After the pink flowers wither away, the base of the flower atop the tall stem begins to swell. This is where each flower's ovules grow and develop into seeds. When ripe, the capsule looks tan and splits open to reveal numerous pink, jelly bean-like seeds.
To begin new plants from seed, sow the seeds immediately after harvest in warm, moist soil. Plant them 1/4 inch deep. Within two weeks, plants germinate and sprout from the soil, according to the PlantzAfrica website.
If you are not interested in raising belladonna lily from seed, horticulturists often recommend cutting off the flower stalks when flowers die. This prevents the bulbs from expending energy on seeds; instead the plant grows foliage or enters winter with lots of stored energy for next spring.