The conidia exhibit various characters which are employed for the separation of the genera. _Pythium_ is the most simple form. The contents of the terminally-formed conidia emerge as a spherical mass and divide into swarmspores. _P. de Baryanum_ lives in the seedlings of many different Flowering-plants, which it completely destroys.--_Phytophthora_ is distinguished by the circumstance that the sparsely-branched conidiophores bear, sympodially, chains of conidia. Besides the Potato-fungus (see above), _Ph. fagi_ belongs to this group; it developes oospores very abundantly, and does great harm to seedlings of the Beech, Sycamore, and Pine trees.--_Peronospora_ generally has conidiophores which are repeatedly forked, and bear a conidium on each of the most extreme ramifications. Many do great harm to their host-plants. _P. viticola_, on Vines, and _P. nivea_, on umbelliferous plants, have swarmspores, which are absent in the following species of this genus: _P. sparsa_, on Roses; _P. gangliformis_, on composites; _P. alsinearum_, on Stitchwort; _P. parasitica_, on cruciferous plants; _P. viciæ_, on Vetches and Peas; _P. schachtii_, on Beets; _P. violacea_, on the flowers of _Scabiosa_; _P. radii_, on the ray-florets of _Matricaria_.--_Cystopus_ (_Albugo_) has the conidia developed in chains, which form a cohesive white layer underneath the epidermis of the host-plant. _Cystopus candidus_, on cruciferous plants, especially Shepherd’s Purse and _Brassica_; the germination commences on the cotyledons, and from this point the mycelium developes together with the host-plant; _C. cubicus_, on the leaves of Compositæ. [Illustration: FIG. 93.--A fly overgrown with _Saprolegnia_.] [Illustration: FIG. 94.--Formation of swarmspores in a _Saprolegnia_: a germinating swarmspores.] Order 3. =Saprolegniaceæ=, _Water-Fungi_ which live as saprophytes on organic remains lying in water, for instance, on dead flies (Fig. 93), worms, remains of plants; but they may also make their appearance on living animals, being frequently found, for example, on the young trout in rearing establishments. [Illustration: FIG. 95.--Oogonium with two antheridia, _Achlya racemosa_.] The thallus is a single, long and branched cell. It has one portion which serves as root, and lives in the substratum, where it ramifies abundantly for the purpose of absorbing nourishment; and another portion projecting freely in the water, and sending out hyphæ on all sides (Fig. 93). The asexual reproduction takes place by swarmspores (Fig. 94), which are developed in large sporangia; these swarmspores generally possess two cilia, and on germination grow into new plants. The entire protoplasm in the oogonium is formed into one or more oospheres, without any surrounding “periplasm.” The oospheres may not be fertilised (p. 100), and then develope parthenogenetically.