OOMYCETES 



325 



contains the more granular particles, and is termed the gonoplasm^ while 

 the other {periplasm) surrounds it. The gonoplasm enters the oosphere 

 through the impregnating tube of the antherid, and thus accomplishes 

 the act of impregnation. Sometimes two, rarely more, antherids arise 

 and apply themselves to the oogone, and this varies both with species 

 and individuals. After impregnation the oosperm assumes a cellulose 

 membrane, and gradually ripens. The fatty contents collect into one 

 body occupying the middle, and the membrane becomes thicker and 

 differentiated into two cellulose layers, the extme and the intiiie. The 

 periplasm develops into a brown, often granulated and wart}' membrane, 

 the extine, enclosing the oosperm, while the original wall of the oogone 

 generally breaks up, but may in some cases persist. 



The oosperms germinate in water after a period of rest generally 

 lasting throughout the winter : and this takes place either by the emission 

 of a germ-tube which gives rise directly to a new thallus like the parent 

 one, or the protoplasm divides into a number oi zoospores, which, extruded 

 together within a globular sac and escaping from it, swim for a short 

 time, and, after settling down, push out each a germ-tube which pro- 

 duces a new thallus. In other species, again, both methods of germina- 

 tion occur, some of the oosperms directly emitting germ-tubes, while in 

 the others the i3roduction of zoospores intervenes. In certain species, 

 the oosperms of which produce a germ-tube directly, a short mycele 

 (promycele) is formed, which, after bearing a few conidiospores, dies, and 

 these conidiospores in turn propagate new thalli. 



The non-sexual organs of propagation {conidiospores) are borne upon 

 special branches of the thallus {sporophores) in a variety of ways cha- 

 racteristic of the genera and in a minor degree of the species. These 

 germinate either by means of a germ-tube directly produced, or the 

 contents break up into zoospores, which, after swarming, settling down, 

 and becoming invested with a membrane, also produce germ-tubes. 



The usual course of life is the production upon the thallus of vast 

 numbers of conidiospores, which propagate the species extensively 

 throughout spring and summer, followed in autumn by the bearing of 

 sexual organs, with which the generation terminates. De Bary points 

 out('Comp. Morph.,'^:c., 1884) that only in the instances above mentioned 

 of the production from the oosperm of a promycele bearing a few coni- 

 diospores, can a distinct alterjiation of generations be recognised. There 

 is indeed merely the succession of one oosperm-bearing generation to 

 another, the propagating spores being only accessor}^ products of the 

 thallus. In such cases as Pythium vexans (de By.) and Artotrogus 

 (]\Iont.), for example, there are no such organs of propagation at all, or 

 at least long-continued research has failed to discover them. Other 



