A 400-million-year-old fossil is revealing how plants grew into giants

In the early 1900s, a small fossil plant was discovered near the village of Rhynie in northern Scotland. Now, a century later, Horneophyton lignieri is changing what we know about plant evolution.

Until not that long ago, scientists widely agreed on the evolutionary journey that plants took from the oceans to the land. They thought that algae evolved into mosses and their relatives, known as the bryophytes, which eventually gave rise to the vascular plants – the group containing ferns, conifers and flowering plants.

However, recent genetic studies have cast doubt on this narrative by suggesting that the common ancestor of plants wasn’t a bryophyte or a vascular plant. It’s a finding that’s provoked strong debate among botanists, as well as making it uncertain what the ancestor of plants would have looked like.

Now, the 407-million-year-old Horneophyton may provide the answer. Research led by Dr Paul Kenrick, one of our fossil plant experts, found that it could shed light on this elusive ancestor.

“Unlike modern plants, which transport water and sugars separately, Horneophyton moves them around its body together,” Paul explains. “This kind of vascular system has never been seen before in any living plant.”

“It suggests that the ancestor of modern plants was more complex than we originally thought and already had some kind of vascular system. It’s a discovery that will help us to interpret how later plants evolved and tie their relationships together.”

The findings of the research were published in the journal New Phytologist.

What are the xylem and phloem, and what came before them?

The vascular system of plants acts a bit like the circulatory system of animals, as it moves water, minerals and other vital substances around its tissues. Without this system plants can’t grow very big because nutrients can’t reach all the parts of their body.

This vascular system is divided into two main tissues called the xylem and phloem. The xylem is a system of tubes made up of dead cells that carries water and minerals from the roots all the way up to the leaves. It’s driven by a variety of processes including transpiration, where evaporation in the leaves draws water up through the plant.

The phloem, in contrast, is a system made of living cells that transports the products of photosynthesis, such as sugars, up and down the plant to where they are needed. These nutrients are sometimes transported in and out of the phloem by specialised cells that have distinctive folds in their cell wall.

When Horneophyton was first discovered and studied, scientists believed it had true xylem and phloem as the extraordinary preservation in the Rhynie Chert seemed to show details that looked like a modern vascular system.

But when Paul and his colleagues took a fresh look at the fossils using more powerful modern microscopes, they realised that this wasn’t the case.

“Using confocal laser scanning microscopy, we were able to create 3D models of Horneophyton’s inner structure,” recalls Paul. “They clearly showed that this plant had a novel conducting tissue that comes from an earlier stage of the vascular system’s evolution.”

“Its vascular system appears to be made mostly of transfer cells that were moving both water and sugars around. It suggests that phloem-like cells seem to have evolved first, and that the xylem only came later. A system like this can only work in small plants.”

If this is the case, then Horneophyton would represent an intermediate stage in the evolution of the plant vascular system.

Evolving plant vascular systems

The origins of the vascular system in plants can be seen in the ancient eophytes. These were millimetre-sized plants thought to have been among the first to move onto land around 420 million years ago.

They had moss-like structures known as food conducting cells that moved nutrients around their bodies and which share certain characteristics with the phloem.

This new type of vascular system now seen in Horneophyton appeared a little after this. It allowed the plant to grow much bigger than the eophytes, with some reaching up to 20 centimetres in height.

But even by this time Horneophyton’s vascular system was an evolutionary relic.

One of its contemporaries, the plant Asteroxylon, had already evolved a sperate xylem and phloem that allowed it to grow to around twice the size of Horneophyton. This paved the way for the eventual evolution of towering trees and sprawling ferns millions of years later.

Other plants in the Rhynie Chert probably had different conducting systems, but they disappeared long ago. Paul is hoping to examine these lost vascular systems as he reinvestigates the Rhynie Chert fossils that we look after.

“These plants have been known about for a long time, but they’ve tended to be shoehorned into pre-existing categories that don’t fit them,” Paul says. “By putting aside our existing ideas and looking at them with modern technology, we can see that their tissues are very different from what we expected.”

“As we drill down into the other plants of the Rhynie Chert, we’ll get a much better idea of how they evolved and transformed the Earth into the world we now know.”