The search engine picks up common names. No persimmon is going to have a common name in the plural form, but a Daylily might have a cultivar name with "persimmons" in the plural form.
Roses and Daylilies have a parent page, so the common name on those pages is "roses" rather than "rose" and "daylilies" rather than "daylily." Most of the genera do not have a parent page, so they aren't ever listed in the plural form.
