White prairie aster symphyotrichum falcatum found primarily in a few western counties but not recorded since 1942 is further distinguished by flowers that are about twice as large and not crowded and more persistent stem leaves below the flowers. The corolla of the ray flower may have 2 tiny teeth opposite the 3 lobed strap or tongue indicating evolution by fusion from an originally 5 part corolla.
Asters Life Cycle Proflowers Blog