Some of the plants with cleistogamous flowers might afford examples of such cases.
"Darwin and Modern Science" by A.C. Seward and Others
The latter part of Letter 591 refers to the cleistogamic flowers of Viola.
"More Letters of Charles Darwin Volume II" by Charles Darwin
Also pale, pouch-like, cleistogamous flowers underground.
"Wild Flowers Worth Knowing" by Neltje Blanchan
In other cases precautions are taken to prevent cross-fertilisation, as in the numerous cleistogamous or closed flowers.
"Darwinism (1889)" by Alfred Russel Wallace
Reference has already been made to cleistogamic species which occur in several genera.
"Encyclopaedia Britannica, 11th Edition, Volume 12, Slice 3" by Various