Peronospora, or to the young mycelia of epiphytic parasites, e.g.
Those due to .Synchytrium, Protomyces, Cysto pus, many Ustilagineae, &c. These cases are not easily distinguished superficially froni the pustular outgrowth of actual mycelia and spores (stromata) of such Fungi as Nectria, Puccinia, &c. The cylindrical stem-swellings due to Calyptospora, Epichloe, &c., may also be mentioned here, and the tyro may easily confound with these the layers and cushions of eggs laid on similar organs by moths.
The simpler mycelia consist of hyphae all alike and thin-walled, or merely differing in the diameter of the branches of various orders, or in their relations to the environment, some plunging into the substratum like roots, others remaining on its surface, and others (aerial hyphae) rising into the air.
Striking instances of coloured mycelia are afforded by Corticiunt sanguineum, blood - red; Elaphomyces Leveillei, yellow - green; Chlorosplenium aeruginosum, verdigris green; and the Dematei, brown or black.
In the first group zygospores can arise by the union of branches from the same mycelium and so can be produced by the growth from a single spore; this group includes Spordinia grandis, Spinellus fusiger, some species of Mucor, &c. The majority of forms, however, fall into the heterothallic group, in which the association of branches from two mycelia different in I nature is necessary for the 2, formation of zygospores.
No sharp lines can be drawn, however, since many mycelia are intercellular at first and subsequently become intracellular (Ustilagineae), and the various stages doubtless depend on the degrees of resistance which the host tissues are able to offer.