What are the bulbs that are on the flower stems on iris plants after the flower has wilted?

the bulbs are green and about the size of a walnut

What are the bulbs that are on the flower stems on iris plants after the flower has wilted?
If the iris has been pollinated, these are actually the seedpods.

You can let them dry out, open them up, and and plant the seeds inside them, but you don't know what color or 'variety' your irises will be, since they have probably been cross-pollinated by the bees. This is how the hybridizers grow iris, they plant the seeds, however, they usually cross pollinate by hand and then puit a bag around the flower so it won't cross pollinate with other irises.

If you want to try growing irises from seed, it may take three or four years to get one to bloom, since most irises are grown by dividing the rhizome (bulb).