For the complicated parasitism of isopods and Sacculinidae on the same hosts Giard and Bonnier (Bopyriens, 1887, p. 197) should be consulted.
The influence of parasitism has so profoundly influenced its structure that its affinities are obscured by the development of specialized and adaptive features.
The group has until recent years been regarded as comprising three classes distinguished by well-marked physiological featuresthe Algae (including the Seaweeds) which contain chlorophyll, the Fungi which have no chlorophyll and therefore lead a saprophytic or parasitic mode of life, and the Lichens which are composite organisms consisting of an alga and a fungus living together in a mutual parasitism (symbiosis); Bacteria were regarded as a section of Fungi.
Natural Protection Against Parasitism The living organism is a rich storehouse of the very materials from which parasites, both animal and vegetable, can best derive their nourishment.
The various degrees of parasitism are to a certain extent explained by the foregoing.
The larch remains free from parasitism so long as its covering is intact, but as soon as this is punctured by insects, or its continuity interfered with by cracks or fissures, the Peziza penetrates, and before long brings about the destruction of the branch.
In epiphytic forms the rhizoids of the epiphyte often penetrate into the tissue of the host, and certain epiphytes are not known to occur excepting in connexion with a certain host; but to what extent, if any, there is a partial parasitism in these cases has not been ascertained.
In its systematized form, as a branch of botanical study, it is of recent date, and, as now understood, the subject first received special attention about 1850, when the nature of parasitism began to be intelligible; but many disjointed references to diseased conditions of plants had appeared long before this.
Commensalism occurs in a few instances, but parasitism either external or internal is rare.
The larch suffers from several diseases caused by fungi; the most important is the larch-canker caused by the parasitism of Peziza Willkommii.