274 Trow. — Observations on the Biology and 
and oospores to the formation of new conidia and ripe 
oospores. 
In August, 1900, it appeared advisable to attempt to secure 
a really pure culture, i. e. not simply a culture of a single 
species of Pythium , derived from a single spore, but a culture 
which in addition was free from all other organisms whatso- 
ever. Two or three species of small Amoebae which multi- 
plied by division and encysted with the greatest regularity in 
the moist-chamber cultures, some Infusoria and Monads were 
got rid of by setting up a series of moist-chamber cultures. 
A culture, while still fresh and vigorous, was thoroughly 
washed with sterilized water and a small fragment used to 
inoculate a second one, the process being repeated a consider- 
able number of times. Bacteria however obviously still re- 
mained, and it was necessary to find some other means of 
getting rid of these. The method used by Klebs to isolate 
Saprolegnia mixta was tried and much time consumed in the 
process, but to no avail, for the Bacteria flourished on the 
nutrient gelatine, and the Pythium made no growth at all. An 
attempt was next made to check the growth of the Bacteria 
by means of acids. A culture on sterilized cress-seedlings in 
a Petri dish was set up, using a 1 °/ o solution of acetic acid 
instead of water. No growth of any kind took place in this, 
either of Bacteria or Pythium. Before proceeding to test 
the effect of other organic acids and weaker solutions, an 
experiment was made by setting up a culture on the acid 
leaves of rhubarb. Very fine cultures were produced in this 
way practically free from Bacteria. But for a fortunate and 
almost accidental observation, I should no doubt have spent 
much time in making and maintaining a pure stock culture 
by the use of rhubarb leaves. In cultures of the Fungus on 
bits of cabbage leaves, parallel to those on rhubarb leaves, it 
was noted that if the leaf projected above the surface of the 
water in the Petri dish, the mycelium in very luxuriant cultures 
developed a slight aerial growth. It was obviously possible 
to exaggerate this by favourable conditions, and, by means of 
cuttings taken from it, obtain the pure infecting material that 
