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trees</title></head><body>
<div><font face="Geneva" size="-1">PNAS Early Edition doi:
10.1073/pnas.1522569113</font></div>
<div><font face="Geneva" size="-1"><br></font></div>
<div><font face="Geneva" size="-1"><b>Revealing catastrophic failure
of leaf networks under stress</b></font></div>
<div><font face="Geneva" size="-1">Timothy J. Brodribb, Diane
Bienaimé, and Philippe Marmottant</font></div>
<div><font face="Geneva" size="-1"><br></font></div>
<div><font face="Geneva" size="-1"><b>Keywords</b></font></div>
<div><font face="Geneva" size="-1">embolism drought xylem vein
leaf</font></div>
<div><font face="Geneva" size="-1"><br></font></div>
<div><font face="Geneva" size="-1"><b>Significance</b><br>
Water sustains photosynthesis and growth of land plants, but it must
be transported from the soil to leaves under high tension. Drying soil
leads to an increase in water tension, exposing plants to the problem
of breakage of the water column, causing embolisms that cut off water
supply, leading to tissue death during drought. The ability of leaves
to resist embolism formation is a key adaptive axis in plant
evolution, and yet the process itself has never been visualized in the
leaf venation. We describe a new optical method that allows the
evolution and spread of embolism in the entire leaf network to be
mapped, thus revealing general rules in the sequence of leaf vein
transport failure.</font></div>
<div><font face="Geneva" size="-1"><br>
<b>Abstract</b></font></div>
<div><font face="Geneva"
size="-1"
>http://www.pnas.org/content/early/2016/04/06/1522569113.abstract</font
></div>
<div><font face="Geneva" size="-1"><br>
The intricate patterns of veins that adorn the leaves of land plants
are among the most important networks in biology. Water flows in these
leaf irrigation networks under tension and is vulnerable to
embolism-forming cavitations, which cut off water supply, ultimately
causing leaf death. Understanding the ways in which plants structure
their vein supply network to protect against embolism-induced failure
has enormous ecological and evolutionary implications, but until now
there has been no way of observing dynamic failure in natural leaf
networks. Here we use a new optical method that allows the initiation
and spread of embolism bubbles in the leaf network to be visualized.
Examining embolism-induced failure of architecturally diverse leaf
networks, we found that conservative rules described the progression
of hydraulic failure within veins. The most fundamental rule was that
within an individual venation network, susceptibility to embolism
always increased proportionally with the size of veins, and initial
nucleation always occurred in the largest vein. Beyond this general
framework, considerable diversity in the pattern of network failure
was found between species, related to differences in vein network
topology. The highest-risk network was found in a fern species, where
single events caused massive disruption to leaf water supply, whereas
safer networks in angiosperm leaves contained veins with composite
properties, allowing a staged failure of water supply. These results
reveal how the size structure of leaf venation is a critical
determinant of the spread of embolism damage to leaves during
drought.</font></div>
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<div><font face="Bookman Old Style" color="#000000">"A new area
of study is the field that some of us are beginning to call<i> social
traps.</i> The term refers to situations in society that contain traps
formally like a fish trap, where men or whole societies get themselves
started in some direction or some set of relationships that later
prove to be unpleasant or lethal and that they see no easy way to back
out of or to avoid."<br>
<br>
John Platt. Social Traps.<i> American Psychologist</i>, August
1973</font><br>
<font face="Bookman Old Style" color="#000000"></font></div>
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