As a drooling young rock gardener, I was told time and time again by the old timers that Acantholimon seed was always 95% empty, infertile seed, for some mysterious reason. I even saw a botanical illustration once that pointed to the dried out flower and called it the seed. Everyone has a story about it: never growing from seed they got from exchanges, only growing from certain czech seed hunters' seed, et cetera. And everyone had a different explanation for why they were rare from seed, leaving most to be grown from cuttings. And yet you'd see the odd seed volunteer plant appearing here and there in folks' gardens.
(Soft pink flowers of Acantholimon armenum are gracefully replaced by similar white clayx which are generously full of lies and disappointment. None of these have been fertile, ever.)
Fifteen years later:
I've beat my head into this wall for long enough to share this:
Some plants produce mostly good seed, others none. Most produce little. I expect it is a pollination thing- is there a bug with the right sized lips or tongue or whatever to do the deed? When I looked into it, I found just one paper in turkish about sweat bees pollinating them, but at APEX, Acantholimon halophilum is covered with mostly honeybees and a few others. Those plants usually produce loads of good seed, last year was an exception. Also, perhaps importantly, it may be necessary for there to be more than one lonesome-george plant so they can pollinate one another. This is conjecture.
Whatever the case, the good news is this: you can check and see if seed is viable- with a plump embryo and endosperm. You need a pudgy parachute. Rumour and oral history says a czech seed hunter used the term "pregnant ballerinas" to describe the look of a good seed. It's just visible. But even better, just smush off the papery calyx from the seed and see if it has the goods:
A thing that looks the size, shape, and color of a black rice grain. Now you know. I wish I'd known years ago.
Thanks to Mike Bone at DBG for being the last person I bugged about this and confirming what I thought I'd figured out.
Now you can grow out sweet little pots of seedlings like Bill Adams' in the picture, or avoid empty pots resulting from empty seed.