A team of scientists led by Yale University researchers believe they have finally figured out why land plants develop such complex vascular systems – a mystery that has been around for nearly a century. When land plants first appeared about 500 million years ago, their vascular systems were extremely simple. The inside of their roots and stems looked like bundles of straw, drawing in water and nutrients from the environment.

However, about 420 million years ago, this simple water absorption system underwent significant changes, gradually breaking “reeds” into more complex shapes, structures, and sizes. For nearly 100 years, scientists didn’t know why evolution favored these meandering interiors, but a new fossil analysis shows a more advanced vascular system is more drought-tolerant.

The authors conclude that a lack of water may be the cause of what makes up the innards of plants.

The first land plants on Earth were small, simple, moss-like forms. They had no root systems, which meant they were devoted to places with plenty of water. As plants began to move further inland, to drier regions, they needed new ways to retain water, sunlight, and nutrients while protecting themselves from evaporation and dehydration.

This is where branches and roots come in handy. And at the same time, these structures created new challenges. During drought, plants can dry out easily and form a steam bubble, a type of embolism that prevents water from flowing up the roots. In a simple and primitive vasculature, an air bubble inside a plant can easily spread to other channels or “straws,” creating a blockage for more water and nutrients. The result can be the death of tissues or even the entire plant.

By modeling the different vascular systems of some modern and extinct plants preserved in the fossil record, the researchers showed that a more complex vascular pattern can overlap with air bubbles. When the straws that make up the plant’s vascular system are separated into patterns, the simulations show that the air bubbles have fewer neighbors to spread to.