Size of the seeds is roughly 2.5 mm long in both. Color of them is very similar as is texture and both have an uncommon "neck" where the seed had been attached to the pod.
Leaves are also similar in that both species show tough fibers. Growing conditions and adaptations... ditto.

I. missouriensis 2n=38
I. lactea 2n=40, 44, 50

Also worth checking: innominata, chrysophylla, longipetala, chinensis. These all look strangely similar to missouriensis. Could longipetala be a natural hybrid of missouriensis or just another branch of the same family group?

Seeds from 2003 cross of I. lactea ILLINI FOUNTAIN seedling X I. missouriensis ex. Odessa, WA.

All Missouriensis were moved in 2003 so there was little bloom but pollen set 3 initial pods on the Lactea. Two pods dropped during the extreme heat of summer but one in partial shade at the back of the Lactea clump held.

2010 has been a rough year for a lot of plants. None of the little seedlings made it through our wild weather. 2 died immediately on planting out and the rest bit the dust during one of the wet cold/warm spells.

Fortunately most of the lactea seedlings were a bit tougher so another attempt is planned. This time we have an exceptionally pretty I. missourensis to use in the cross. She's the product of a cross between a solid pale blue type from Montana and one of the more common white & lavender with stripes type from Odessa, WA. Unlike the rest of the missouriensis whose bloom lasted only a week or two, she bloomed for just over a month. Not bad.