F 7?NGI- - PERONOSPOREA P.. 



95 



extremity of the branch, by two if in any intermediate portion of it, from the rest 

 of the branch (Fig, 57, og). Then near it begins the formation of antheridia ; usually 

 a lateral protuberance, like the rudiment of a branch, makes its appearance on the 

 filament that bears the oogonium or on an adjacent branch of the mycelium, and 

 grows in a curve in the direction of the oogonium. Its extremity which is in contact 

 with the oogonium is now divided off by a transverse wall, and forms the antheridium 

 (Fig 57, an). The processes of formation of the oosphere and of fertilisation have 

 been recently made known to us by the researches of De Bary. The interior of the 

 oogonium is at first uniformly filled with dense finely granular protoplasm. But the 

 whole contents of the oogonium are not employed to form the oosphere, as is the case 

 in the Saprolegnieae. The dark granular portion of it retreats from the periphery and 

 collects into a ball (Fig. 57, os), the outer surface of which is separated by a broad 

 interval from the wall of the oogonium. This ball is the oosplierc, and its granular 

 mass is bounded by a layer of thinner hyaline protoplasm. The portion of the pro- 

 toplasm which is not expended in forming the oosphere is called \\-\z periplasm^, and 

 remains as a pale turbid substance, with fine granules unequally distributed through 



FIG. 58. Formation of oospores and fertilisation in the Peronosporeae. / /"/. Pythium gracilf, successive 

 states of an oogonium. / Oogonium full-grown, to the right an antheridia! branch applied to it but not yet divided 

 off. //. Antheridium cut off by a transverse wall ///. Formation of spherical oosphere complete in the oogonium ; 

 a narrow zone of periplasm lies between the oosphere and the wall of the oogonium. IV. The antheridium has put out 

 the fertilisation-tube ; a clear receptive spot may be seen in the oosphere. V. Passage of the gonoplasm from the anthe- 

 ridium to the oosphere. VI. Ripe oospore invested with a thick cell-wall, almost entirely filling the cavity of the 

 oogonium, the figures magn. about 800 times. VII. Peronospora arboresceiis ; an oogonium with an antheridium 

 applied to it, which has put out a fertilisatinn-tube. The oospore is invested with a thick cell-wall, and outside it is 

 a comparatively broad zone of periplasm ; the periplasm is contracting to form the episporium round the oospore ; magn. 

 600 times. After De Bary. 



it, filling the space between the oosphere and the oogonium (Fig. 58,77, VI, Vlf). From 

 this stage the course of procedure varies somewhat in the different genera of the 

 group. We will take Pythium first, and especially P. De Baryanum (Fig. 58, 7- VI). 

 As the oogonium is formed changes also begin in the antheridium, in preparation 

 for fertilisation. At a spot in the surface of the antheridium, where it impinges on the 

 oogonium, there appears a blunt cylindrical or conical protuberance, the fertilisation- 

 tube, which grows through the wall of the oogonium straight towards the oosphere, 

 till its extremity is in close contact with it. The tube at first contains only homo- 

 geneous protoplasm ; but as soon as its surface impinges distinctly on the oosphere, a 

 sudden separation is visible in the protoplasm of the antheridium. A thin layer, the 

 periplasm, remains, lining the wall of the antheridial cell- ; the larger part, \\\& gonoplasm, 



1 It corresponds to the portion of the protoplasm not used to form the oosphere but ejected from 

 the oogonium in Vaucheria. 



' This circumstance too finds its parallel in the antheridia of Tauc/icria, where a large part of 



