Mycology is the scientific study of fungi, a vast kingdom of organisms that includes yeasts, molds, and mushrooms. Fungi belong to the domain Eukarya, meaning their cells contain nuclei and other membrane-bound organelles, but they are neither plants nor animals. Genetically, fungi are actually closer to animals than to plants, and this relationship is reflected in some of their biochemistry. A key structural difference is that fungal cell walls are made of chitin, the same tough polysaccharide found in arthropod exoskeletons, rather than the cellulose used by plants. Fungi also obtain nutrients very differently from most other kingdoms: they digest their food externally by secreting enzymes into their surroundings and then absorbing the broken-down organic molecules, a process called absorptive nutrition.
Fungi fill several ecological roles that make them indispensable to life on land. As decomposers, often called saprotrophs, they break down dead organic matter and recycle the nutrients locked within it. As mutualists, many fungi form partnerships with plants, animals, or other fungi in which both partners benefit. As pathogens and parasites, certain fungi cause disease in plants, animals, or humans; parasitic fungi like honey fungus (Armillaria) can attack and kill living trees. These varied roles depend on whether the fungus feeds on dead material, lives in symbiosis, or harms a host. Fungi also span a remarkable range of sizes, from single-celled yeasts just a few micrometers across to mycelial networks that stretch for miles underground. Although fungi share a eukaryotic cell plan with animals and plants, and although there is a clear difference between symbionts and parasites based on whether the host benefits or suffers, fungi are distinct enough that scientists have recognized them as their own kingdom since the twentieth century.