The sight of bright red or milky white liquid oozing from a mushroom can understandably conjure images of a creature bleeding, leading to the common question of whether fungi possess a form of blood. Mushrooms are not animals; they lack a circulatory system, blood cells, and the hemoglobin that colors human blood red. The striking liquid droplets are not blood but metabolic byproducts, often water mixed with various compounds. This release is a natural function of the fungal fruiting body as it interacts with its environment.
The True Nature of Mushroom Exudates
The liquid released by mushrooms, known as exudate, is classified into two distinct types based on how it is produced. The first is a passive release called guttation, where droplets form spontaneously on the surface of the mushroom. Guttation fluid is typically a mixture of water, sugars, proteins, and secondary metabolites, which are often concentrated and can taste bitter or acrid. This fluid can appear clear, yellow, or red, depending on the compounds the fungus is producing.
The second type of exudate is a more active release known as latex or milk, characteristic of mushrooms in the Lactarius genus, commonly called milk-caps. This fluid is stored under pressure in specialized cells and flows out only when the tissue is damaged or cut. Latex is a viscous fluid, often white and milky, but it can also be vividly colored, including shades of orange, blue, or red. Unlike guttation, this substance acts primarily as a defense mechanism against herbivores.
The Science Behind Fluid Release
The spontaneous fluid release in guttation is caused by hydrostatic pressure created by the extensive network of fungal filaments, the mycelium. When the surrounding soil is saturated with water and the air humidity is high, the fruiting body cannot lose moisture quickly through evaporation. The mycelium continues to absorb water and nutrients, creating high internal turgor pressure within the fungal structure. This pressure forces the excess water and dissolved metabolic compounds out of the mushroom’s surface, typically at the actively growing edges.
The physical mechanism for this expulsion is not fully understood in fungi, but it is believed to be a way for the mushroom to manage the high volume of water produced as a byproduct of its rapid metabolism. The release of latex, conversely, is a simpler process involving the rupture of specialized, pressurized internal cells called lactifers upon injury.
Notable Examples and Edibility Concerns
The most visually striking example of guttation is the “Bleeding Tooth Fungus,” Hydnellum peckii, which is known for exuding bright red drops that look exactly like blood on its white cap. The red color comes from a pigment called atromentin, a compound with anticoagulant properties. Although the fungus is not poisonous, its extremely bitter taste and tough, cork-like texture make it inedible.
In the Lactarius genus, the blue-latex-producing Lactarius indigo and the orange-latex-producing Lactarius deliciosus are famous examples of mushrooms that release colored milk when cut. These species, along with many others in the milk-cap family, are often collected for food.
However, the presence of colored exudate, whether it is guttation or latex, is not a reliable indicator of whether a mushroom is safe to eat or toxic. Many milk-cap species with bitter or acrid latex are considered inedible or require special preparation. Therefore, anyone identifying a mushroom for consumption must rely on a full range of characteristics, not just the color of the liquid it releases.