They form a superficial mycelium on the surface of the plant, the hyphae not usually penetrating the tissues but merely sending haustoria into the epidermal cells.
A and B, mycelium (m), the mix t ure over the affe c ted with haustoria (h).
Mycelium with haustoria (h); 2, Erysiphe; A and B, mycelium (m), with haustoria (h).
Appressoria are also formed by some parasitic fungi, as a minute flattening of the tip of a very short branch (Erysiphe), or the swollen end of any hypha which comes in contact with the surface of the host (Piptocephalis, Syncephalis), haustoria piercing in each case the cell-wall below.
Such obligate parasites may be epiphytic (Erysipheae), the mycelium remaining on the outside and at most merely sending haustoria into the epidermal cells, or endophytic (Uredineae, Ustilagineae, &c.), when the mycelium is entirely inside the organs of the host.