Researchers have solved a century-old mystery about plants

The earliest terrestrial plants were small – only a few centimeters tall at most – growing in moist swampy areas
around streams and ponds.
However, about 400 million years ago, plants developed vascular systems to extract water from soil more efficiently and use it for photosynthesis, a shift that would change Earth's atmosphere and ecosystems
forever.
A team of researchers has now solved a 100-year-old paleontological mystery: How did ancient plants grow from swamps and river banks to new habitats with limited water?

In a new paper published in Science, Craig Brodersen, a professor of plant physiology and ecology at Yale University, and his research team, including first authors Martin Bouda, PhD, and Kyra A.
Prats, PhD, found that a simple change in plant vascular systems made them more drought-tolerant, opening up new prospects
for exploration.

The study was sparked by a century-long debate about why the simple cylindrical vascular systems of the earliest land plants quickly transformed into more complex shapes
.
In the 2020s, scientists noticed an increasing complexity of the fossil record, but they couldn't pinpoint the causes—if any
—of evolutionary changes.

Over the past decade, Brodersen and his colleagues have explored how modern plant vascular systems are constructed, especially in arid environments
.
When plants begin to dry out, bubbles get stuck in the xylem, the tissue
responsible for transporting water and nutrients from the soil to the stems and leaves.
Air bubbles impede the flow
of water.
If left unchecked, they can spread throughout the network, separating the plant from the soil and eventually causing the plant to die
.
Today, avoiding the formation and spread of these bubbles is essential to endure drought, and the team used the same idea to explain vascular patterns in the fossil record
.

The cylindrical vascular systems of the earliest land plants, resembling a bundle of straw, initially served them
well in their early aquatic environments.
But when they migrate to land with less water resources, the plants must overcome the bubbles
caused by drought.
Early terrestrial plants prevented the spread of bubbles by transforming the cylindrical xylem of their ancestors into more complex shapes
.

Historically, observations of increased vascular complexity in the fossil record were considered incidental and of little significance, a byproduct
of plant volume growth and the development of more complex structures.
The new study disproves that notion
.

"It didn't happen
by chance.
There's actually a good evolutionary reason for that," Bouda said
.
"The intense pressure of drought has contributed to all this
.
It's a century-old mystery that we've now found the answer
.
”

Bouda notes that the composition of the research team that co-authored the study, including a paleobotanist, plant physiologist and a hydrologist helped them provide the techniques and perspectives that allowed them to discover the causes of the
complex vascular structures that appeared in Devonian plants.
The team used microscopy and anatomical analysis to observe the internal structure of plant specimens, including fossil specimens from the Peabody Museum at Yale University, as well as living plants
from Yale University's Myers Woods, Swamp Botanical Garden, New York Botanical Garden, and the University of Connecticut.
Using this information, the team predicted the structure of blood vessels that could tolerate drought and illustrate how seemingly simple shape changes led to significant improvements
in drought tolerance.

"Every time a plant deviates from the cylindrical vascular system, every time it changes a little bit, the plant is rewarded
for its ability to resist drought.
If this reward persists, then it forces plants to move away from the ancient cylindrical vascular system to these more complex forms," Brodersen said
.
"With these very small changes, plants solve problems that they had to solve early in Earth's history, otherwise the forests we see today wouldn't exist
.
"

These changes occurred fairly quickly—on the paleontological time frame, that is—between about 20 million and 40 million years
.
The driving forces behind changes in plant vascular structure can help research to breed drought-resistant plants, help build resilience to the effects of climate change, and address food insecurity
associated with production.

"Now that we have a better understanding of how vascular systems fit together and how it affects a plant's drought tolerance, that's what can be used as a goal for a breeding program — for example, to breed better root systems, to breed better plant vascular systems
," Brodersen said.

Hydraulic failure as a primary driver of xylem network evolution in early vascular plants