Anyway, I popped the spent flowers off the tips of the stems, and stored them overnight in a cabinet, hoping they might dry out a bit. They didn't, but I decided to harvest the seeds from them anyway.
Here is a picture of the spent flowers:
Here's what the mature seeds inside looked like, once they were separated from the chaff:
The term chaff is a botanical term meaning "all the useless bits." When I first opened the flower head, the seeds were still connected to the chaff. I needed to separate them, so they could dry out and be stored without getting moldy. When I first separated the seeds, they were waxy and a bit slimy. They tended to stick together, to each other, to the chaff, to my fingers, to the plate.... They looked like this:
I just fanned them out and picked them apart. Some flower heads had a lot of seeds, some had only a few. Sometimes they were all around the outside edge of the flower, sometimes they were in a cluster in the center. It was a treasure hunt.
Sometimes there were what I think of as "fake me out" seeds. They look sort of like seeds, but not as big, and they didn't have that thicker part on one end.
These seeds are from the pretty red and white Dahlia, called 'Harlequin', that is pictured at the top of the page with my title. I also saved seeds from a yellow and pink one, a red one, and a streaky red, pink and white one. I want to grow loads of these next year in my new garden.