GROUP I. THALLOPHYTA: FUNGI: OOMYCETES. 



293 



zoogonidia. The germination of the oospore has not been observed. These 

 observations require confirmation ; if they are accurate, Monoblepharis is the 

 only Fungus which has spermatozoids, and an oogonium which opens. 



In the genus Peronospora, which is represented by many species (P. para- 

 sitica on Capsella, P. calotheca on KubiaceaB, etc.), only one gonidangium is 

 borne by each branch of the gonidiophore which protrudes through a stoma. 

 In Phytophthora the sporangia are displaced laterally by branches which arise 

 from the hyphas bearing the sporangia, at their points of origin. To this genus 

 belongs P. infestans, which produces the 

 potato-disease. The tissues of the host un- 

 dergo decomposition in the infected parts and 

 turn black : the mycelium of the Fungus ex- 

 tends from the circumference of these spots, 

 and throws up gonidiophores through the 

 stomata (Fig. 203). The gonidangia of the 

 parasite are carried by the wind to healthy 

 plants and infect them : the zoogonidia also 

 penetrate tbrough the soil to the tubers, and 

 the mycelium which is developed from them ex- 

 tends into the young Potato-plant which grows 

 from the tuber. No sexual reproductive organs 

 have been observed in this Fungus as yet. 

 Phytophthora omnivora infects and destroys 

 the seedlings of the Beech and other plants. 

 In Cystopus (C. candidus on Capsella and other 

 Crucifers, C. cubicus on Composite) gonidio- 

 phores bearing numerous sporangia are formed 

 in great numbers close together under the epi- 

 dermis, and cause its rupture. 



Order 3. Saprolegniaceae. The Sapro- 

 legniacesB all live in water, and are mostly 

 saprophytic, though some are parasitic ; one 

 species causes the Salmon-disease. 



Asexual reproduction of the gametophyte is 

 effected entirely by zoogonidia, but in Aplanes 

 Braunii they are not set free but germinate 

 within the gonidangium ; they are formed in 

 terminal but not otherwise especially differen- 

 tiated gonidangia (Fig. 204). On coming to 

 rest they germinate to form a mycelium. 

 They are, in some forms, surrounded by a thin 

 cell-wall at their first formation. 



The oogonia and pollinodia (when present) 

 resemble those of the Peronosporaceas. The 

 number of oospheres in the oogonium varies 

 widely in different individuals; sometimes there is only one (Leptolegnia, 

 Aphanomyces) ; but as a rule there are many, as many as 30-40 ; in either 

 case there are developed from the whole of the protoplasm of the oogonium. 



FIG. 204. Gonidangium of an 

 Achlya. A Closed. B The zoo- 

 gonidia are escaping ; c a lateral 

 branch. ; a zoogonidia just es- 

 caped ; b empty membranes ; 

 e swarming zoogonidia. (x 550: 

 after Sachs.) 



