Pethybridge and Murphy — On PhytophtJiora infestans. 579 



with the stopcock, is run into this in a covered dish for half an hour. This 

 cooks the material without burning, and at a uniform temperature. The 

 coarse sediment of the oats is then strained ofE through an ordinary fine wire 

 strainer, and 10 grams agar is added to the liquid, wliich is again treated to 

 the steam for half an hour to melt the agar thoroughly. Some water passes 

 over with the steam during tiiese cookings, so tliat what little, if any, is needed 

 to bring it up to the required 500 c.c, is added after the whole is drained into 

 a graduated cylinder. After the added water is uniformly distributed by 

 repouring, the medium is placed in the test-tubes, and these are sterilized in 

 the autoclave for fifteen minutes under 7 to 10 lbs. pressure." 



We followed these directions closely, except that steam was not generated 

 from an autoclave, but from a glass flask, and it was passed into the mixture 

 in the first instance for rather less than the half hour, since with the 

 oats we used there was a tendency for the medium to become too stiff to 

 be poured easily if steamed for this length of time. We also found that the 

 half hour's steaming after addition of the agar was not sufficient to dissolve 

 the whole of it thoroughly ; but by keeping the mixture well stirred during 

 pouring into tubes it could be distributed fairly uniformly through tliem, 

 and subsequent sterilization completed its solution. 



When this medium is inoculated with eonidia, the early gi-owth of 

 mycelium is confined to its surface, but in due time aerial mycelium becomes 

 visible to the unaided eye. The period wliich elapses before this occurs varies 

 from tliree to nine days, the usual interval being from five to seveu days. If 

 Petri dishes are used, the whole surface becomes covered with a thick growth 

 of mycelium in about a week after it has become visible to the naked eye, at 

 room-temperature. On slants in test-tubes during a similar period a fairly 

 dense felt of mycelium is also formed, which completely fills the space 

 available in the tube for some considerable distance up the slant. Conidia 

 are produced in large numbers; and the fungus remains alive in a tube 

 of this medium for several months. Oogonia were also observed, but 

 only sparingly ; out of about 150 cultures carried out on it during a period 

 of seven months these bodies were only abundantly present in one case. No 

 antheridia were developed in any of our cultures on this medium ; but, in 

 spite of their absence, apparently normal thick-walled oospores were 

 developed in some cases. (See tigs. 5 and 6, Plate XLY.) 



There are certain disadvantages connected with this medium which caused 

 us to give up its use in favour of the one next to be described. It was trouble- 

 some to prepare and difficult to pour in a clean fashion into tubes. Its most 

 serious drawback, however, was that certain starch-containing cells in which 

 the grains had become swollen during cooking and sterilization, but not 



