[Illustration: FIG. 86.--_Chytridium lagenula._ Zoosporangium _a_ before, _b_ after the liberation of the swarmspores.] [Illustration: 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. [Illustration: FIG. 88.--_Polyphagus euglenæ. A_ with smooth, _B_ with thorny oospores; _m_ and _f_ the two conjugating cells.] [Illustration: FIG. 89.--_Peronospora alsinearum._ Mycelium with egg-cell and antheridium.] [Illustration: FIG. 90.--_Phytophthora infestans_ (strongly magnified). Cross section through a small portion of a Potato-leaf (the under side turned upwards): _a_ the mycelium; _b b_ two conidiophores projecting through a stoma; _c_ conidia; _e_ the spongy tissue of the leaf; _g_ the epidermis.]