The Illinois Native Plant Society is “dedicated to the preservation, conservation, and study of the native plants and vegetation of Illinois.” They were founded in 1982 and have grown to encompass eight chapters throughout the state with about 500 members that are involved with both the state and local chapters.
Erigenia is a peer-reviewed journal. The most recent issues are available for subscribers only, the archives are also available online.
The Harbinger is a quarterly newsletter available to members. You can see an archive of past issues here while the most current issues are available to members only.
According to their website, Grand Prairie Friends is an all-volunteer, not-for-profit, conservation organization and land trust composed of people from many walks of life who share a commitment to preserve and restore tallgrass prairie and woodlands in east-central Illinois.
Norma Pfeiffer (1889-1989) was a botanist and expert on lilies. She discovered one of the only endemic plants of Illinois, the Thismia Americana on a low prairie on the south side of Chicago in 1912. The entire plant is smooth and white with a touch of blue green at its base and only reached 4-6 mm in height. This discovery earned her a Phd. at the age of 24 from the University of Illinois. This plant was only seen for five years and is now thought to be extinct. Image scanned from the "Morphology of Thismia americana" by Norma Pfeiffer from the Botanical Gazette, vol. LVII, no. 2.
Wildflowers of Illinois Woodlands by 
Vascular Flora of Illinois by 
Henry A. Gleason (1882-1975) was a botanist associated with the New York Botanical Garden for 32 years. He explored Illinois and wrote extensively on the vegetation of the Midwest. Being geographically diverse from the Tall Grass Prairie to a true swamp with impressive Cypress trees--Illinois offers it all. There are many rivers and lakes throughout Illinois that flow into the Mississippi River; early explorers traveled these rivers to reach the west.