The mystery seeds from last week were from a daffodil, Narcissus sp.
Gardeners usually plant daffodils as bulbs, but the plants also make seeds.
The seeds form in capsules.
Why don't we plant more daffodils from seeds? It turns out that germinating daffodil seeds can be a bit tricky. The seeds often have special structures called "elaiosomes" that attract ants. The ants carry the seeds to their nest, where they eat the elaiosomes and toss out the seeds into their trash heap, which is usually a chamber in the soil. The conditions in an ant mound are often ideal for seed germination and the seeds eventually grow.
The relationship between the ants and the plants is quite specialized. The presence of the elaiosomes may even inhibit germination in some species, so the seeds won't germinate until the elaiosome has been removed (by an ant). Cool!
It turns out that many plants, especially our spring wildflowers like Trillium and Viola, are planted by ants.
It's amazing what you can learn by being on the look out for seeds.