Image Courtesy of Court Johnson.
Like any great puzzle, the initial setup seemed deceptively simple. What two botanists noted at the 1920 Royal Society of Edinburgh meeting was just that: larger plants had more complex vascular systems. The bigger the plant, the more shapes its bundles of xylem and phloem would take on to exchange water with its root systems. Yet, as with all unsolved mysteries, the pair of scientists could not explain why this relationship existed. The scientific world would go on to assume that increasing vascular tissue complexity was nothing more than a morphological quirk of plant size, not considering that there might be an evolutionary layer to the problem.
A recent study published in Science from the Brodersen Lab at the Yale School of the Environment might just have fit the pieces together. Through simulations, modeling, and paleobotany, they uncovered how certain vascular tissue arrangements could have offered the earliest plants a survival advantage as they migrated from the comforts of their watery habitats onto dry land. One century and two years later, we have an answer.
A Balancing Act
The earliest forebears of plants today were likely a small and scraggly bunch—most fossil reconstructions give them the look of tiny, mushroom-like hairs rather than anything that remotely resembles a fern. But they faced many of the same challenges as their present-day descendants: accessing light, finding enough carbon, weathering droughts. At its heart, the struggle for survival is also about photosynthesis.
“One of the core parts of how plants work is that they exchange water for carbon,” said Jonathan Wilson, professor of environmental studies at Haverford College and an author of the study. To acquire atmospheric carbon, plants must open their stomata. These tiny pores on the undersides of leaves release precious water vapor in exchange for the carbon dioxide in their environment. What follows is usually a tightrope walk of delicate tradeoffs: keep water sources steady, and you slowly deplete your carbon reserve; open a stoma too wide or for too long, and you might die from drought.
There’s another catch: opening stomata also runs the risk of succumbing to another kind of slow, languishing death. In extremely dry surroundings, the atmosphere can pull from the plant’s exposed water reserves harder than usual. Water molecules would normally follow like a chain or rope, tugged along by hydrogen bonds. Yet in some instances, these hydrogen bonds can break to cause what Wilson explained as a ‘cavitation’: a bubble of air within the vascular tissue. Like a clogged artery, the consequences are often fatal. This ‘embolism’ blocks the xylem, which is responsible for transporting water from the roots to the leaves, leading the obstructed parts to waste and wither away. Left unsolved, it only worsens. “These gas bubbles can spread through the vascular system where connections exist, which means that the connectivity of the vascular network becomes a key feature of drought tolerance,” said Craig Brodersen, professor at the Yale School of the Environment and principal investigator of the study.
While some vascular plants—namely, trees—can either grow new xylem or dissolve the air bubble, this is not always an available option. “The tricky part about this is [that] in a lot of places, water stress-induced embolism is a limiting factor in plant growth,” Wilson said.
Piecing It Together
During their search through the early fossil records, the researchers noticed a stunning variety of patterns: there were xylem tissue cross sections that appeared like neat circular bundles, three-lobed stars, tapered lines, and, in other cases, haphazard U-shaped streaks of paint. “We see this diversity of arrangements early on in plant evolution, and then quickly that […] diversity gets kind of winnowed down a little bit,” Wilson said.
By the time nature finished dry running its designs, most surviving species seemed to have undergone a sudden spike in vascular system complexity. Xylem cells arranged in the form of narrow, curled arcs or warped asterisks had somehow taken the place of the contiguously bunched circles. Something was afoot.
Making sense of this problem required turning to a mix of math and microscopy. Some researchers including Wilson imaged fossilized plant stems from four hundred million years ago with electron microscopy. Another group simulated the evolutionary changes in the primordial xylem arrangement by incrementally adding nodes and branches to create complex, spiraling shapes that could approximate the kinds found in the fossil record. Others conducted experimental drought trials on currently existing plants to gather data for their models.
The findings teased out a surprising advantage: xylem tissue arrangements that were more structurally complex fared better under drought stress. In the narrower, thinner groupings of vascular tissue, each xylem cell was surrounded by fewer neighbors and therefore less prone to embolism. Highly lobed, intricate xylem tissues offered fewer paths through which the embolism could spread. The simulation results suggested that advantageous xylem tissue arrangements could have potentially decreased plant mortality two-fold.
