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Determining Seed Set

Much of the work I have been doing up to this point has been to determine a single number for each head—the proportion of all achenes on a given head that contain a fully formed seed, or seed set. This gives a good indication of how successful that plant was in terms of reproduction. The most likely reason that an achene does not contain a seed is that the flower did not receive compatible pollen, either due to a lack of mates or due to a limitation on the part of the pollinators.

In order to determine seed set, I need two numbers: the total number of achenes and the number of achenes containing an embryo. While the achenes could be counted by hand, this would be a tedious and error-prone process. Instead, the achenes were placed on a glass tray and scanned into the computer and counted digitally.

It is possible to determine whether or not an achene contains a seed by several methods. Germination experiments are useful because every achene that germinated certainly contained a seed, but they can be time-consuming and demand lots of attention and resources. Another possibility is to weigh the achenes. Heavier achenes are much more likely to contain a seed, and lighter achenes are most likely empty. We chose to use x-ray, which allows us to see directly inside of each achene. When achenes are x-rayed, empty achenes are barely visible while seeds show up as opaque. Ideally, all achenes could be easily categorized into “empty” or “full,” but some achenes are partially full, likely meaning they were initially fertilized but full seed growth was not entirely successful.

Together, these numbers are very important in allowing us to make inferences about what conditions are best for Echinacea reproduction.

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Randomized achenes ready to be x-rayed.

 

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This is what x-rayed achenes look like. Achenes that contain a seed show up with a white oval in the center.

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Full, partial, and empty achenes are counted on the computer and entered into an spreadsheet.

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