102 THE PHYCOMYCETES 



which is approximately as long as the diameter of the sporangium. 

 This vesicle is the inner sporangial wall. After 10 to 15 minutes 

 the zoospores become differentiated \\ithin the vesicle and move 

 about within it, at first sluggishly and then ^\•ith increasing 

 rapidity. At length the vesicle ruptures, and the zoospores da'rt 

 aw^ay at high speed. After a few minutes they come to rest, en- 

 cyst, and then orerminate by the formation of a tube. 



Phytophthora. The Genus Phytophthora (the name signifies 

 "plant destroyer"), well known to all plant pathologists, includes 

 17 species and 1 variety, according to the monograph by Tucker 

 (1931). The name was first employed by de Bary in 1876 as a 

 result of his studies of P. infestavs, \\hich causes late blight and 

 tuber rot of potatoes. This organism \\as introduced into Eu- 

 rope, where it has produced serious losses year after year. How 

 it hibernates remained a controversial question for years, but the 

 mycelium is now known to survive within the "seed" tubers. 

 Smith (1875) early maintained that it forms, within the potato 

 foliage, oospores by means of \\hich it survives from year to 

 year, but de Bary demonstrated that the oospores observed by 

 Smith were those of Pythium. Jones, Giddings, and Lutman 

 (1912) obtained oogonium-like structures in pure cultures. An- 

 theridia were not observed, however, and Clinton (1911), who 

 secured ooo-onia and antheridia on oat acjar, believed the struc- 

 tures seen by Jones and his associates might as well have been 

 chlamydospores. 



Microscopic examination of diseased potato foliage shows that 

 the mycelium of P. injestans is intercellular and that haustoria 

 penetrate the host cells. Slender, sparsely branched hyphae in 

 small groups, the sporangiophores, emerge from the stomata. 

 Ovoid to pear-shaped sporangia are formed at the tips of the 

 sporangiophores, after which the hyphae continue to elongate, 

 thus causing the sporangia to appear laterally attached. Sporangia 

 are readily detachable and may be carried by rain-splash or air 

 currents to other leaves to initiate new infections. 



The rv^pe of germination of sporangia is determined by tem- 

 perature and moisture. The sporangium may form a germ tube 

 that penetrates the host directly, or else its content becomes 

 divided to form biciliate zoospores. Zoospores escape through 

 an apical pore, and after 2 or 3 hours, have settled, produced a 

 germ tube, and penetrated new host tissue. Entrance occurs 



