74 



COMPARATIVE MORPHOLOGY OF FUNGI 



ally fill up the whole host cell like a knot. Biologically, from the simpler 

 to the higher forms, there is an increased adaptation to parasitism. 

 Pythium and Phytophthora kill the infected tissue, Albugo and Perono- 

 spora only stimulate it to hypertrophy or to storage of food. Some 

 species of both these genera winter over in the rhizomes of the host and 

 penetrate the whole shoot. Some species of Peronospora in flowers 

 seem to grow up the stem as intercellular parasites and only develop 

 their conidiophores on the corolla. Furthermore, most species of Pythium 

 and Phytophthora are plurivorous and may be cultivated on artificial 

 media; Albugo and Peronospora are much more specialized and will not 

 grow saprophytically. 



Asexual reproduction takes place by sporangia which, even in the 

 most highly metamorphosed form, are cut off as multinucleate structures 



nit 



7< 



Fig. 44. — Phytophthora Syringae. 1. Mycelium. Phytophthora Arecae 2 to 7. 

 tion of zoosporangia. (X 215; after Rosenbaum, 1917.) 



Germina- 



at the ends of hyphae or hyphal branches; nuclear divisions do not occur 

 in them as in members of the Saprolegniaceae (Ruhland, 1904, disputed 

 by Istvanffi and Palinkas, 1913). Within the family they undergo 

 notable changes which were first recognized by Bary (1863). There are 

 three stages of development corresponding to the tribes Pythieae, 

 Albugineae and Peronosporeae. 



Pythieae. — In this tribe Pythium and Phytophthora take the lowest 

 position. In the simpler species of Pythium (Subgenus Aphragmium) , 

 true zoosporangia do not occur. Occasional hyphae emit their proto- 

 plasm into a germ sac (remains of a first swarm stage with terminal 

 flagella) where it changes to zoospores without being ab jointed from the 

 rest of the mycelium (Fig. 45, 2 to 5). For this lower degree of develop- 

 ment, it is further characteristic (as in the holocarpic Chytridiaceae) 

 that, in proportion to their thallus, they can produce a very large number 

 of spores; e.g., in Pythium gracile, almost the whole content of the hypha 



