84 COMPARATIVE MORPHOLOGY OF FUNGI 



to protoplasm and the length of life, however, their oogonia show a con- 

 tinuous development, due to retardation of the degeneration of the 

 superfluous nuclei in the periplasm. While in the Saprolegniacae and 

 in Phytophthora this begins before mitosis (i.e., before the gametangial 

 nuclei are activated as sexual nuclei) and continues after it; in the Perono- 

 sporeae and in the Albugineae, as far as is known, it begins after mitosis; 

 thus at fertilization the superfluous nuclei are still intact and, in con- 

 junction with the periplasm, are adapted to creative activity. The 

 transition from Phytophthora takes place gradually; thus in Plasmopara 

 and Sclerospora the periplasm, as in Phytophthora, has only a slight struc- 

 ture and even during mitosis is not sharply separated from the ooplasm. 

 Especially in the last named genus the periplasm is always in several 

 layers around the oogonial wall which persists and surrounds the oospore 

 for a considerable time. In some species of Peronospora it behaves 

 similarly; in others it increases in importance and becomes much denser 

 and more homogeneous than the ooplasm; instead of the oogonial wall 

 becoming strengthened, however, it deposits on the oospore itself a 

 strong epispore with verrucose or reticulate sculptures; thus the oogonial 

 wall remains thin and transitory and collapses after spore maturity. 

 Only here the oospores have become true independent resting spores. 

 Furthermore, in Peronospora there collects in the center of the oosphere, 

 a deeply staining dense mass of protoplasm (the coenocentrum) in the 

 vicinity of which the female nucleus is placed; this will be discussed in 

 detail under the Albugineae. 



The germination of the oospores in Plasmopara takes place through 

 zoospores, in P. viticola under certain conditions also with a germ tube 

 ending in a large conidium (Ravaz and Verge, 1913), in Sclerospora 

 and Peronospora through a germ tube which develops in the host to a 

 mycelium. 



Opinions differ regarding the position of meiosis in each of these three 

 genera. According to Wager, Rosenberg and Ruhland it takes place 

 just before fertilization; according to Kriiger it takes place just after 

 fertilization in the first division of the zygote nucleus. In rare cases 

 (e.g., Peronospora Ficariae) it is connected directly with nuclear fusion, 

 but ordinarily takes place much later in the germination in the oospores. 

 According to present conceptions, and according to what is known con- 

 cerning change of nuclear phase in other fungi, Kriiger's theory might 

 easily be the correct one. In any case, further investigation of the 

 question would be desirable. 



The Albugineae, because of their special position in regard to conidio- 

 phores and fertilization, afford unique types, which are the most primitive 

 known in the Peronosporaceae. They are further noteworthy in that in 

 the same genus (Albugo) they go through fundamental changes while the 

 form of the conidiphore remains entirely constant. 



