The botanical vampire
Greater Broomrape (Orobanche rapum-genistae) is a plant that has abandoned photosynthesis entirely. It contains no chlorophyll, the green pigment that allows plants to create their own food, and instead lives as an obligate holoparasite. This means it derives all of its water and nutrients by stealing them from other living plants. Its appearance is striking and unusual. It produces a single, thick, honey-brown or purplish stalk that can grow up to 80 or 90 centimeters tall. Instead of leaves, it has small, brown, scale-like structures, mostly near the base of the stem.
The plant's entire existence is dedicated to siphoning resources from its hosts. In the United Kingdom, its primary targets are leguminous shrubs, especially Common Broom (Cytisus scoparius) and European Gorse (Ulex europaeus). It attaches to the host's root system using a specialized structure called a haustorium. This organ penetrates the host's tissues and taps directly into the xylem and phloem, the vascular "pipes" that transport water and sugars throughout the host plant. The Greater Broomrape then acts as a powerful sink, drawing away the resources the host needs for its own survival.
A life of chemical espionage
The life cycle of the Greater Broomrape begins with a feat of extreme patience. A single plant can produce tens of thousands of minuscule, dust-like seeds which can remain viable in the soil for more than 20 years. These seeds lie in a state of deep dormancy, waiting for a very specific signal. They will only germinate when they detect the presence of chemical compounds called strigolactones, which are released by the roots of a suitable host plant.
Once this chemical trigger is received, the seed springs to life. A small radicle emerges and grows towards the host root. Upon making contact, it develops the haustorium to establish its parasitic connection. Most of this early life cycle occurs completely underground, hidden from view. After establishing a secure connection and drawing sufficient nutrients does the broomrape send its flowering stalk above the soil. The flowers, which appear between May and July, are pinkish-purple with two lips and bright yellow stigma lobes. After flowering, the stalk dies but often remains standing as a dark brown, withered spike. This plant is increasingly uncommon in the UK and is listed as Near Threatened.