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.
