Phylum Thallophyta - The Algae - Phycomycetes
The Phycomycetes are Fungi in which the thallus is composed of single cells or of filaments of cells forming hyphae. Frequently these hyphae are coenocytic, transverse septa occurring either rarely, in the older hyphae, or exclusively at the separation of the reproductive organs. The hyphae may be multinucleate and the nuclei are frequently extremely small.
Sexual reproduction is either by motile isogametes, in the lowest order, or by means of an oogonium and antheridium. The oogonium may produce from one to many oospheres. The antheridium rarely produces free antherozoids, and fertilization is normally effected by direct contact through a fertilization tube. The oospore is usually thick walled and functions as a resting spore. It may germinate directly to form hyphae or by the development of zoospores. In the :\Iucorales peculiar conditions obtain, sexual reproduction being by isogametes or anisogametes \yhich are im'ariably non-motile.
Asexual reproduction in the lower groups is by zoospores, which may have either one or two flagella, a character to which some workers attach considerable importance and use as a basis of classification. In the higher members either sporangia are produced containing spores \yhich germinate directly into a hypha or aerially distributed conidiospores may be abstricted in chains from the apices of special hyphae termed conidiophores.
The species are predominantly aquatic and may live either as parasites or saprophytes. Some occur in soil and others as endoparasites of higher plants, and it is in these latter forms that we see the transition from an aquatic to an aerial method of spore distribution being evolved. By virtue of their aquatic habit and their marked similarity to certain Algae they were at one time regarded as Algae which had lost their chlorophyll and were classified after their supposedly corresponding algal groups. The name Phycomycetes or " Algal Fungi" emphasizes this idea. This view now receiyes but little support from mycologists.
Consider examples from the following orders ;-
- Chytridiales (e.g., Rhizophidium, Synchytrium).
- Saprolegniales (e.g., Saprolegnia).
- Monoblepharidales (e.g., Monoblepharis).
- Peronosporales (e.g., Pythium, Cystopus, Peronospora).
- Mucorales (e.g., Mucor).
