OOMYCETES 73 



on account of their surroundings, zoospore emission cannot take place, 

 such as in aerial mycelium in gelatin cultures (Minden, 1916). Appar- 

 ently they have a biological significance similar to that of gemmae : they 

 germinate like ordinary sporangia. 



King (1904) investigated the cytological development of A. pulchra. 

 As in the Saprolegniaceae, zoospores arise by cleavage. At the time the 

 oogonium is cut off, its protoplasm forms a homogeneous network with 

 35 to 50 nuclei, but whether this large number is caused by division is 

 still unknown. They migrate to the periphery, thus causing a vacuola- 

 tion of the interior of the oogonium (Fig. 43, 1). Thereupon most of 

 the protoplasm migrates back to the middle without the nuclei, causing 

 large vacuoles at the edge. The central mass of protoplasm becomes the 

 egg (Fig. 43, 2). In the middle is differentiated a reticulate, strongly 

 staining mass of protoplasm which apparently corresponds to the coeno- 

 centrum of the Peronosporeae. Meanwhile the nuclei at the edge have 

 divided once. One of them migrates back into the egg and lies in the 

 neighborhood of the coenocentrum (Fig. 43, 3); the others remain in 

 the peripheral layer of protoplasm, the periplasm. 



In the antheridium also, the nuclei divide. Only one of them is 

 functional and at fertilization passes over into the oogonium. Indi- 

 vidual peculiarities, such as the absence of a conjugation tube and the 

 formation of a receptive spot, are reminiscent of the Peronosporaceae. 

 The egg surrounds itself with a thick wall and becomes an oospore 

 (Fig. 43, 4). The fusion of the two nuclei takes place only at germination. 

 The periplasm changes into a peculiar faveolate sheath. The further 

 fate and germination of the oospore is unknown. 



Peronosporaceae. — A few primitive forms are saprophytic on earth 

 or fresh water; the higher ones are exclusively parasitic on land plants. 



The mycelium is ramose and consists of hyaline, generally compara- 

 tively thick hyphae which are coenocytic when young or when well 

 nourished, but which become septate in age. In some species of Pythium 

 and Phytophthora, they may unite into growths apparently adapted to 

 resist to external influences (Fig. 44, 1). Several species form thick- 

 walled gemmae. In saprophytic forms, the mycelium is both intra- and 

 extramatrical and in the latter case forms (particularly in Water) Sapro- 

 legniaceous clumps; in the other cases, there is no differentiation into 

 extramatrical hyphae and rhizoids. The parasitic forms develop exclu- 

 sively within the substrate and only liberate conidiophores. In the 

 simple representatives of this group, as in Pythium and Phytophthora, 

 the mycelium is both intercellular and intracellular and generally forms 

 no special haustoria penetrating the host. In the other genera it is 

 exclusively intercellular and penetrates the host cells by characteristic 

 haustoria, which in Albugo, Plasmopara, Bremia, etc., are like short 

 sacs and in some species of Peronospora branch digitately and occasion- 



