Culturing Science – biology as relevant to us earthly beings

Posts Tagged ‘leaf veins

Nature-inspired network design: recent studies in slime mold and leaf veins

This post was chosen as an Editor's Selection for Nature-inspired design:  this phrase makes me think of shark-skin swimsuits, velcro, and an endless assortment of coffee tables using natural knots and tree branches.  There is logic behind design reliant upon natural elements.  After all, organisms have been undergoing evolution for millions of years for the sake of efficiency.  If anyone knows how to cut through the water cleanly, it would be a shark, wouldn’t it?

When thinking about designing human networks, it thus seems “natural” to turn to nature, which certainly has expertise in the subject.  Every organism has to have a way of transporting nutrients and water around its body, systems which become more complicated as the organisms do.  Think about your circulatory system: all those veins and arteries and capillaries seemingly spread through your skin and innards at random.  But this network of transport tubes has gone through great evolution in order to be so intricate that it seems random to us.

Three recent papers have highlighted the use of organisms to help plan civilized networks: that is, highways and train systems.  The first two papers feature one of my father’s favorite organisms, the slime mold, while the third looks at leaf vein systems.  (For my post on leaf veins as drivers of plant evolution, click here.)

Slime molds are plasmodial protists — their cellular structure is undefinable.  While a slime mold can live as a unicellular and uninuclear organism, if it runs into another of its own species, the two will join by their cellular membranes.  If there are many around, the organism can potentially become a huge amorphous sac with many nuclei, spreading over a surface foraging for food.  (The question is: is it the singular cell membrane or the singular nucleus that makes an organism “unicellular?”)  Here’s a quick video by John Bonner, a Princeton slime mold specialist, showing the beauty of this unfortunately-named creature:

Edit: A commenter pointed out that John Bonner’s “slime molds” are different than the ones used in the research discussed below.  The slime molds in this video, Dictyostelium discoideum, do in fact form a multicellular organism and don’t exhibit the same sort of networking behavior.  The video is still worth watching, but be aware of these differences!  (Thanks, Iain and class!)

When foraging, the blob spreads over a surface looking for sugar.  Once food is located, it will redirect most of its mass elsewhere, leaving a vein behind leading to the food source.  After millions of years of evolution, one would expect that this vein would be the most efficient path between the two points.

Independently, two groups of researchers took advantage of this assumption to use slime mold to compare its foraging network between food sources to our networks connecting cities.  (For the record: one of the papers was published in the International Journal of Bifurcation and Chaos, which is an obsession-worthy title.)  Each team laid out oat flakes simulating the layout of major cities (one oat flake each!) in their region of choice – the UK and Tokyo metro areas.  The question: will the slime mold trails emulate our own systems, thought out by our grand human brains for efficiency?

Slime mold networks compared to actual city planning. Image maniputed; see papers for original figures

The answer: for the most part, yes!  As you can see in the above figures, the authors of each paper saw very similar networks to our own.  However, the slime mold lacked a more circular structure, connecting the outer hotspots to one another.  And why would it have these:  due to its impermanent nature, a slime mold does not need the forethought to create multiple connections to the same spot.  If one is broken, they can simply create a new line leading straight through.

If we’re trying to use these organisms to help us plan transportation networks, obviously this is not a perfect fit, as weather and technical problems cause blocked lines frequently.  We do need this sort of forethought when laying out our cities!  But we are not the only ones.  A paper published last week in Physical Review Letters (open access here) looked at branching patterns in leaf venation.

Just like in our own circulatory system, the network of veins bringing nutrients and water throughout a leaf cannot be simply retracted and reformed, but is a permanent structure.  With herbivores knawing holes all the time, you betcha that plants have evolved “looping networks,” as the authors put it, to ensure that a blockage in one pathway doesn’t deprive the entire leaf of nutrients.  Traditionally modelling has shown these looping networks to be inefficient compared to non-circular, tree-like networks.  Do you think these authors believed that?

As support for their premise, they punched a hole through the central vein of a lemon leaf (see image above) and injected fluorescent dye below.  And behold!  The dye was able to spread throughout the leaf despite this disruption.  They then created a model that incorporated the network’s (a) resilience to damage and (b) variation in load.  Separately and together, the models agreed: looping networks are actually more efficient in the long run.

What do these studies put together show us?  That neither alone is good enough.  We need our slime-mold, treelike networks for basic structure, with some excess leaf-vein looping for support during damage control and tourist influx.  (You never know when you’ll host the Olympics.)  The circularity might seem excessive, but when bad times hit, it will be worth it.  (The citizens will appreciate it too.)

This may seem like second nature to many of you.  But sometimes it’s nice to get some support from “first nature,” amiright?

(Endnote:  I really am curious about the grammar for slime mold.  Are slime mold(s) awesome or is a slime mold awesome?  Guesses/answers in the comments, if you are so inclined.)

