It is lowest class of Eumycetes or True fungi. Plants of this class may be aquatic like saprolegina, some may be amphibious which may be saprophytes in moist soil or parasites on higher plants. Many plants are terrestrial and may be saprophytic or parasitic e.g. Nucor (Saprophyte) and Albugo (Parasite). Some of them cause serious diseases of crops besides spoiling our food.
Thallus or plant body is simple and ranges from simple isolated cell to a richly branched mycelium. Hyphae are un-separate and coenocytic, septa occur either rarely or at the time of formation of reproductive organs.
Reproduction: In phycomycetes vegetative, sexual and asexual reproductions occur.
(1) Vegetative reproduction: It is affected by fragmentation, budding and oidia.
(2) Asexual reproduction: It takes place by zoospores or aplanospores produced in sporangia. Sporangia develop at tip of reproductive hyphae the sporangiopore. Zoospore may be uni or bi ciliate. In some higer forms spores are non motile and secrete a wall all around known as aplanospores. In some forms spores are formed exogenously in chains (e.g. Albugo) on conidiosphorangia. These spores are called conidiosphores.
(3) Sexual Reproduction: It takes place by fusion of gametes and varies from isogamy to oogamy. The mycelium may be either homothallic or heterothallic. In primitive forms like olipiduim viciae and synchytrium endobiotricum it takes place by fusion of free swimming flagellated isogametes. In Allomyces Javanicus there is anisogamy of two fusing flagellated gametes one is larger than the other. In members of oomycetes like ALbuno there is oogamy in which non motile oospore is fertilized by free swimming flagellated antherozoid.
Product of sexual fusion is a zygote or zygospore or oopspore which secretes a thick wall around itself and un-goes a period of rest on germination it may give rise to zoospore or many produce a mycelium or sporangiosphore.
Classification:
There is considerable difference of opinion regarding classification of Phycomycetes.
Brefeld in 1881 divided phycomycetes into oomycetes and zygomycetes.
Sparrow in 1943 divided phycomycetes into uniflagellate, Biflagellate and Aplanate based on presence of absence of flagella and on their number and arrangement on the spores.
Gwyne Vaughan and Barnes in 1926 classify the class phycomycetes according as the mycelium is a simple, coenocytic, protoplasmic mass or an extensive coenocytic filamentous mycelium.
Some consider classification of phycomycetes directly into eight orders as chytridiales, blastocladiales, monoblepharidiales, ancylintales, saprolegniales, Permosporales, mucorales and entomophthorales.
Recently Alexopuls divides Phomycetes into six classes. They are chytricomycetes, hyphochytridiomycetes, oomycetes, plasmodiophoromycetes, zygomycetes and trichomycetes.
In all above mentioned schemes of classification the bone of contention is arrangement of orders into classes and subclasses. The number of orders and families is particularly the same.
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