254 E. J. BUTLER. 



which then dissolved, and the mass at once broke up into individual 

 zoospores, as did the remaining one-quarter still within the sporan- 

 gium at the moment of rupture of the vesicle. Sometimes the vesicle 

 is formed in the ordinary way but the zoospores are almost mature 

 when they leave the sporangium and the vesicle early dissolves 

 to liberate them. 



The third mode of germination is a regression on the last. The 

 zoospores mature within the sporangium and when the papilla opens 

 are unable, from one cause or another, to escape. After a variable 

 period of free movement within the sporangium they come to rest 

 in situ, round off, become clothed with a wall and germinate by germ- 

 tubes which pierce the sporangia! wall (plate IV, fig. 9). Here also 

 intermediate cases between this and the last type occur. Sometimes 

 a portion of the protoplasm escapes in a mass and breaks up at once 

 into zoospores which swim away in the water. Blocking of the 

 tube of exit from any cause then prevents the remainder from es- 

 caping, and this segments within the sporangium to form zoospores 

 which ultimately germinate in situ as just described. Even when 

 complete segmentation within the sporangiimi occurs, some of the 

 resulting zoospores may creep out through the opening while others 

 fail to escape. 



The last type of germination is that in which zoospore forma- 

 tion is entirely suppressed, and the unopened sporangium germinates 

 by putting out one or several germ-tubes in the manner of a coni- 

 dium. Conclusive evidence was given in the previous account of 

 the genus to show that this is merely a further reduction on the zoo- 

 sporal type. It is one of the main adaptations to a terrestrial life 

 which the semi-aquatic Phycomycetes have developed in the process 

 of establishing themselves on land. Successful dissemination by 

 zoospores requires the presence of water in bulk, and is unsatisfac- 

 tory in species which are only intermittently irrigated. Germina- 

 tion by germ-tubes merely requires a moist atmosphere, and has en- 

 tirely replaced the older zoospore method in the higher fungi. In 

 Pythium the transition is clearly seen, and it is almost equally main- 

 tained in Phytophthora, all the species of which still preserve the 



