Studying Leaves With Physics And Fluorescent Dye

Leaves have an intricate web of veins that transport nutrients and water and provide structural support. But what determines the pattern of venation? Physicists Marcelo Magnasco and Eleni Katifori investigated this question using sophisticated algorithms and a little glow-in-the-dark dye.

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IRA FLATOW, host:

I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.

And right now, Flora is here, hi. Flora Lichtman is here with our Video Pick of the Week. Hi, Flora.

FLORA LICHTMAN: Hi, Ira.

FLATOW: What have we got this week?

LICHTMAN: This one sells itself. Okay, let me just set it up for you.

FLATOW: Sells itself - just like a big squid.

LICHTMAN: Yeah, exactly. It's the equivalent of a big squid for us. Fluorescent dye in the veins of leaves.

FLATOW: Whoa.

LICHTMAN: Just imagine that, and then you can go to the site and watch researchers...

FLATOW: Wow.

LICHTMAN: ...sort of inject fluorescent dye into all these different leaves. And what it shows is the vasculature, the venation of the leaves underneath their surface. It's beautiful.

FLATOW: It's - yeah, I was going to say that. I saw the videos on sciencefriday.com, our Video Pick of the Week up there in the left corner. And they watch the, you know - reminded me of a really fancy, remember the old celery in the water...

LICHTMAN: Yes, it is that.

FLATOW: ...experiment and the colored water experiment.

LICHTMAN: Actually, I should say, the researchers - there were two physicists, Marcelo Magnasco and Eleni Katifori at Rockefeller said, you know, if you're a - if you, like, have a high school education in biology, you could easily recreate this experiment yourself with something like celery or other plants.

FLATOW: Yeah, but they've done it with some gorgeous leaves. And what have they discovered about why the leaves do what they do with this stuff?

LICHTMAN: Well, they're interested in - from a sort of physics/math perspective of this pattern, the pattern of veins beneath the leaf. And the premise of their work is sort of that evolution has molded this geometry, and that this geometry is sort of the best solution or the optimal answer to the constraints on the leaf. And so what they're trying to figure out is what constraints create the sort of beautiful pattern in the leaf.

FLATOW: Why a leaf would choose to make that kind of pattern over another?

LICHTMAN: Right. Why go left her and then right, and specifically they looked at lemon leaves. And lemon leaves have this sort of beautiful looping structure. So the point is that it's not just an efficient tree where you have a direct path to one point on the leaf, you have lots of different paths. And the, you know, and - under the rules of the economy of scale, that's not the most efficient way, so why do we have these loops in the veins. And they think that you can sort of look at this from a mathematical perspective to understand.

FLATOW: Mm-hmm. And they found, from looking at your video, that the straight line is not the best - it's not the healthiest for a leaf to have the veins go in a straight line.

LICHTMAN: Right. And you can imagine, if you have damage to one of those veins, which we see in this video, you want other paths to get to the tip of the leaf. I mean, if you only had one path and then you damaged it, the leaf would be in trouble.

FLATOW: Mm-hmm.

LICHTMAN: And if you - and what the video shows is exactly all these sort of looping interconnected paths.

FLATOW: And they're - you have different kids of leaves from different plants.

LICHTMAN: Yes, lilies and roses, just for Valentine's Day. Happy Valentine's Day.

FLATOW: I see roses on there. I saw that one.

LICHTMAN: Yes.

FLATOW: And lilies and roses, and there was...

LICHTMAN: And lemons.

FLATOW: Lemons.

LICHTMAN: Oh, and one more which is sort of the interesting exception, which is that ginkgos, these - which are really ancient plants - dont have this looping structure. So, you know, you can survive without it, apparently.

FLATOW: So but - perhaps this was, in evolutionary speaking, a better leaf pattern, right...

LICHTMAN: That...

FLATOW: ...to go - go in a circle than go on a straight line.

LICHTMAN: It seems like that could be true. And, you know, they are physicists so they're hesitant to make any biological claims. But, that may be.

FLATOW: It's cool. It's gorgeous.

LICHTMAN: Thanks.

FLATOW: And if you want to...

LICHTMAN: Not to me, to them.

(Soundbite of laughter)

FLATOW: So - this is our SCIENCE FRIDAY Video Pick of the Week. It's Flora's video, and she made a gorgeous one over there. We've done that. Where do these leaves happen? Who's doing these experiments?

LICHTMAN: Oh, it's at Rockefeller.

FLATOW: It's Rockefeller University here in New York.

LICHTMAN: Yes.

FLATOW: And you can see them as our Video Pick of the Week up there on sciencefriday.com, up there in the left corner. Thank you, Flora.

LICHTMAN: Thanks, Ira.

FLATOW: Flora Lichtman, our video producer.

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