Answer: The mystery of the grape leaf polygonally changing color

 Fascinating... 

AS YOU RECALL.. 

I was emailing with my artist friend Lynne Garell who captured a set of photos that are striking. I've never seen any leaves change color quite like this: 

A grape leaf closeup. P/C Lynne Garell, from her series of grape leaf images.  

 How does this happen? How can a polygonal piece of a leaf all change at once?  Is this the way all leaves change?  What's going on here? 

Another image from her series: 

What's going on here isn't at all obvious... 

Here are the Challenges I set out last week.  

To begin with, I really had no ideas, but I realized that I needed some basic terminology.  So I dealt with Challenge 2 first... 

2. To answer this question for myself, I thought I'd first find out what each of those polygonal sections is called, and then search on that term.  But I couldn't find it!  So, a Challenge for you:  What is that part of the leaf called?  Here's an image that I made to illustrate the question: 

That thing...  what's it called?  These polygons, like the ones above in Lynne's photo, would seem to be fairly obvious structural features of a leaf.  But I can't figure out what they're called.  Can you?  (I realize that up close this looks like a giraffe's skin, but I assure you, this is a closeup of a leaf.)  

I started with an easy search:  

     [ leaf veins ] 

and just read around the results for a bit.  The first hit is to a comprehensive (and fascinating) summary of what leaf veins are and how they're organized.  (Leaf venation: structure, function, development, evolution, ecology and applications in the past, present and future.  Sack, L., & Scoffoni, C. (2013). New Phytologist, 198(4), 983-1000.

Really interesting, but while I learned a lot about venation structures (first-order, second-order, arrangement of xylem and phloem conduits within veins!), I did not learn what the spaces in-between the veins are called.  

But I DID learn that grapes are dicots, which have a reticulate venation pattern (that is, it's a web-like structure between the veins), and that "veins transport water throughout the lamina mesophyll."  

That is, the leaf (as a whole) is the lamina, while the veins are made up of xylem and phloem cells surrounded by bundles of sheath cells. The vein xylem transports water from the base of the leaf throughout the lamina mesophyll, and the phloem transports sugars out of the leaf to the rest of the plant.  Right? Water comes in through the xylem; nutrients go out via the phloem. 

Generically, the stuff in the middle is the mesophyll, but that's too generic a term. What's a more specific term for those polygonal structures we see? 

I went back to the SERP and found the Wikipedia article on Leaf, wherein I found this: 

"The areas or islands of mesophyll lying between the higher order veins, are called areoles. Some of the smallest veins (veinlets) may have their endings in the areoles, a process known as areolation..."  

Now we've got a specific term: areole.  Let's use that in our search: 

     [ leaf areole ] 

Which gives the satisfying image search result of: 

That image on the far right is from a text about fern leaf architecture, and is basically the same drawing I did above!  

On to the next Challenge! 

1. What is this effect called?   (That is, the polygons changing color independently of the surrounding leaf to create this kind of pattern.)  

This has turned out to be pretty hard. 

I spent a long time trying various queries, looking at lots of image results of grape leaves... and finding results like this, nice, but not polygonal: 

What's more, I was NOT able to find grape leaves that change color areole-by-areole.  As you can see from the images above, generally speaking, grapes change color on a fine-grain size, not by the entire areole changing color at once.  Even the top image in the above set isn't quantized--if you look carefully, you'll see that the areoles change color across a gradient--an entire areole doesn't change all at once.  

I was so surprised by this that I went out and picked up a few grape leaves from the neighborhood (with permission!) for a closer look.  Here are a few of those images at increasing magnification:  

  

In the bottom image you can see the individual cells in the mesophyll (technically, the palisade cells).  The bright out-of-focus lines are the veins.  

And, as you can see, not all of the cells in the areole are of the same color--there's a gradient across the areole from brown (on the far left), through red, to green just before the cells hit the vein.  

So the big Challenge boils down to this: Why do these areoles all change color simultaneously?  

I have to admit that I struck out on this, despite searching extensively on Scholar, News, and Images.  These polygonal red patches are fairly rare!  

Part of the clue might be in the structure of the color changes in Garell's photos.  Here I've annotated a couple of her leaves with purple lines to show that the areoles that change color are the ones farthest from the primary veins. 

What causes leaves to change color in the first place?  As you know, in the fall, because of changes in the length of daylight and changes in temperature, the leaves stop their food-making process. The chlorophyll breaks down, the green color disappears, and the yellow to orange colors become visible. The amount and kind of color depends on the carotenes and xanthophyll pigments in the leaf, along with development of red anthocyanin pigments.  

So, in Garell's leaves, the areoles that are most distant from the veins are the ones that change color first. 

While I wasn't able to find a documented explanation for the polygonal areole color changes, I'd be willing to bet that these particular grape leaves had some kind of environmental or biological stress (e.g., a bacterial infection) that let the areoles that are least connected to the main veins lose their chlorophyll first.  Whatever the stressor was, it affected the entire areole, rather than just a part.  Was it a hard freeze during a cold snap?  Or what it something else?  

At this point, we don't know the deep reasons, but just have a suggestion that these particularly lovely images capture a fairly rare color event in the French fall.  Most grape leaves in France don't seem to have these beautiful polygons, but look a bit more generic.  Here's a photo from near Lyon--nice, but no polygons.  

Many thanks to Lynne Garell for letting me use (and annotate) some of beautiful images.  If you liked these images, please check out Lynne's Etsy site for her photography.  It's all beautiful.  

SearchResearch Lessons

Well... there are at least two here, perhaps more... 

1. You won't always succeed.  I'm pretty sure there's an answer out there, somewhere, but I wasn't able to find it this week, despite multiple hours on the task.  It was fun, but I didn't find an answer.  But I WILL keep my eyes open! 

2. Finding a specific term (e.g., areole) can help target your search much better.  And yes, I'll eat my words if someone manages to find the answer to this Challenge and doesn't use areole somewhere in their search process.  

I'll keep looking and will give an update if I learn more about the geometry of color-changing French grape leaves. 

Laissez les bons temps rouler!  

 (I realize this is Cajun French, but it seems appropriate...)  

Search on!