234 CYSTOPUS 



system has been created to supply the material necessary for 

 the production of gonidia. 



Sexual reproductive organs are usually produced towards the 

 end of the host's flowering period (i.e. when the supply of nutri- 

 ment probably becomes deficient), and arise in the interior of 

 the infected regions. They consist of spherical oogonia (Fig. 

 123, D, o.), generally situated at the ends of the same hyphae as 

 bear the club-shaped anther idia (a.) at a slightly lower level, 

 although in some cases the two kinds of sexual organs are formed 

 on neighbouring hyphae. Both are multinucleate and, during 

 development, undergo differentiation of their protoplasmic con- 

 tents into a denser central and a less dense peripheral region 

 (Fig. 123, E) ; the former constitutes the egg in the case of the 

 oogonium and the male gamete in the case of the antheridium, 

 while the outer region plays no part in the sexual fusion. 



The antheridium becomes applied to the female organ and 

 puts out a slender tube which, piercing the oogonial wall, pene- 

 trates through the peripheral cytoplasm up to the egg (Fig. 

 123, D, E). The tip of the tube thereupon opens and the male 

 gamete passes through it to fertilise the ovum, the process in- 

 volving nuclear and cytoplasmic fusion in the usual way. The 

 product becomes invested by a thick dark-coloured wall (Fig. 

 123, F). After the decay of the host the oospores, which con- 

 stitute the resting-stage in the life-history, may remain dormant 

 in the soil for a considerable period. When conditions suitable 

 for germination occur, the contents divide to form numerous 

 zoospores which, after rupture of the thick wall, infect seedlings 

 in the way already described. 



The Potato Blight (Phytophthora infestans) and the damping- 

 off Fungus (Pythium debaryanum) have life-histories very similar 

 to that of Cystopus, except that their gonidia can, under certain 

 circumstances, germinate direct into a new plant (i.e. without 

 forming zoospores). In both cases the mycelium is intercellular, 

 and the asexual reproductive organs alone appear on the surface 

 of the host. In the Potato Blight the oval or elliptical gonidia 

 are formed singly at the ends of branched hyphae, which emerge 

 through the stomata of the diseased leaves (Fig. 124, A). If 

 blown on to the leaves of another Potato-plant, the gonidia 

 grow out direct into an infecting hypha ; whilst, if they fall on 



