300 Trow. — Observations on the Biology and 
indefatigable botanist, to whose labours we owe so much of 
our knowledge of the Peronosporaceae. 
As the species stands at the end of a series indicating 
greater and greater adaptation to a terrestrial existence, as 
evidenced by the fact that it alone has lost all power of 
producing zoospores, I propose for it the name Pythium 
ultimum. The specific name will thus serve to call attention 
to its position in the genus. The species may be defined as 
follows : — 
Pythium ultimum , n. sp . 
Mycelium saprophytic on boiled potatoes, house-flies, cab- 
bage leaves, and other vegetable and animal substrata, never 
parasitic ; either aerial and then very luxuriant, and snow-white 
like cotton ‘ wool ’ (as in potato cultures), or aquatic and 
inconspicuous ; extramatrical and intramatrical ; hyphae very 
long (6 cm.) and slender, the diameter varying from 6-5 \x to 
1*7 n (average of sixteen measurements = 3-8 /u), much branched, 
septate in old cultures, the septa separating the older empty 
parts of the hyphae from the younger. 
Conidia chiefly terminal and spherical, with a diameter 
varying from 28 \i and more to 12/x and less (average of 25 
measurements — 20 \i), but occasionally intercalary and barrel- 
shaped, with the dimensions varying from 27-8 ju x 22*9 \x to 
17/xx Tq/x; set free by the decay of the supporting hyphae 
and germinating at once in nutrient solutions (in an hour if 
placed in cabbage-water) with the formation of one or more 
germ-tubes ; remaining at rest in exhausted nutrient solutions 
or distilled water for so long a period as seven months with- 
out loss of germinating power, and showing no tendency to 
form zoospores either in distilled water or running tap-water ; 
multinucleate. 
Oogonia , terminal and spherical, very rarely intercalary, 
with a diameter ranging from 22*9 \x, to 19 *6//, (average of 14 
measurements = 20-6 ju) ; smooth. 
Antheridia , generally one to each oogonium, arising from 
the stalk of the oogonium immediately below its boundary 
