Phyllum Thallophyta - The Algae - Phycomycetes - Peronosporales
The Peronosporales are Phycomycetes in which the vegetatiye mycelium is composed of branched coenocytic hyphae, septa occurring rarely in older hyphae or in the formation of the sex organs. Sexual reproduction is oogamous, but only one oosphere is formed in each oogonium. Fertilization is effected by a fertilization tube. Asexual reproduction may be by sporangia, which either give rise to zoospores or produce spores which germinate by a germ tube. In the higher members conidiophores are produced which abstrict conidiospores, which also germinate directly by a germ tube. Intermediate stages between the two types are known. Where zoosporangia are produced the contents pass out into a vesicle in an incompletely diffentiated state and then divide into reniform zoospores with lateral flagella.
The species are characteristically parasitic on higher plants, though a few occur as parasites of Thallophytes. Some can live as saprophytes after the death of the host, but in the higher members this faculty is absent, nor can they be cultivated on artificial media.
Consider three examples of this order, Pythium , Cystopus and Peronospota , each of which represents a separate family and shows an apparent advance on the previous one in adaptation to a land habit.
