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What to do When the Weight Test Fails: Methods to Distinguish between Full and Empty Achenes in Echinacea angustifolia

By Marie Schaedel

Most Echinacea heads have achenes with a clear weight gap between full and empty, which is useful for lab purposes. Sometimes, however, there is no clear difference in weight for full and empty achenes. This winter, I took a closer look at achenes from Echinacea heads that had a continuous pattern of variation in weight to see if there was another way to distinguish between full and empty achenes when the weight test failed. To do this, I reweighed achenes from 20 samples that lacked an obvious weight cutoff. After plotting the new weights in R, I gathered X-ray images of the achenes closest to my best-guess cutoff weight for each sample. I found that more than half of the samples I tested had an intermediate transition weight that divided the full from empty populations of achenes. For ambiguous cases in which there is no clear difference in weight between full and empty achenes, this transition weight can be used to predict the cutoff value.

Read more:What to do when the Weight Test Fails.pdfAppendix

Figures

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X-Ray Images

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Photograph Images

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12139.1_top_red: Close-up of two achenes with a transition weight

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14142.1_top_wht: Close-up of two achenes with a transition weight

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