PYTHIUM. 117 



sporangia, however, may first pass through a resting period. 

 Sexual reproduction consists in the impregnation of an egg-cell 

 by means of a fertilization-tube from an antheridium. The 

 oospores are formed singly in each oogonium, and are liberated 

 ■only after decay of the oogonium walls and the tissues of the 

 host-plant containing them. After a resting period they pro- 

 duce a germ-tube, which penetrates into the host-plant and 

 becomes a . delicate branched colourless intercellular mycelium. 

 Hibernation is accomplished both by these oospores and by resting- 

 conidia, which remain amongst the decaying plant-debris on the 

 ground. 



Humphrey has observed sickness and death of cucumber 

 seedlings as a result of Py. de Baryanum,. Wittmack found a 

 species {Py. Sadebeckianum) very destructive on peas and lupines ^ 

 in various localities ; it has been observed frequently since. 



Py. gfacile is parasitic ou algae. 



Py. dictyospermum, Eac. occurs in Spirogyra. 



Py. cystosiphon is found on species of Lemna. 



Py. intermedium frequents prothalli of vascular cryptogams. 



Phytophthora. 



The mycelium is at first non-septate though much branched. 

 It grows both between and through the host-cells, and in some 

 species, {e.g. Ph. omnivora), has small haustoria. 



The conidiophores branch and produce a large number of conidia 

 or sporangia in succession. The first conidia are terminal, but 

 are displaced towards one side and thrown off by further 

 growth of the conidiophore to produce other conidia. 



The sporangia distribute their contents as swarming cells with 

 two lateral cilia; the conidia produce a hypha directly. The 

 ■egg-cells are developed one in each oogonium, and are fertilized 

 by an antheridium. The spherical oospores germinate in spring 

 by means of a germ-tube. 



Phytophthora omnivora, De Bary^ (syn. Ph. fagi, R Hartig.) 

 This is a destructive enemy to the seedlings of conifers, and even 

 more deadly amongst naturally-sown beech-seedlings. Death of 

 the beech is preceded by brown-spotting of stems, cotyledons and 



^ Verein z. Be/, d. MoorkuUur, 1891. 



-R. Hartig, Zeitschrifi f. ForM. u. Jagd-wesen IS7 5; also, Untersuchungen aua 

 ■d. forstbotan. hist. Munchens, 1880. 



