The tiny seeds of the obligate root parasitic plants of the Orobanchaceae do not germinate unless they detect chemical signals from a host plant. Joel et al. study seed ultrastructure and find that mucilage swelling in the extended labyrinthine endothelium walls leads to opening of the micropyle. The perisperm cells underneath this opening mediate between the rhizosphere and the embryo, and are the likely to be the location for the receptors of germination stimuli. Other seed tissue characteristics are described in relation to water absorption and nutrient transport before and during germination.