In 1916, Johan Hagström published "Critical researches on the Potamogetons" from detailed measurements of dozens of vegetative, anatomical and reproductive characteristics such as fruit beak, peduncle rigidity and leaf apex across hundreds of specimens. It was a pioneering work that preceded the development of cladistics by more than 30 years, and his clustering of groups into broad-leaf and linear-leaf is still considered the framework for all subsequent classification systems of "Potamogeton." Hagström's work was developed further by Merritt Fernald (1873-1950) who created a detailed phylogeny for the linear-leaved species, and by Eugene Ogden who did so for the broad-leaved species.