In autumn of 1875, Francis Darwin was busy researching aggregation in the tentacles of Drosera rotundifolia. This phenomenon occurs when colored particles within either 'protoplasm' or the fluid in the cell vacuole (the cell sap) cluster together. Charles Darwin had theorized in the book Insectivorous Plants that these aggregated cellular masses consisted of living protoplasm. Inspired by his investigation of Drosera, Francis set out to examine the cup-like receptacles formed by the leaves of Dipsacus fullonum (synonym D. sylvestris), commonly known as fuller's teasel. Francis went on to suggest that the common teasel might be a carnivorous plant, acquiring nutrients from decaying insects drowned in its water-holding basins. Since then, other biologists have evaluated these claims with little experimental success. This paper provides a historical overview of Darwin's experiments with Dipsacus based on extant correspondence, discussing his results in the context of later studies that examine this unusual adaptation.