A Glimpse Into The Past
The intimate association between xylem shape and drought resistance reveals telling insight about the past. The team drew upon a wide range of species for their analysis, sampling everything from lycophytes—a plant lineage that had once spawned one-hundred-foot trees in cold swamps three hundred million years ago whose modern descendants are low creepers—to everyday ferns.
Comparing the xylem shapes between past and present specimens showed an evolutionary trajectory shaped by an arc of drought resistance. Statistical analyses determined that the least drought-resistant xylem arrangements were found entirely among extinct Paleozoic species; even configurations that were fairly common among plants at the time are hardly seen today.
“These plants [worked] very, very well for their environment. But we also find that some early land plants had vascular systems that would have allowed […] relatively mild drought events to harm them,” Wilson said. Xylem cells in sampled modern-day ferns have at most three neighbors. Among their Paleozoic predecessors, that number would have hovered closer to around four or five. In other words, the study suggests that some constant, evolutionary pressure has continued to shape xylem tissue arrangement.
Species in reliably moist environments varied widely in their xylem arrangement. Only in xeric conditions—where drought is a constant, daily threat—did the researchers come across plants with consistently resilient xylem arrangements.
The findings dispel the age-old assumption of size and its inevitable complexity. The shapes of xylem tissue were not biological oddities or products of some unexplainable physiological rule of thumb. “There’s lots of different arrangements of a vascular system that could support larger plants,” Wilson said. “[But] the fact that we don’t see a uniform distribution of these strategies in plants [is] telling us that there’s an environmental selection on top of it.” The study instead suggests that there were real evolutionary advantages for having differently shaped xylems, and that xylem tissue shapes continue to be sculpted by the complex interplay of environmental factors: water supply, soil moisture, and atmospheric humidity.
The project also gives us a window into an evolutionary period where even the slimmest of advantages must have made a difference. “Nobody had really looked at [xylem tissue] from this kind of eco-physiological perspective before,” Wilson said. Plants arrived on land anywhere from five hundred to seven hundred millions of years ago, but the first vascular organisms—most of the plants we readily recognize today—wouldn’t appear until a few hundred million years later. That leap from mossy bryophytes to stemmed plants posed formidable challenges: the earliest vascular organisms would have had to develop new modes of transporting water that went against the forces of gravity. The researchers also noticed that xylem tissue diversification coincided with the Devonian period, a time in which scarce levels of atmospheric CO2 would have forced these plants to negotiate the razor-thin margins of survival even more rigorously than before. For the first vascular pioneers, the terrestrial world was an unforgiving one.
“The main takeaway is that plants developed this complex inner plumbing, and it protected them from drought, and allowed them to colonize and spread on the land surface,” Wilson said. “We wouldn’t have vegetation on land, if plants hadn’t […] figured out these particular evolutionary strategies.”
Towards The Future
The team’s findings have immediate importance. Unlocking the secrets of xylem arrangement and water uptake could allow the agriculture industry to develop plants better prepared for an increasingly erratic climate. “We believe that by understanding how the earliest plants overcame the limitations of living on land. We can also better understand how plants will respond to drought in the future,” Brodersen said.
But the sprawling, rich history of plant evolution cannot be distilled into a single study. The findings offer no more than a slice of the roughly 320,000 plant species that have since made themselves at home on this planet. Wilson expressed potential interest in comparing the water uptake processes found among their specimens of study to those of flower-bearing angiosperms, which connect their xylem cells with special structures called pits.
The researchers hope that their work offers just a start to decoding other evolutionary puzzles, too. Xylem tissue is, after all, only one trait among a vast selection of others. “I think everybody is in this collaboration is quite interested in thinking about interesting evolutionary novelties in plants,” Wilson said.
For now, though, they close the one-hundred-year-old puzzle with a four-hundred-million-year-old story. Plants effectively terraformed early Earth, but also changed themselves. They tell a story about the power to shape and be shaped, all the while tucking their heritage and history within themselves.