Cytology of Pythiuni ultirnum, n. sp. 277 
distilled water, (3) tube-germination takes place, although 
slowly and irregularly, in material placed in running tap- 
water, (4) in suitable nutrient solutions each conidium may 
give rise to four or five germ-tubes whose place of origin is 
quite indefinite, and (5) empty conidia have never been seen, 
although very numerous experiments have been made on 
germination. There can be no doubt that in this species the 
conidium has completely lost its sporangial character. Pythium 
furnishes us with a series of forms — P. megalacanthum , P. de 
Bar 'yanum , P. vexans , and P. ultimum , roughly parallel to 
the corresponding series in Peronospora described by von 
Tavel (’92), viz. P. nivea , P. densa , P . Lactucae , and P . Radii , 
in which the stages in the conversion of a typical sporangium 
into a conidium are preserved for us. 
The conidium generally becomes vacuolated if kept in 
distilled water, or in exhausted nutrient solutions. They 
have been kept under observation for seven months. What- 
ever may be the age, germination takes place at once in 
appropriate nutrient solutions. 
Growth of the mycelium. Growth is extremely rapid, as 
may be seen at once by reference to Figs, i and 2 . The 
hyphae indeed, in a few hours, grow out of the drop alto- 
gether. 
Branching is of course monopodial, but very irregular. In 
order to keep the same culture under observation for any 
period of time greater than twenty-four hours, it is necessary to 
remove the cover glass, and by suitable manipulation bring 
the wandering hyphae once more within the limits of a small 
drop of nutrient solution. The hyphae are so delicate that 
it is difficult to do this without causing serious injury. Under 
careless manipulation the protoplasm may be seen coursing 
down the older hyphae in the most violent way. Such 
injury however is not irreparable, for the protoplasm in the 
apical parts is fairly stable and not readily destroyed, and 
fresh growth takes place at once. In about two days in 
warm summer weather (day temperature averaging about 
22° C.) the small drop is traversed in all directions by a net- 
