116 PHYCOMYCETES. 



oogonium wall.^ N"o formation of spermatozoids occurs, as is the 

 case in VoMcheria and other groups of algae showing close rela- 

 tionship to these fungi. In certain eases the formation of 

 swarm-spores in sporangia does not take place, but conidia 

 are produced, which germinate directly into a mycelium. 



Preventive measures against the whole group consist in 

 destruction (by burying or burning) of diseased and dead parts of 

 host-plants which contain the hibernating oospores ; by change of 

 crop on infected fields ; and by treatment with copper reagents 

 (see Chap. VI.). 



To the Peronosporeae belong the genera Pythium, Phytoph- 

 thora, Cystapus, Basidiophcra, Plasmopara, Sclerospora, Bremia and 

 Peronospora. 



Pythium. 



The mycelium possesses no haustoria, and grows both between 

 the host-cells and inside them. Cross-septa are not present at 

 first, but later these may be found at irregular intervals. 

 Pythium lives as a parasite in living plants, or as a saprophyte 

 on a dead substratum. The conidia are of various forms, and 

 either germinate directly into hyphal filaments, or discharge their 

 contents into a bladder where zoospores are developed and 

 liberated as free-swimming spores with two lateral cilia. The 

 oogonia contain only one ovum-cell, which is fertilized by means 

 of an antheridial tube applied to the oogonium. The thick - 

 walled oogonia on germination produce hyphae or discharge zoo- 

 spores. 



Pythium de Baryanum, Hesse ^ (Britain and U. S. America). 

 This parasite is injurious to the seedlings of various plants in 

 gardens and fields. Some of its commoner hosts are maize, 

 clover, mangel, millet, and many species of the Cruciferae ; it has 

 also been found on the prothalli of Equisetum and Lycopodium.^ 

 It may also attack living or dead leaves and tubers of potato. 



The sporangia have a lateral beak-like outgrowth, into which 

 the plasma passes and divides into biciliate zoospores. The 



'In many species the fertilization-tube remains closed e.g. Plasmopara 

 viticota. 



^ Hesse, Pythium de Baryanum, Halle, 1874. Atkinson {Cornell Univ. Af/ric. 

 Expt. Stai., Bull. 94, 1895), describes and figures tliis and other fungi causing 

 "Damping-off." (Edit.) 



^Sadebeck, NattirforscJi.-Versammlu7ig., 1876. 



