Lathraea squamaria

When it comes to parasites, for most people it is a case of "out of sight, out of mind", and that especially applies to holoparasitic plants that spend most of their lives hidden underground or inside the tissue of their host plant, and only make their presence known when their flowers emerge from the ground or their host. Lathraea squamaria is a parasitic plant that infects many different types of deciduous trees including beech, hazel, aspen, and ash. Also known as the common toothwort, it is found in ravine forests and spends most of its life underground as rhizomes clinging to the roots of its host.

Left: A whole Lathraea squamaria plant, Centre: L. squamaria flowers in bloom, Right: Cross-section of a L. squamaria fruit with developing seeds.
Photos from Fig. 1, Fig. 3, and Fig. 5 of the paper.

While the word "parasite" often has a negative connotation, there are many parasites that perform important ecosystem services, and L. squamaria is no different. When few other plants are flowering, the toothwort blooms, providing bees with an important early-season food source when food for such pollinators are relatively scarce. They also release nutrient-rich saps that are savoured by small invertebrates and other plants in the forest.  While common toothwort has been studied for almost two centuries, because it exists almost entirely underground, many of the most basic aspects of this plant's biology and structure remain a mystery.

This post features a study where researchers set out to provide a detailed description of this parasite, particularly the haustoria, a very important part of parasitic plant as this is the organ they use to cling to and dig into their host. They collected L. squamaria from parts of the Czech Republic and Ukraine, sampling toothworts at different stages of development, from newly sprouted rhizomes to fully matured fruiting plants. Throughout the toothwort's life, the only part of this parasite which emerges aboveground are its flowers, which are mainly pollinated by specialised bees that can tolerate the parasite's extremely alkaline nectar. While the nectar of most flowers is generally slightly acidic, nectar from the common toothwort has a pH level of 11.5, which is comparable to soap.

As with other plants, once they are pollinated and produce mature fruit, they need to disperse their seeds, and like other holoparasitic plants, its seeds are transported by ants. Lathraea squamaria recruits their service by providing them with a treat, in the form of a tiny tag on each seed called an elaisome. This fleshy tag is irresistible to ants, which carries the whole seed back to its nest to rip off this tasty morsel. The ant then tosses the seed into the nest's communal refuse pile, but by doing so, they also end up planting the seed where it can sprout and find a new host.

The seed begin sprouting when exposed to chemicals which are extruded by the host tree's roots, and when the toothworth initially emerges from its seed as a rhizome, it can extend itself quite a long way beneath the ground in search for a host, before latching itself around the roots of a tree. Once it does, it sends out fine tendrils that tap into the host's vascular system, not just to draw nutrients, but also to send proteins, hormones, and signalling molecules to the host plant to make it more compliant. But sometimes they can also get a bit too overzealous and end up wrapping those tendrils around their fellow parasites, or even themselves. This has also been observed in other parasitic plants.

While called the "common" toothwort, researchers have found that they're not found in just any ravine forest, but mostly in mature forests which have not been disturbed. This is comparable to some horsehair worms which help keep trout well-fed with drowning crickets - some of these worms are only found in old growth forests. While one is a plant and the other a worm, both of those parasite provide important ecosystem services. For all that those parasites may take from their hosts, their actions also help support many other inhabitants of such forests.

Reference:

Krasylenko, Y., Teixeira-Costa, L., Sosnovsky, Y., Šamajová, O., Ovečka, M., Sytschak, Horielov, O., Brücknerová, N., Polláková, K.  & Ioannou, E. (2026). Persephone’s Flower: Morphology and anatomy across the life cycle of the forest root holoparasite Lathraea squamaria (Orobanchaceae). Flora 337: 152948.