GROUP 1. THALLOPHYTA : FUNGI : OOMYCETES. 



291 



The sexual organs are fairly differentiated oogonia and pollinodia, and it is 

 on this account that this order is removed from the Chytridiacese, with which it 

 is closely allied, and is included in the Oomycetes. In fertilisation, the whole 

 contents of the pollinodium enter the oogonium and fuse with the whole con- 

 tents of the oogonium to form an oospore. The plants are either monoecious 

 (Myzocytium, Lagenidium), or dioscious (Ancylistes). The germination of the 

 oospore is known only in Ancylistes, where it developes into a hypha which 

 bores into a host. 



Order 2. Peronosporaceae. The forms comprised in this order are 

 mostly parasitic, chiefly on Phanerogams, but some species of Pythium inhabit 

 the dead bodies of plants and animals. 



The asexual reproduction of the gametophyte is effected, in most forms, by 

 gonidangia developed at the ends of the branches of the simple gonidiophores 



(Fig. 201 ; Fig. 203 A) : no such 

 organs have, however, been observed 

 as yet in Pythium vexans or P. Arto- 

 trogus. In some forms (Planoblastae) 

 the gonidaugium gives rise to zoo- 

 gonidia either before or after it has 

 fallen off the gonidiophore (Fig. 

 203 B, C) ; whilst in other forms 



FIG. 201. Part of a section of a Potato- 

 leaf infested by Phytophthora infestans : s 

 the gonidiophore passing out into the air 

 through a stoma ; c the gonidangia. 



FIG. 202. Phytophthora omnivora. An 

 oogonium (Og), containing an oospore 

 (sp) ; a a pollinodium which has fertilised 

 the oosphere. ( x 400.) 



(Siphoblasta?) it falls off and germinates as if it were itself a gonidium, 

 growing out into a hypha, and so into a mycelium. 



The ooaonium is spherical, and remains closed (Fig. 202). The protoplasmic 

 contents undergo differentiation into a single oosphere which is surrounded by 

 the remainder of the protoplasm, the periplasm. The oosphere is multi- 

 nucleate in some forms, though it has been described as ultimately uninucleate 

 in Pythium. 



The pollinodium is developed terminally, either on a hypha springing from 

 beneath the oogonium, or on an adjacent hypha, and is club-shaped. Its pro- 

 toplasmic contents undergo differentiation into a male cell (aplanogamete) and 

 into periplasm. 



At the time of fertilisation, the pollinodium is closely applied to the oogonium 

 and sends out a delicate tube which penetrates through the wall of the 

 oogonium and reaches the oosphere. The tube then opens, and the male cell 

 passes out of the pollinodium into the oosphere and fertilises it. The oosphere 

 then surrounds itself with a proper wall and becomes the oospore. In some 



