1. SUBCLASS OOMYCETES.
432. These are the egg-spore fungi. They include the water mold (Saprolegnia), the downy mildew of the grape (Plasmopara), the potato blight (Phytophthora), the white rust of cruciferous plants (Cystopus = Albugo), the damping-off fungus (Pythium), and many parasites of the algæ known as chytrids, as Olpidium, Rhizophidium, Lagenidium, Chytridium, etc.
The two following orders are sometimes placed in a separate subclass, Archimycetes.
Fig. 250.
Monoblepharis insignis Thaxter. End of hypha bearing oogonium (oog) and antheridium (ant). Sperms escaping from antheridium and creeping up on the oogonium. (After Thaxter.)
433. Order Chytridiales (Chytridineæ).—These include the lowest fungi. Many of them are parasitic on algæ and lack mycelium, the swarm spore either with or without minute rhizoids, developing into a globose sporangium (Rhizophidium, Chytridium, Olpidium, etc., [fig. 249]), or the swarm spore attached to the wall of the host develops into a long sword-shaped body with a sterile base, which proliferates and forms a new sporangium in the old one (Harpochytrium), or with slight development of mycelium in aquatic plants (Cladochytrium). Some are parasitic in leaves and stems of land plants. Synchytrium decipiens is very common on the trailing legume, Amphicarpæa monoica.
434. Order Ancylistales (Ancylistineæ).—The members of this order have a slight development of mycelium and many are parasitic in algæ (Lagenidium, [fig. 249]).
435. Order Saprolegniales (Saprolegniineæ).—These include the water molds (Saprolegnia). See [Chapter XIX].
436. Order Monoblepharidales (Monoblepharidineæ).—These are peculiar water molds, related to the Saprolegniales, but motile sperm cells are formed (Monoblepharis, etc., [fig. 250]).