The hyphae composing the mycelium are provided with haustoria which project into the cells of the affected part (fig.
A and B, mycelium (m), the mix t ure over the affe c ted with haustoria (h).
In Physma, Arnoldia, Phylliscum and other genera the gonidia are killed sooner or later by special hyphal branches, haustoria, which pierce the membrane of the algal cell, penetrate the protoplasm and absorb the contents (fig.
Synalissa, Micarea, the haustoria pierce the membrane, but do not penetrate the protoplasm (fig.
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.
The other genera are more purely parasitic; the mycelium usually sends haustoria into the cells of the host and puts out branched, aerial conidiophores through the stomata, the branches of which abstrict numerous "conidia"; these either germinate directly or their contents break up into zoospores (fig.
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.
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.
Endophytic parasites may be intracellular, when the fungus or its mycelium plunges into the cells and destroys their contents directly (Olpidium, Lagenidium, Sclerotinia, &c.), but they are far more frequently intercellular, at any rate while young, the mycelium growing in the lacunae between the cells (Peronospora, Uredineae) into which it may send short (Cystopus), or long and branched (Peronospora Calotheca) haustoria, or it extends in the middle lamella (Ustilago), or even in the solid substance of the cell-wall (Botrytis).