Andrew Adamatzky, & Jeff Jones (2009). Road planning with slime mould: If Physarum built motorways it would
route M6/M74 through Newcastle International Journal of Bifurcation and Chaos arXiv: 0912.3967v1

Katifori, E., Szöllősi, G., & Magnasco, M. (2010). Damage and Fluctuations Induce Loops in Optimal Transport Networks Physical Review Letters, 104 (4) DOI: 10.1103/PhysRevLett.104.048704

Tero A, Takagi S, Saigusa T, Ito K, Bebber DP, Fricker MD, Yumiki K, Kobayashi R, & Nakagaki T (2010). Rules for biologically inspired adaptive network design. Science (New York, N.Y.), 327 (5964), 439-42 PMID: 20093467

Written by Hanner

February 11, 2010 at 5:55 am

Leaf veins as the driving factor in angiosperm evolution (Brodribb and Feild 2009, Ecology Letters)

ResearchBlogging.orgThe news would have us believe that we are living on earth at a time of high carbon dioxide levels.  And it’s true — carbon dioxide levels have been rising since the industrial age, quickly enough to suggest that the effect is human-derived and we may be about to destroy this world for ourselves.

Image by Mark Boucher

But life has existed at much, much higher carbon dioxide levels.  Right now, carbon dioxide is about 400 ppm (parts per million) in the atmosphere; 60 million years ago, it was at 3500 ppm.  There is fossil evidence for a loss of atmospheric carbon dioxide through the Cretaceous period (145-65 million years ago), and scientists have long suspected that the evolution of the angiosperms, or flowering plants,  could provide more evidence.  The word “angiosperm” derives from Greek, meaning “enclosed seed” referring to the flesh surrounding their seeds, i.e. they produce fruit.  The first fossil evidence for angiosperms is pollen from around 130-140 mya (million years ago), and they diversified rapidly in the mid-Cretaceous period, about 100 mya.  As you can see from the below figure, spanning time through the entire Cretaceous period, angiosperms really took off, increasing in number and dominating other plant types.  (Pteridophytes are ferns; Conifers are cone-bearers such as pines; Cycads are something else!)  Their complete take-over is evident at 80 mya.

(Figure from JC McElwain, KJ Willis, and R Lupia's chapter "Cretaceous CO2 decline and the Radiation and Diversification of Angiosperms" from the book A History of Atmospheric CO2 and its effects on Plants, Animals and Ecosystems by TE Cerling and MD Dearing. Page 135.)

Angiosperms have an advantage because they are more productive — that is, they go through photosynthesis more efficiently, allowing them to grow faster and more effectively compete for resources.  There are many aspects of their biology that could affect this rate: greater food and nutrient import by their roots, or greater water flow through the plant, greater nutrient flow, greater ability to absorb carbon dioxide.  The question is: which one of these features was the causal change that allowed angiosperms to take over the world?

Drs. Brodribb and Feild believe they have found the answer: leaf veins, as published in the early edition of Ecology Letters (doi: 10.1111/j.1461-0248.2009.01410.x).  They argue that leaf veins are a proxy for greater photosynthetic capacity, via greater gas exchange capacity. While the veins in a leaf typically carry water, each vein opens to the air through a hole in the leaf (stoma, pl. stomata), through which both water and carbon dioxide enter.  In a world of decreasing carbon dioxide, the leaf would need to keep their stomata open longer or develop more of them in order to absorb enough carbon dioxide from the air to maintain their growth rate.  However, the longer you keep your stomata open, the more water you lose. In order to keep them open to absorb carbon dioxide but also maintain hydration, the authors argue that the plants evolved more complicated leaf venation systems, to deliver water throughout the leaf more efficiently.  Thus we can use the density of these veins to track the evolution of carbon dioxide uptake of angiosperms.

The authors looked at 759 different plant species total: 504 living angiosperms, 89 extinct non-angiosperms, and 166 living non-angiosperms (for comparison).   They modeled the phylogeny (evolutionary tree) of the samples from two well-supported “backbone topologies,” which they consider to be conservative estimates of plant evolution.  They saw a pattern of increased vein density as each of these plants species evolved, but no change in non-angiosperms, demonstrating angiosperm-specific evolution.   The authors then assigned photosynthetic rates to 35 species for which they had evolutionary data using modern leaf samples.  These data showed that, under today’s atmospheric conditions, the plants which evolved later had higher photosynthetic capacities, suggesting  directional evolution towards greater photosynthesis in an atmosphere with lower carbon dioxide.  The non-angiosperm data once again showed no change, demonstrating that they did not go through a similar evolutionary revolution, which perhaps let the angiosperms take over.

Often, in order for a great evolutionary leap, an organism needs some sort of pressure, or hard-times.  For an organism like a plant that consumes carbon dioxide and sunlight, life 100 mya would have been great times — too much carbon dioxide and sun, and a warm climate, removing the necessity for any sort of winter-time adaptations.  Plenty of resources to go around.  Even low-efficiency parts were able to keep the plants alive with ease, so there was no need to evolve more sophisticated photosynthetic machinery.

