572 Scientific Proceedings, Royal Dublin Society. 



needles were sterilized without becoming unduly heated, and without suffering 

 from the corrosion which follows from the frequent strong heating of a steel 

 needle in a flame. Platinum needles are, of course, much too flexible for this 

 work. 



The oogonia being of a distinct brown tinge were easily discernible 

 in the media under a low-power dissecting microscope, although they are just 

 beyond the limit of vision by the unaided eye. In examining them, small 

 portions of the media containing them were removed ; and, as a general rule, 

 each individual oogonium or oogonium and its adhering antheridium was 

 dissected out of the medium under the dissecting microscope, mounted in a 

 drop of water and covered with a cover-slip. Excess of water, if any, was 

 removed by means of blotting-paper, and the preparation was then irrigated with 

 a drop of a 2| per cent, solution of caustic soda. This tended not only to clear up 

 somewhat any small opaque portions of still adhering medium, but to some 

 extent cleared away the rather deep-brown colouring-matter in and around 

 the oogonium. When the clearing process had gone far enough, the soda 

 was neutralized by irrigating the preparation with a drop of weak acetic acid 

 solution. If the clearing was allowed to go too far, and particularly if no 

 spore was present in the oogonium, the latter was frequently apt to swell up 

 and burst. 



More or less successful attempts were made in our earlier preparations to 

 obtain the oogonia free from the semi-opaque starchy medium (oat-agar), in 

 which alone they developed, by a process of digestion of the latter with malt- 

 extract. This plan did not, however, offer any special advantages ; and the 

 method described of first mechanically dissecting away under the microscope 

 the greater part of the medium from around the oogonium proved itself, 

 with a little practice, to be in the end the simplest and most satisfactory. 



It is perhaps unnecessary to add that the strictest control was exercised 

 over the cultures, both by microscopic examination and by control cultures 

 to obtain and to keep them pure; and there is no room for doubt but 

 that the sexual organs described do belong to Phytophihora uifestans, and to 

 no other fungus. 



III. Pure Cultures on Media in which no Sexual Organs were 



FOUND. 



(1) Growth on sterile, raw Potato. — It is commonly but erroneously 

 supposed that P. infestans produces a more or less soft wet rot in potato 

 tubers. As a matter of fact, however, tubers when infected with this fungus, 

 whether naturally or artificially, remain hard and firm, showing the well- 

 known and characteristic dark and sunken areas on the skin, unless the 



