Fig. 87.—Obelidium mucronatum: m mycelium; s swarmspores.
Order 2. Rhizidiaceæ. Mycelium present. Zoospores and resting-spores.
Chytridium (Fig. [86]). Obelidium (Fig. [87]) is bicellular; the one cell is the mycelium, the other the zoosporangium; found on insects. The species of Cladochytrium are intercellular parasites on marsh plants. Physoderma.
Order 3. Zygochytriaceæ. Mycelium present. Zoospores and oospores. The latter are the product of the conjugation of two cells (Fig. [88]).
Polyphagus euglenæ on Euglena viridis. Urophlyctis pulposa on species of Chenopodium.
Family 3. Mycosiphonales.
The mycelium is bladder-like or branched. Zoospores. Sexual reproduction by oospores, which are produced in oogonia. The latter are fertilised, in some forms, by the antheridium.
Order 1. Ancylistaceæ. The entire bladder-like mycelium is used for the construction of zoosporangia, oogonia, or antheridia. Lagenedium is parasitic on Spirogyra, etc.
Order 2. Peronosporaceæ. Almost entirely parasites. The unicellular, often very long and abundantly branched mycelium lives in the intercellular spaces of living plants, especially in the green portions, and these are more or less destroyed and deformed in consequence. Special small branches (suction-organs, “haustoria”) are pushed into the cells in order to abstract nourishment from them. Both oospores and conidia germinate either immediately, or they develope into sporangia with swarmspores, having always two cilia. Only one oospore is formed in each oogonium; its contents (Fig. [89]) divide into a centrally placed egg-cell and the “periplasm” surrounding it; this is of a paler colour and on the maturity of the oospore forms its thick, brown, external covering.
