers such as pine, spruce and fir, with their needles; ginkgos, with their fleshy seeds; and cycads, with their large palm-like leaves and prominent cones.

Around 300 million years ago a global ice age hit the planet, and the Earth became progressively drier and cooler as great bodies of ice formed at the poles and locked away precious water vapour from the atmosphere. The reduction of atmospheric moisture caused vast areas of tropical forests and swamps to shrink and dry out, and with their ability to disperse their seeds and colonise drier environments, gymnosperms soon replaced ferns as the dominant plants on the planet. In time the higher-altitude regions of the planet became regions of cold-climate peat lands and swamps, which would have resembled something similar to the boreal taiga of modern-day Siberia. In the milder lowlands, deciduous swamp forests were dominated by the seed ferns of Glossopteris and Gangamopteris, along with large clubmosses and immense horsetails.

The first seeds

The development of the seed saw gymnosperms become the dominant plant group between 290 and 145 million years ago.

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By the end of the Permian period the main continents of Earth’s land masses had all fused together into one supercontinent called Pangaea, and parts of the planet had become arid with little rainfall, creating extreme desert landscapes. As deserts expanded and coastlines shrank, this extreme climate shift began to push many life forms to the brink, and by 248 million years ago, 95 per cent of the plant and animal species that had evolved by this point were wiped out. This marked the largest extinction ever known, and for the next 500,000 years complex life on Earth teetered on the brink of complete extinction. The 5 per cent of life that remained was sheltered from the extreme climate, in habitats that remained temperate and moist enough. These pockets of life harboured the fundamental DNA that had evolved so far. Over the following 50 million years, as the global climate became more amenable once again, plant life would bounce back to colonise the planet. Slowly plants began again to create temperate woods, tropical forests and dry savannahs.

As the Jurassic swamps and prehistoric woodlands began to spring back to life, plants continued to increase and diversify. Seeds, leaves and pollen became more specialised, and the world of plant life provided an abundance of food for the dinosaurs. Plants gave rise to fast-growing bamboo and shade-giving palms until 140 million years ago, when the plant world would be changed completely.