My question: would a gradual change in carbon dioxide result in the drastic evolutionary changes which Brodribb and Feild have shown?  I looked into the carbon dioxide record for the Cretaceous period and discovered a potential cause for these changes: oceanic anoxic events. Oceanic anoxic events (OAE) were periods of great climatic upheaval, caused by overproduction.  In a hot, carbon dioxide-rich environment, plants went through periods of drastic growth, accumulating organic carbon which then was unable to decompose.  In effect, this sequestered large amounts of carbon, decreasing the amount of carbon dioxide in the atmosphere.

The most severe OAE I know of, and thus a good example,  is the  Azolla Event (Brinkhuis et al. 2006, doi:10.1038/nature04692).  About 50 million years ago, there was a dramatic drop in carbon dioxide levels from 3500 ppm to below 1000 ppm.  (See Pearson 2000 for carbon dioxide reconstructions for 60 million years ago. doi:10.1038/35021000)  In the Arctic Ocean during this time, there were huge blooms of the aquatic fern, Azolla, which absorbed incredible amounts of carbon dioxide from the atmosphere during photosynthesis and sequestered it on the bottom of the ocean when it died and sank.  (Anoxic soil conditions kept the plant matter from decomposing and re-releasing the trapped carbon, instead fossilizing, giving us evidence that this event occurred at all.)  This event was a major cause of the drop in carbon dioxide and temperature during and since the Eocene.

Smaller versions of the Azolla Event occurred throughout the Cretaceous.  These events lasted less than a million years – just a drop of time in the large scale, but the loss of carbon dioxide for a million years can create great evolutionary pressure on organims.  Below is a figure showing three different reconstructions of carbon dioxide levels during the Cretaceous period, the scale in relation to current carbon dioxide concentrations.  The solid line (Berner 1994, American Journal of Science, link) and dashed line (Berner and Kothalva 2001, doi:10.2475/ajs.301.2.182) show similar, downward trends in carbon dioxide levels throughout the Cretaceous.  I’m more interested in the dotted line (Tajika 1999, doi:10.1046/j.1440-1738.1999.00238.x), which takes into account these OAE, evidenced by high-carbon black shale.  During each of the estimated times for these OAE, there is a drastic drop in carbon dioxide levels, after which it rises back up to similar levels as previously, while the total trend is downward.

(Figure from JC McElwain, KJ Willis, and R Lupia's chapter "Cretaceous CO2 decline and the Radiation and Diversification of Angiosperms" from the book A History of Atmospheric CO2 and its effects on Plants, Animals and Ecosystems by TE Cerling and MD Dearing. Page 139)

The estimated times for oceanic anoxic events align well with estimated times for angiosperm diversification and expansion.  The first OAE, around 115 mya, immediately preceded angiosperm diversification, while the second and third precede clear angiosperm dominance, as seen in the first figure.

These oceanic anoxic events, with their severe but temporary drops in atmospheric carbon dioxide, would provide pressure to cause swift evolution in angiosperms toward a higher photosynthetic rate, as Brodribb and Feild showed in their paper.  The alternation during the Cretaceous period between these carbon dioxide droughts and high levels of carbon dioxide from volcanic activity stemmed from tectonic movement from the breakup of Pangea, would give plants cause to evolve these new mechanisms, but also allow them to reap the benefits during times of carbon dioxide wealth and spread rapidly.

To me, Brodribb and Feild’s paper doesn’t suggest that the change in leaf vein density caused the evolution and spread of angiosperms.  This incredible evolution was simply a reaction to the changing carbon dioxide in the environment.  This adaptation allowed them to grow and diversify much more efficiently than non-angiosperms, but I would be surprised if it were the only one.  I’d be interested to know how other carbon dioxide-related mechanisms in plants evolved over time, and if they were also developed to cope with changing atmospheric conditions.

Besides being fascinating, studying evolution in relation to climate change is pertinent right now.  If we can learn more about how plants and other organisms have reacted to changing atmospheric conditions in the past, we could have more information to predict how they will involve in the face of human-induced climate change.  A lot of science can be incredibly forward-thinking, which is why some scientists blow-off detailed studies of evolution.  But biology is all patterns; the more we can learn about its history, the more tools we have to study modern biology and predict the future.

Leaf vein image by Mark Boucher, Flickr

Graph figures from JC McElwain, KJ Willis, and R Lupia’s chapter “Cretaceous CO2 decline and the Radiation and Diversification of Angiosperms” from the book A History of Atmospheric CO2 and its effects on Plants, Animals and Ecosystems by TE Cerling and MD Dearing, available here on Google Books

Brodribb, T., & Feild, T. (2009). Leaf hydraulic evolution led a surge in leaf photosynthetic capacity during early angiosperm diversification Ecology Letters DOI: 10.1111/j.1461-0248.2009.01410.x

Written by Hanner

December 6, 2009 at 3:20 pm

Posted in Journal Article

Tagged with , , ,