Researchers Solve Hundred-Year-Old Botanical Mystery that was Key to the Spread of Plant Life on Land

Amazing stuff! It was hydraulic failure stemming from plant embolism that was overcome!

"The earliest land plants were small — just a few centimeters tall at most — and restricted to moist, boggy habitats around streams and ponds. Around 400 million years ago, however, plants developed vascular systems to extract water more efficiently from the soil and use it for photosynthesis, a transition that would forever alter the Earth’s atmosphere and ecosystems. ...
When plants begin to dry out, air-bubbles get stuck in the xylem, which is the specialized tissue that transports water and nutrients from the soil to stems and leaves. The bubbles block the movement of water. Left unchecked, they spread throughout the network, disconnect plants from the soil, and ultimately lead to plant death. ...
“Every time a plant deviates from that cylindrical vascular system, every time it changes just a little bit, the plant gets a reward in terms of its ability to survive drought. And if that reward is constantly there, then it’s going to force plants in the direction away from the ancient cylindrical vascular system toward these more complex forms ...
These changes happened rather rapidly — in paleontological timeframes, that is — over approximately 20-40 million years. ..."

"Since plants colonized land, they have developed increasingly complex vessel architectures to carry water from their roots to their highest leaves. Vascular plants now display a diversity of xylem strand shapes in cross section, from elliptical to linear to many lobed. Bouda et al. investigated whether selection from drought, which causes vessel cavitation and embolism, drove the complexity of xylem strand shape as plants inhabited drier climates. By simulating embolism spread between vessels across varying shape and complexity, including those seen in extant lycophytes and ferns and extinct plant fossils, the authors found that evolutionary changes in xylem strand shape have reduced embolism spread and made plants less vulnerable to drought."

From the abstract:

"The earliest vascular plants had stems with a central cylindrical strand of water-conducting xylem, which rapidly diversified into more complex shapes. This diversification is understood to coincide with increases in plant body size and branching; however, no selection pressure favoring xylem strand-shape complexity is known. We show that incremental changes in xylem network organization that diverge from the cylindrical ancestral form lead to progressively greater drought resistance by reducing the risk of hydraulic failure. As xylem strand complexity increases, independent pathways for embolism spread become fewer and increasingly concentrated in more centrally located conduits, thus limiting the systemic spread of embolism during drought. Selection by drought may thus explain observed trajectories of xylem strand evolution in the fossil record and the diversity of extant forms."

Researchers Solve Hundred-Year-Old Botanical Mystery that was Key to the Spread of Plant Life on Land YSE-led research has discovered the answer to a 100-year-old paleontology mystery — how early plants emerged from their watery habitats to grow on land through changes to their vascular systems.

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