Peronosporaceae are a group of endophytic parasites - about ioo species - of great importance as comprising the agents of "damping off" disease (Pythium), vine-mildew (Plasmopara), potato disease (Phytophthora), onion-mildew (Peronospora).
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.
Entoderma sp., endophytic in Ectocarpus.
In both Psilotum and Tmesipteris the functions of the root-system, which is completely absent, are performed by leafless rhizomes bearing absorbent hairs and inhabited by an endophytic fungus.
All the saprophytic prothalli contain an endophytic fungus in definite layers of their tissue.
In the roots of Ophioglossum and Botrychium and in the first formed roots of Helminthostachys an endophytic fungus is present, forming a mycorhiza - the stele in the larger roots has the usual radial arrangement of xylem and phloem; monarch roots occur in Ophioglossum.
Pedunculosum, as observed by Mettenius, subsequently reached the surface and produced green lobes; those of the other species known are wholly saprophytic, and contain an endophytic fungus.
An endophytic Fungus referred to the latter group (Peronosporites antiquarius, W.