GEOUP 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 Kubiaceae, etc.), only one gonidangium is 

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

 In Phytophthora the gonidangia are displaced laterally by branches which arise 

 from the hyphse bearing the gonidangia, 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 m} T celium of the Fungus ex- 

 tends from the circumference of these spots, 

 and throws up gonidiophore s 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 through 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 Compositae) gonidio- 

 phores bearing numerous gonidangia are formed 

 in great numbers close together under the epi- 

 dermis, and cause its rupture. 



Order 3. Saprolegniaceae.. The Sapro- 

 legniaceas 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 Aplane 

 Eraunii 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 Peronosporaceaa. The 

 number of oospheres in the oogonium varies 

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

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

 case they are developed from the whole of the protoplasm of the oogoniuni. 



Pis. 204. Gonidangium of an 

 Achlya. A Closed. JB The zoo- 

 gonidia are escaping ; c a lateral 

 branch ; o zoogonidia just es- 

 caped ; b empty membranes ; 

 e swarming zoogonidia. (x 650: 

 after Sachs.) 



