OOMYCETES 32c 



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

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

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

 new plants, and thus the disease is propagated. ' Countless numbers o: 

 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 spreac 

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

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

 eluding the tubers, in which the mycele often remains in a resting con 

 dition throughout the winter (as in certain species of Peronospon 

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

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

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

 part of it utterly worthless. 



Pythium (Pringsh.).— 'Several species of this genus are saprophytes 

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

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

 The thallus and sexual organs are of the type described, The oospermi 

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

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

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

 hyphae. A terminal cell is cut off by a transverse wall, and usually become: 

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

 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 othe 

 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 algse, P. Equiseti (Sad.) and P 

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

 •seedlings of different phanerogams and fern-prothallia. The last-namec 

 is capable of attaining full development as a saprophyte on both dea( 

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

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

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

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

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

 vexans is found in diseased potato tubers. 



