CHAPTER XLVIII. 

 FUNGI {continued). 



PHYCOMTCETES. 



Sub-class ii. — Oomycetes. 

 I. In this sub-class the mycelium resembles that of the 

 Zygomycetes in being formed of non-septate hyphae, but sexual 

 reproduction is carried on by oospores, which are generally 

 formed as the result of a fertilisation act between more highly 

 differentiated reproductive organs, namely, between an oogonium 

 and an antheridium, as described below in the account of the 

 fungus causing the 'damping-off' of seedlings. 



Asexual reproduction takes place by means of conidia, and 

 also by means of motile zoospores, which are produced within 

 sporangia of various forms. On account of their power of rapid 

 movement in water the zoospores are specially adapted for dis- 

 tribution in dew and water generally. Dampness of soil and 

 atmosphere greatly encourage the vigorous development of 

 almost all species belonging to the Oomycetes, a fact which must 

 be borne in mind when attempts are made to curtail their 

 ravages. 



Unlike the Zygomycetes, the Oomycetes are chiefly parasitic 

 in habit, and the group includes some of the most destructive 

 species of fungi which are known. 



The genera worthy of especial mention are Pythium, Phyto- 

 phthora, Plasmopara, Peronospora, Bremia and Cystopus. 



2. The fungus causing the disease known as ' damping-ofiF' may 

 be studied as a type of the genus Pythium belonging to the 

 Oomycetes. 

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