The evolution of plants part 2: Birth of the forests





Editor and Artist
We live on a green planet. Today we may take them for granted, but plants are the most important living things on Earth. Their colonization of land made it possible for all animal life to survive, from the smallest ants to the biggest dinosaurs. After their first foray onto land, how did they turn into nature’s skyscrapers?

We like to talk about the largest animals in the world. The blue whale tips the scales at over 150 tons while the largest dinosaurs held their heads far above the ground. Yet some of us manage to overlook the largest of all plants. Growing over 60 to 90 meters (200 to 300 ft) tall, the world-famous redwoods of the west coast of North America dwarf even the biggest animals by a huge margin.

As impressive as their size is, the largest plants in Earth's history have left a bigger mark on the planet. Pioneering plants altered the landscapes and fueled the growth of some truly massive and charismatic animals, man included. But how exactly did the small early plants grow into trees? What adaptations allowed them to reach for the sky and stay there for so many millions of years? How did the deep roots, woody stems and photosynthetic leaves come about?

During the Late Devonian roughly 380 million years ago, there was a new revolution in the story of plants. A new type of tissue began to emerge — wood. This added more strength to the plants of the era, allowing them to outgrow their ancestors.

The earliest plants would perform transportation of water between cells by way of an inter-cell system. Unfortunately this was not always helpful and limited the size of these early conquerors. Things changed with the development of lignin, a chemical polymer that allows for the formation of bark and wood. Lignified cells are often dead and empty, allowing for an excellent water transportation method within the plant.

Cellulose is another key compound found in plants. It is hard to digest and is highly resistant to damage. This, mixed with that lignified outer layer makes for an incredibly tough cover for an ambitious plant. The stems of the early plants soon became coated with these dead lignified cells and transformed into thick trunks.

The new trees, primitive though they may be by our standards, began to spread across the floras of the world, growing ever larger and ever more recognizable. Trees began to grow ever taller and finally this growth culminated in one of the biggest organisms to inhabit the Late Devonian period.

During this time in the planet’s history, the skyline was practically dominated by the titanic crowns of Archaeopteris. It was one of the earliest true trees with a wooden trunk over a meter in thickness and with huge branches that supported frond-like leaves. At a distance it might have appeared somewhat like a cross between a gigantic fern and a conifer tree of today. But hidden among these great boughs was another adaptation that would stay with plant kingdom to this day – the evolution of the axillary bud.

These buds exist in the axil, the intersection of a leaf and its stem, and produce either new shoots or new branches. The presence of these growth buds meant that the tree could shed its leaves and even whole leafy branches during times of hardship, for there was always an insurance policy around. This makes Archaeopteris perhaps the first deciduous tree, although whether it shed annually or just a few times a month is not yet known.

An intepretation of Archaeopteris by Kylie Langton

As the land plants grew, their roots began to get tougher and deeper. They managed to dig themselves deep into the earth, breaking up rocks into smaller particles and eventually soil. Plants speed up this process by producing organic acids that enter through the leaf litter and mingle chemically with the ground below. This adaptation enabled them to move away from the lowlands and onto drier country, while also increasing the spread of organic soils. This is one of the key events of the Devonian, but the emergence of roots was not the end for them.

While clubmosses and moss-like plants in aquatic environments had simple leaves with a single unbranched vein, the more complex leaves developed at the crowns of much more advanced trees and plants. These leaves were filled to the brim with chlorophyll and became the main photosynthesizers while the wooden trunks were left bare of any green. The development led at least partially to the fall of atmospheric carbon dioxide in the Late Devonian.

With new inventions under and above the ground, plants altered and shaped the very landscape. One change in the old order was the reduction in soil erosion and stability of the banks of meandering rivers. Although their courses are constantly shifting, deep-rooted plants slowed their changes. The advance of the high-rising plants allowed the spread of the wetland environment, fueling one of the greatest developments to occur in the evolution of vertebrates.

By that time, fishes had conquered fresh water. As falling leaves and plant parts decompose, they clog and deoxygenate the water. So freshwater fish have to find new ways to get their oxygen. One way is to take it directly from the air and develop a lung or lung-like organ. For a small fish wishing to escape predators, hiding among the shallows is also a good option to explore.

A group of lobe-finned fish with fleshy, muscular fins went one step further and evolved features to increase their strength — a shoulder joint, wrist bones, and even separate fingers. These were the first of the early tetrapods, or four-footed land vertebrates. They were all predators with heavy jaws and sharp teeth which took advantage of a glut of food on land, consisting of insects, spiders and millipedes. These plucky bugs had been safe from any predatory fish that stalked in deeper waters offshore, until now.

The trees also had a huge hand in the rise of complex vertebrates on land, especially when it came to purifying the air. The lungs of the land vertebrates would never have survived an environment cloaked in carbon dioxide. By the Late Devonian, the incredible rise in oxygen would keep on going until the air would grow thick with it. As the forests altered the atmosphere, the land became filled with huge swaths of swamps in the next chapter of their history, the Age of Coal.

This is part two of a five-part series on the evolution of plants.

1: The first conquerors of land

2: Birth of the forests

3: The Age of Coal

4: A tale of flowers and seeds

5: The grassland empire