Science answers a century-old mystery in botany

Our green world would not have been possible
without the hidden changes in plant bodies over the past 400 million years.
Rowing distances of more than a few centimeters in the wettest places on land, plants must rearrange their water transport tissues to keep them away from drought
.
A new study by Martin Buda of the Institute of Botany of the Czech Academy of Sciences, published in the Wall Street Journal of Science, shows how a century-old solution to a century-old debate in botany sheds light on key adaptations
for plant colonization in arid lands.

Background: All but the smallest plants require vascular tissue to provide moisture to the whole body and avoid drying out by absorbing carbon from the surrounding air
.
If a plant suffers from drought, the chain of water molecules pulled to the stem breaks to form emboli: a bubble-like gas that blocks the transport
of water throughout the vascular ducts.
If the embolism spreads from this duct to the entire tissue, the plant's water supply vessels will be effectively blocked, and the plant will dry up and die
.

This suggests that the original arrangement of vascular tissue — the cylinder in the center of the stem — becomes increasingly susceptible to embolism that spreads with size
.
"If all the ducts are entangled, the plant may face an exponential spread
of embolism on the resulting vascular network.
If they are strung together in elongated shapes, the embolism would have to cross many successive cell walls to travel very far, which could save plant lives during drought," said Dr.
Buda, lead author of the study

The first vascular plants were only a few centimeters tall and could only live where
there was water.
In order to grow taller and start exploring the land, they must first find a way
to replace the arrangement of blood vessels of their ancestors.
"To our surprise, very few plants were able to maintain the original layout of the stem, where vascular tissue was placed in a cylinder in
the center.
This superficial detail is actually the key to deciphering the entire evolutionary process," Bouda added
.

The fossil record shows that stems are combined in increasingly diverse ways, just as
plants radiate outward from a water source.
Vascular tissue is arranged in a variety of shapes from ellipses to stellar rings – divergent in form and convergent in function
.
Plant lines that succeed on land must find their own solutions
to the embolization problem.
The intensity of this evolutionary pressure increases
with the size of the plant.

This study solves a century-old problem
in botany.
In larger plants, vascular tissue is increasingly complex in shape, a discovery first discovered by F.
O.
Bauer (president of the Royal Society of Edinburgh) and his student C.
W.
Wardlaw
.
Bauer presented their findings in his opening address at the 1920 meeting of the Society, but could not explain the findings
.
A century of debate has culminated in an uncomfortable consensus that the complexity of xylem arrangement only happens to increase
as plants grow and branch.
New research shows that plants maintain drought-resistant vascular alignment
by limiting the width of tissues.
As the volume increases, the tissue must take on elongated, narrow, and increasingly complex shapes, which provides the answer
to Bauer and Waldlaw's puzzle.

To evaluate their hypothesis, the team of scientists sampled the xylem of living and extinct seedless vascular plants that evolved over a period of more than 400 million years
.
They examined the arrangement of conduction cells of different vascular bundle shapes and analyzed the topology of the resulting ductal network
.
Numerical simulations of how drought-induced embolism spreads to lethal through the vascular network of real and ideal plants confirms that hydraulic damage should choose narrower, increasingly complex shapes
.
"By developing new methods to quantify how the topology of the catheter network affects embolic diffusion, and applying these methods to the early fossil record and living plants, we were finally able to ask this question in the right way, Dr.
Buda concluded
.
"

This fundamental advance includes the potential to
ensure drought resistance in crop breeding programmes to combat climate change.
"Now that we have a better understanding of how vascularity fits together and how it affects a plant's drought tolerance, that's what can be targeted for a breeding programme," Professor Brodson said
.

Follow-up research will explore how plants can circumvent the limitations of the newly discovered findings to achieve woody growth forms
.