Born and educated in Massachusetts, Riddle made significant contributions to lichenology, leveraging extensive scientific collections at Wellesley College and Harvard University.
[2] With access to the collections of cryptogamic botanist Clara Eaton Cummings at Wellesley and the substantial resources at Harvard University, Riddle aspired to significantly advance the study of lichens.
[2] In 1913, Riddle took a year's leave from Wellesley to study lichen collections and collaborate with botanists in various European cities including London, Paris, Geneva, Uppsala and Helsingfors.
[4] Riddle's premature demise was deeply felt by the scientific community due to his valuable contributions, expertise, and dedication to botanical research.
[2] In an obituary jointly authored by Harvard colleagues Winthrop John Van Leuven Osterhout, Roland Thaxter, and Merritt Lyndon Fernald, they wrote "In the circle which mourns him his careful scholarship was widely esteemed by his professional associates; he was honored by all for his inspiring ideals, and, beyond the lot of most men, he was sincerely beloved".