OOSPORE-PRODUCING ALGAL FUNGI III 



experience of the writer, immersion of the diseased fish in strong brine 

 in many cases brings about a cure, if the growth of the fungus is not 

 too great. Petersen^ observed a sick bream in the lake of Fare So 

 with a wound quite overgrown ■with Saprolegnia hj'phae and he has 

 found frog eggs which were attacked, the h},-phae growing in the jelly 

 around the eggs, penetrating into them. The fungus can be raised in 

 the laboratory on dead fishes by allowing tap water to slowly flow over 

 them in a jar. A few days are necessary to secure a copious growth. 

 Frogs which die under the ice in winter for lack of oxygen float to the 

 surface in the spring entirely covered by thi'; fungus. It thrives 

 best in the early stages of decay, for as putrefaction advances bacteria 

 and infusoria increase to such an extent as to check the growth of the 

 fimgus. When air insects, such as gnats, fall into lake or pond water 

 in great numbers, species of Saprolegnia, Achyla and Aphanomyces 

 appear in great numbers and seem to form a gray felt on the smiace. 



The vegetable materials on which the S.'vprolegniace^ mostly live 

 are branches and shoots of trees, except Salix, owing to presence of 

 saUcin, which fall into the water. Second in importance are haJf- 

 rotten rhizomes of Call a, half -rotten leaves and leaf stalks of Nuphar 

 and Nymphaa and other parts of aquatic plants which float on the 

 surface. Species of the genus Achlya are mostly associated with such 

 materials. Achlya polyandra have been repeatedly found by me on the 

 fruits of Osage oranges which have fallen into the pond at the Univer- 

 sity of Pennsylvania. The most favorable en\-ironniental conditions 

 seem to be the absence of air about the hyphae, quiet, still, pure water, 

 that does not contain much iron and a relatively op>en light surface. 

 Low temperature conduces to the formation of oogonia, .which also 

 keeps in check other competing organisms (Fig. 35"). 



Family 3. Peronosporace^. — This family is rich in parasitic 

 forms which may be accounted as the cause of important diseases of 

 cultivated plants. The hyphae of the myceUa are irregularly and copi- 

 ously branched and are found mainly in the intercellular spaces of the 

 host tissue sending short branches called haustoria into the adjoining 

 living cells. These haustoria may be globular {Albugo = Cystopus), 

 dub-shaj)ed (Peroiwspora corydo/is), branched {Plasmopard) (Fig. 36), 

 or branched and snarled {Peronospora). Septa are absent except 



'Petersen, Hexninc E.: An Acw>unt of Danish Fresh- water Phycomycetes. 

 Annalcs M^xologici, \-iii, Xo. 5, 1910. 



