he presented a classification based on longevity of the plant, power of vegetative propagation, duration of tillers, hypogeous or epigeous type of shoots, mode of wintering, and degree and mode of branching of rhizomes. The observation were made while raising wild plants from seed under garden conditions. In the late 1880s, after Warming's return to Copenhagen, he swopped research topic with his student Christen Raunkiær, who had traveled along the North Sea coast from Jutland to the Netherlands and published on the phytogeography of coastal vegetation. Warming now worked on plant adaptations in dunes and salt marshes, while Raunkiær studied the morphology of Danish plants, eventually leading him to his plant life-form scheme. Nevertheless, after Raunkiær had published his life-form scheme, Warming return to this topic in the work