80 



COMPARATIVE MORPHOLOGY OF FUNGI 



In Phytophthora (Fig. 45, 2 to 7), Albugo, Basidiophora, Sclerospora and, 

 in part, Plasmopora (Fig. 48, C), the stage of the germ sac has almost 

 entirely disappeared. Here the mature zoospores swarm directly out 

 of the conidium. In other forms, the formation of swarm spores is 

 suppressed and replaced by the direct development of conidia to a myce- 

 lium. Thus in Plasmopara pygmaea, the undivided content of the conid- 

 ium swells out of the top, rounds off and puts out a germ tube. In 

 Bremia, the conidium germinates directly by a germ tube which has 

 grown out of a papilla at the tip; in Peronospora the papilla is lacking and 

 the germ tubes come from any portion of the conidium. 



Wherever zoospores are formed in the Peronosporaceae they are reni- 

 form and possess two lateral fiagella (Fig. 49, 4; 44, 5); thus they are 





i 





Fig. 49. — Pythiogeton transversum. 1. Hyphae with three zoosporangia, one of which 

 is discharging. 2. Protoplast has formed an elongate mass on emerging. 3. Zoospore 

 formation. 4. Single swarming zoospore. {After Minden, 1916.) 



connected directly to the Saprolegnieae of the Achlya type. Those of 

 Pythiogeton swim about for a while (up to two days) then come to rest, 

 surround themselves with a membrane and germinate to a mycelium. 

 Butler (1907) has observed in Pythium diacarpum and Murphy (1922) in 

 Phytophthora infestans the shedding of the membrane, as in Dictyuchus; 

 the zoospores which have found no suitable substrate come to rest and 

 surround themselves with a membrane. After a certain time they 

 again slip out with the same reniform appearance and swarm further. 

 The phenomenon has further been described by Bary for Pythium 

 proliferum and by Atkinson for P. intermedium; ocasionally the bifiagel- 

 late zoospores may split into two uniflagellate ones; this appears to be 

 only a morbid specimen, however, since Butler could not find in his 

 material the process described by Atkinson. 



In addition to this asexual reproduction, sexual organs, antheridia 

 and oogonia are known in most of the Peronosporaceae. With the 



