Toothwort is a highly unique plant: all Broomrape family plants steal nutrition from neighbouring plants and also assimilate themselves, but toothwort is the only one that is fully parasitic. The species’ scientific name means ‘hiding’, and indeed most of this completely non-chlorophyllous plant’s life happens out of sight under the ground. Toothwort’s rootstock’s branches have sucking nodules (haustoria) between the branches which attach early in the spring to the host plant’s rootstock. This thankless task is usually performed in Finland by the hazel shrub, or occasionally a linden tree, ash, maple or alder. Scaly leaves next to the ground help toothwort dissipate water, which improves the plant’s ability to suck nutrition from the host plant.
In spring toothwort’s pale purple shoots push through the ground as they begin to flower. The seeds develop quickly and fall to the ground. They only sprout in the vicinity of a suitable host. The aerial shoots wither already at midsummer – in autumn the underground haustoria wither too and the plant rests. Toothwort is so highly adapted to life underground that it is able to flower and produce seeds without breaking the surface. Especially in boggy habitats it can grow from pieces of stem in the ground, which demonstrates its ability to survive independently, at least for a short while.(link)