Geranium petal: unexpected structure of cell boundaries

[rjlittlefield]

For no particular reason, I looked at the petal of a geranium flower through a microscope.

What I saw was a big surprise. I was expecting simple polygonal cells with smooth boundaries.

But what I actually saw (see HERE) was a collection of small "fingers" around the edge of each cell.

Then of course I got curious. Are those "fingers" totally internal to the cells, or do they also show as part of the surface structure?

So I made a cast of the petal surface, using the simple Bondic method described HERE.

The answer is yes, the "fingers" do show on the surface also. Here is a photo of the cast, no actual petal remaining.




Here is an actual petal -- not the same one but at the same scale:




Here are closer views.






Colors that appear in the cast image seem to be due to prismatic effects from the Bondic cast. Each cell has a roundish top that forms a strongly concave lens in the cast, and around the edges of those lenses there is a ring of strongly colored darkness. I presume that these are areas where most of the light gets directed away from the objective used for viewing, with a disproportionate amount of long wavelengths being bent not quite as much, so more of that gets through. The rings shift with focus and get broadened in the focus stacking process. In addition the colors are made more vivid here because of curves adjustments made to increase the contrast and show the structures better.

Shot with Mitutoyo M Plan Apo 20X objective on Raynox DCR-150 tube lens, on Canon T1i APS-C sensor. Roughly 100 frames per stack, 0.002 mm focus step. Cropped here to 0.7 mm field width for the wider views.

Illumination is by a single flash diffused by paper behind the specimen. The pink petal is illuminated broadly and symmetrically by placing the flash centered several inches behind the paper. The clear cast is illuminated more directionally and oblique, by placing the flash close to the paper and off-center.

--Rik

[Edit: to rearrange photos for impact]

[rjlittlefield]

Here are some other blossoms from the same plant:



This image is an actual-pixels crop from a stack shot in 2015 that I periodically use for presentations. It was shot using a CamRanger setup as discussed in http://zerenesystems.com/presentations/ ... 181110.pdf , starting at slide 34.

--Rik

[rjlittlefield]

Colors that appear in the cast image seem to be due to prismatic effects from the Bondic cast. Each cell has a roundish top that forms a strongly concave lens in the cast, and around the edges of those lenses there is a ring of strongly colored darkness. I presume that these are areas where most of the light gets directed away from the objective used for viewing, with a disproportionate amount of long wavelengths being bent not quite as much, so more of that gets through. The rings shift with focus and get broadened in the focus stacking process. In addition the colors are made more vivid here because of curves adjustments made to increase the contrast and show the structures better.

To illustrate that description, here is a short sequence of frames, every 4th frame as shot (so, spaced by 0.008 mm focus position), cropped to an interesting region.




In addition to the colored rings, which expand in a rather simple way as focus changes, more complicated portions of the structure strongly exhibit the "squirming around" effect that I mention from time to time. Here is a two-frame animation for illustration. Note the fine structures on the left sides of the big "hill" and the one above right of it. These structures look for all the world like they are dramatically changing shape. But in the physical world, nothing changes between these two photos except for 0.008 mm focus position.




Surely this effect deserves a long and detailed technical discussion --- which I will be happy to write as soon as I figure out how to do that.

--Rik

[simplejoy]

Fascinating and beautiful details! Well spotted and captured. I've never even tried getting as close to the structure of flowers, but I can imagine it being quite challenging.