He returned after suffering from malaria and then taught at Leiden University (1904-1909), as a lecturer in Systematic Botany.

He became director of the State Herbarium (Rijksherbarium) 1906–1909, then Secretary of the Hollandsche Maatschappij van Wetenschappen.

[1] Lotsy founded the Association internationale des Botanistes and was editor of the Botanisches Centralblatt and the Progressus rei botanicae.

[5] India (1895–1900), the United States (1922), Australia and New Zealand (1925), South Africa (1926–27), and Egypt (1930).

Lotsy argued that the monocotyledons were diphyletic, with the Spadiciflorae being derived from the dicotyledons (specifically Piperales) and the remainder from a hypothetical ancestor, the Proranales.