DIV. I 



THALLOPHYTA 



433 



chains on the branches of the mycelium under the epidermis of the host plant, 

 and produce in water numerous swarm-spores. 



The sexual organs of the Peronosporeae resemble those of the genus Vaucheria 

 (Fig. 348). They arise within the host plant the oogonia as spherical swellings 

 of the ends of certain hyphae, the antheridia as tube -like outgrowths arising as 

 a rule just below the oogonia. Both are cut off by transverse walls and are multi- 

 nucleate (Fig. 375). The several species exhibit interesting differences as regards 

 the nuclear changes. In Peronospora parasitica, Albugo Candida, and A. Lepigoni, 

 Pythium, Plasmopara, and Scler'ospora, a single large central egg-cell or oosphere 

 becomes differentiated in the protoplasm of the oogonium ; this contains a 



FIG. 375. Fertilisation of the Peronosporeae. 1, Perono- 

 spora parasitica. Young multinucleate oogonium (og) 

 and aiitheridium (an). ~'. Albugo Candida. Oogonium 

 with the central uninucleate oosphere and the fertilis- 

 ing tube (a) of the antheridium which introduces the 

 male nucleus. 3, The same. Fertilised egg-cell (o) 

 surrounded by the periplasm (p). (x (566. After 

 WAGER.) 



FIG. 376. Rhizopus nigricans ( = 

 Mucor stolonifer). Portion of the 

 mycelium with three sporangia ; 

 that to the right is shedding its 

 spores and shows the persistent 

 hemispherical columella. (x 38.) 



single nucleus in a central position, while the remaining nuclei pass into the 

 peripheral layer of protoplasm (periplasm). The antheridium now sends a process 

 into the oogonium, which at its apex opens into the oosphere and allows a single 

 male nucleus to pass into the latter. The oosphere then becomes surrounded with 

 a cell wall, and nuclear fusion takes place, while the periplasm is utilised in form- 

 ing the outer membrane of the spore (episporium). In Peronospora parasitica the 

 ripe oospore has a single nucleus, in Albugo it becomes multinucleate as a result 

 of nuclear division. In Albugo Bliti and A. portulacae there is also a central 

 oosphere surrounded by periplasm, but the oosphere contains numerous nuclei, 

 which fuse in pairs with a number of male nuclei entering from the antheridium. 

 A multiuucleate oospore thus arises from the compound egg-cell. The behaviour 

 of these two species can be regarded as primitive, the uninucleate oospheres of 



2F 



