OOMYCETES 329 



wafted away, and, falling on other potato leaves, there produce zoospores, 

 or germ-tubes directly, in drops of water formed by dew or rain. The 

 germ-tubes penetrate the epiderm, setting up fresh growths of mycele in 

 new plants, and thus the disease is propagated. Countless numbers of 

 such propagating cells, each potentially the mother of a number of zoo- 

 spores, may thus be set free from a few diseased plants, and the spread 

 of infection and destruction of tissue in warm moist weather is almost 

 inconceivably rapid. The disease extends to all parts of the plant, in- 

 cluding the tubers, in which the mycele often remains in a resting con- 

 dition throughout the winter (as in certain species of Peronospora 

 mentioned above), and from which a fresh start is made in the following 

 year. The interest attaching to the subject is mainly economic, and an 

 extensive literature bearing upon it has grown up — by far the greater 

 part of it utterly worthless. 



Pythiiun (Pringsh.). — Several species of this genus are saprophytes, 

 inhabiting the dead bodies of plants and animals, while others are true 

 parasites on fresh-water algae, on prothallia, and on flowering plants. 

 The thallus and sexual organs are of the type described. The oosperms 

 of P. proliferum (de By.), like those of Phytophthora omnivora, form a 

 promycele ; while of P. vexans (de By.) the oosperms only are known. 

 The formation of propagating spores occurs at the end of simple thallus- 

 hyphce. A terminal cell is cut off by a transverse wall, and usually becomes 

 a zoosporange. This body expands at the apex into a thin globular 

 sac, into which the whole of its protoplasm empties itself. There zoo- 

 spores are differentiated, and, bursting the sac, escape and germinate. 

 There is some variation according to species in the forms of the zoo- 

 sporanges ; sometimes they are round or oval and sometimes elongated. 

 They have not the definite arrangement which characterises the other 

 genera. P. intermedium (de By.) and P. de Baryanum (Hesse) some- 

 times form spores which emit a germ-tube instead of the usual zoospo- 

 ranges. P. gracile (Schenk), P. entophytum (Pringsh.), and P. Chloro- 

 cocci (Lohde) inhabit fresh-water alg^, P. Equiseti (Sad.) and P. 

 circumdans (Lohde) attack prothallia, while P. de Baryanum infests 

 seedlings of different phanerogams and fern-prothallia. The last-named 

 is capable of attaining full development as a saprophyte on both dead 

 plants and animals. P. intermedium, also saprophytic, becomes a 

 parasite on fern-prothallia. It is worthy of note that these fungi are 

 parasitic only on seedlings, prothallia, &c., which contain abundance of 

 water ; and though P. de Baryanum causes local injury to grown plants, 

 this power may be raised to one of destruction under water. Pythium 

 vexans is found in diseased potato tubers. 



