the effect of mounting media on size

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iconoclastica
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the effect of mounting media on size

Post by iconoclastica »

Fern spores are frequently measured to distinguish between closely related species that are hard to tell apart otherwise. It has been known for quite some years that the average length of the spores may vary up to 10% attributed to the mounting medium. I only know about this effect from the scarce references in fern-related literature, but I expect it might be a well-known phenomenon. Is it the refractive index of the medium that causes the size-difference? Is it predictable if you know the RI?

For a test, I measured two samples of spores, coming from the same fern, one dry and one mounted in glycerine (n=44 and 60). The latter averaged 8% longer, highly significant. Measured by imageJ's particle analysis.
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Lou Jost
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Post by Lou Jost »

My first guess would be that mounting a dry spore or seed in a liquid medium is going to make it expand. That's the spore's whole purpose in life. There are probably all kinds of biophysical adaptations to get liquids into the spore from the outside.

iconoclastica
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Post by iconoclastica »

An interesting thought. It should be a physical effect then, for it happens too with old spores from herbariums in poor nourishing media such as KOH. A swelling of 25% by volume within minutes should be observable.

I'll see what happens if I can moisten the spores without having them actually swimming in fluid.
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Lou Jost
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Post by Lou Jost »

Yes, this should be easy to test.

iconoclastica
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Post by iconoclastica »

I have tried a few. Storing the slide with spores in a warm, humid enclosure had no effect whatsoever. Overflowing it with IPA resulted in an 4% increase after evaporation. So, not an optical effect. Hm.
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Lou Jost
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Post by Lou Jost »

This might also depend on the relative humidity of the environment where the dried spores are stored. If they are kept very dry, like 30-40% RH, they might show measurable expansion when placed in a hot humid environment with 100% RH.

iconoclastica
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Post by iconoclastica »

So it seems that I have to do the measurements myself, and to be useable by others (amateurs), I should mount them in water.

I am writing now the chapter on Polypodium for the new Dutch flora. I find it pretty amazing that hardly any author who published spore sizes for these species took the trouble to mention the mounting medium. Even though it has been known since at least 1933 that it has a marked effect.
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Lou Jost
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Post by Lou Jost »

Good for you for questioning this. It would have been easy to just assume everything is ok since all the experts do it.

Biology is full of nonsense, even in the most advanced and most quantitative sub-fields, like population genetics and mathematical ecology. Even quite straightforward things are sometimes completely wrong, for decades at a time, with no one noticing.

The best example is how people measure the genetic differentiation between species or populations. This is a fundamental thing with huge practical and theoretical impacts. Yet the most popular measures (Fst and Gst) can be nearly 100% wrong. That is, two species or populations that were completely different at a given locus--sharing no alleles-- could show almost zero genetic differentiation at that locus, according to the standard measures. See this article I wrote about that problem if you are interested:
https://onlinelibrary.wiley.com/doi/ful ... 08.03887.x

iconoclastica
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Post by iconoclastica »

See this article I wrote about that problem if you are interested
You bet I am! Have some unfinished history there... Thanks!!

Wim
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Lou Jost
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Post by Lou Jost »

The paper is behind a paywall. I can send you the pdf if you send me a PM with your address.

iconoclastica
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Post by iconoclastica »

Thanks, but no need. I made a tiny hole in the wall once that's still open.
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