284 Trow . — Observations on the Biology and 
cavity of the oogonium, and then perish. A profile view of 
the bursting has not yet been obtained, although about six 
specimens have behaved in this way under direct observation. 
For these reasons figures illustrating the germination of the 
oospores are for the present withheld. The actual outgrowth 
of the germ-tubes has been traced under the microscope in 
about thirty cases, occurring in two sets of cultures in two 
successive years. 
Interesting differences presented by the three types of 
cultures. The foregoing remarks refer almost exclusively to 
observations carried out on moist-chamber cultures. These 
are apt to be abnormal for the reasons already given, and it 
was thought wise to grow the plant in Petri dishes in abun- 
dance of water to check the results obtained by direct obser- 
vations of small cultures under the microscope. In these 
cultures dead house-flies, which had been kept in an air-dry 
condition for twelve months, and bits of cabbage leaves were 
generally used to furnish the requisite nutrient material. 
Both kinds of food were sterilized by steaming for an hour on 
each of three successive days. Having set up at least sixteen 
series of Petri-dish cultures, each with four or more examples, 
one can safely make the following generalizations. The cul- 
tures on cabbage leaves were more luxuriant than those on 
the flies, and they normally produced oospores only. The 
house-fly cultures, on the other hand, produced with few 
exceptions conidia only. In cabbage-leaf cultures conidia 
might occasionally be seen, but always in relatively small 
quantity, and similarly in house-fly cultures a few oogonia 
might be found. By using these two types of cultures for 
imbedding, it was obviously easy to avoid confusing sections 
of oogonia and conidia. Such an aid was, however, found to 
be quite unnecessary, for these organs are much more readily 
distinguished in the stained than in the fresh condition. 
It has already been said that pure cultures were made in 
August, 1900, on potatoes. These cultures were remarkable 
not only for the luxuriant development of an aerial mycelium, 
but for the fact that they remained sterile for weeks. At 
