Phylum Thallophyta - The Algae - Phycomycetes - Chytridiales
The Chytridiales are Phycomycetes in which sexual reproduction is effected by the fusion of isogametes and no trace of oogamy occurs. The vegetative thallus may be either a plasmodium devoid of a cell wall or it may be unicellular, while in the highest groups it consists of a multicellular or a coenocytic hypha.
The Chytridiales are Phycomycetes in which sexual reproduction is effected by the fusion of isogametes and no trace of oogamy occurs. The vegetative thallus may be either a plasmodium devoid of a cell wall or it may be unicellular, while in the highest groups it consists of a multicellular or a coenocytic hypha.
The group contains the most primitive members of the Phycomycetes. A few live as parasites in the tissues of higher plants, but the majority are ectoparasites or saprophytes on Algae. In some the life-history is complicated by special adaptation to the parasitic habit and to overwintering in the host, but in many the life-history is remarkably simple and demonstrates the way in which the group may have evolved. In general, it may be said to consist of two stages, the first an amoeboid vegetative phase, followed by a flagellate reproductive stage. As we pass up the group to the more complex types the tendency appears to have been to reduce the amoeboid or plasmoidal stage and to enclose the protoplast in a wall, which then becomes the wall of a sporangium in which eventually the reproductive organs arise.
'vVe shall consider two types, the one Rhizophidium globosum, representing the ectoparasitic type, and the other Synchytrium endobioticum, the endoparasitic mode of nutrition.
Rhizophidium globosum
This little organism is by no means well known, though actually it is very common. It occurs with other closely allied species on the filaments of Green Algae, particularly Spirogyra and Cladophora, and may be found at almost any time of the year in ponds or streams.
The mature plant consists of a spherical sac attached to the surface of the algal filament by a short rhizoid. The extent of development of this rhizoidal system is a specific character; in some species it is little more than a peg which does not
penetrate the cell wall of teh host, in other species the wall is penetrated and a system of rhizoids
develops inside which may ramify over the chloroplast of the host cell, suggesting that food material is withdrawn through the rhizoids.
REPRODUCTION
The whole body of the Fungus functions as a zoosporangium and its contents divide up into a large number of uniflagellate, spherical zoospores which escape through an operculum formed at the top of the sporangium. Whether these bodies ever fuse in pairs or not is unknown though this has been recorded in allied genera. The motile cell soon settles down by its flagellated end, and the flagella are withdrawn. From the attached end the rhizoidal system is developed, "vhile the remainder of the cell enlarges and grows into a sac resembling the parent, which will in time become a fresh zoosporangium.
An interesting development which is worth referring to here is seen in certain allied genera. In some, after the development of the rhizoidal system, a swelling appears immediately inside the host wall which is termed the subsporangial swelling. In certain genera this remains small and it is the cell outside which forms the zoosporangium. A series of examples, however, are known which lead up to a condition where the subsporangial swelling becomes the zoosporangium, while the original external cell remains quite small and after penetration may disappear. At the same time rhizoids develop not merely from the base but all over the surface of the new sporangium. Thus we see one way in which an ectoparasitic fungus may become an endoparasitic one.
The life-cycle of Rhizophidium is so simple that it will not be necessary to express it in the form of a diagram.
