A team of scientists, led by researchers at Yale University, think they have discovered why land plants evolved complex vascular systems – a mystery that has stood for nearly a century.
When land plants first appeared about 500 million years ago, their vascular systems were very simple.
Inside their roots and stems looked like bundles of grass, which drew water and nutrients from the surrounding area.
About 420 million years ago, however, this water-absorbing system underwent a major change, gradually dividing the ‘grasses’ into shapes, forms, and sizes.
For almost 100 years, scientists did not know why evolution favored the interior of these things, but a new study of fossils shows that the modern nervous system can withstand drought.
The lack of water, the authors conclude, is probably what started the process inside the plants.
The first terrestrial plants on Earth were small, harmless, moss-like structures. They didn’t have roots, which meant they only lived in areas with a lot of water.
The first vascular plants were centimeters tall and were found only in the wettest areas.
One of the problems they had to overcome to get out of the river was how to keep water transport through their xylem from being blocked by embolism during drought.
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– Martin Bouda (@BoudaLab) November 10, 2022
As plants move upland into more arid regions, they need new ways to obtain water, sunlight, and nutrients while protecting themselves from evaporation and dehydration.
That’s where the branches and roots came in handy. And yet, at the same time, the design also introduced new challenges.
During a drought, plants can easily dry out, creating vapors, like an embolism, that block water from passing through the roots.
In a simple and unaltered vascular system, air bubbles within the plant can spread to other channels or ‘grasses’, trapping additional water and nutrients. The effects can cause tissue death, and can kill the entire plant.
Drawing the various vascular systems of some modern and extinct plants preserved in fossilsresearchers have now shown that the vascular system can block air bubbles.
When the grasses that make up the vascular system of plants are divided into structures, simulations show that air bubbles have fewer neighbors to spread.
The videos below show the difference between an embolism that spreads through the veins of a simple, primitive plant versus a complex, modern one.
These findings have led researchers to conclude that drought is a “nonsensical” determinant of vascular stress in plants.
“Every time a plant leaves a cylindrical vascular system, every time it makes a small change, the plant gets a reward for its ability to survive drought,” explains plant biologist Craig Brodersen of the Yale School of the Environment.
“And if the reward is always there, then it will force the plants to move away from the old cylindrical vascular system to these more complex types.
“By making these small changes, plants solved this problem that had to be figured out very early in the history of the earth, otherwise the forests we see today would not exist.”
Not only do the findings reveal interesting facts about the ancient world, but they also help to explain how many types of nerves seen in modern plants evolved and discuss future solutions.
This new understanding of how plants respond to drought may one day help researchers prepare plants for rapid climate change.
If scientists can figure out how to grow good roots and blood vessels, other plants can feed us in the future.
This study was published in Science.
