The structure of the vegetative growth stage of fungi is called vegetative body. The vegetative bodies of most fungi are branched filaments, and a single filament is called mycelium. Many hyphae are collectively called mycelium. The hyphae growing on the substrate are called hyphae. Microscopically, the mycelium is tubular, with cell walls and cytoplasm, colorless or colored. Mycelium can grow indefinitely, but the diameter is limited, generally 2-30 microns, and the maximum can reach 100 micron. The hyphae of lower fungi are called septate hyphae, and those of higher fungi are called septate hyphae. In addition, the vegetative bodies of a few fungi are not filamentous. But heteroplasmic plasmodium without cell wall or oval single cell with cell wall. Fungi parasitic on plants usually spread from cell to cell in the form of mycelium.

When the mycelium comes into contact with the host cell wall or protoplasm, nutrients enter the mycelium due to osmotic pressure. After some fungi, such as living vegetative organisms, invade the host, hyphae form a special mechanism to absorb nutrients in the host cell, which is called haustorium. The shape of haustorium varies from species to species, for example, the haustorium of powdery mildew is palm-shaped, the mold is filamentous, the rust is finger-shaped, and the white rust is spherical. The mycelium of some fungi grows to a certain stage and can form loose or tight tissues. The tissue of seedling silk mainly includes sclerotia, matrix and rhizome. Sclerotinia sclerotiorum is a dormant body formed by closely intertwined hyphae. The inner layer is sparse and the outer layer is quasi-thin-walled. The epidermal cell wall is thick, dark and hard. The function of sclerotia is mainly to resist adverse environment. However, when the conditions are suitable, sclerotia can germinate to produce new vegetative hyphae or form new propagules from them. The shape and size of sclerotia vary greatly, usually like mung beans, rat droppings or irregular. Matrix is a mat-like structure formed by hyphae interwoven on the surface or under the epidermis of the host, sometimes combined with the host tissue. The main function of the base is to form a mechanism for holding children, but it also has the function of overcoming harsh environment. The funicle is a long rope-like structure composed of parallel mycelium, which is similar in shape to the roots of plants, so it is also called rhizome. The bacteria rope can resist the harsh environment and help bacteria spread on the substrate.

Some cells in hyphae or spores of some fungi swell and become round, protoplasm is concentrated, and cell walls are thickened to form chlamydospores. It can resist adverse environment and germinate into mycelium when conditions are suitable.