W. Robinson 127 



mentioned that infected seedlings show ''damping off," and this is 

 invariably due to a species of Phytophthora, the Fusarium not being 

 present in the tissues in the early stages of the disease. This Phyto- 

 phthora always occurs in the tissues of infected asters even when the 

 latter are mature and the characteristic mycelium is easily recognised 

 in sections made near the upper limit of the diseased part of the stem. 

 It will be convenient to describe first the method by which this Phyto- 

 phthora was isolated and grown in pure culture, then to describe its 

 main characters, and finally to give an account of inoculation experi- 

 ments both with this fungus and the Fusarium. 



The PhytopJithora was first detected by the following experiment. 

 A diseased seedling was cut off through the hypocotyl so that a small 

 portion of the infected region was included in the separated part. This 

 seedling was then placed with the cut end in water and examined at 

 the end of 24 hours. In this time an abundant crop of characteristic 

 pear-shaped sporangia had developed from near the cut surface. The 

 similarity of these bodies to the sporangia of certain species of Pythium 

 and Phytophthora led to the suspicion that one of these Phycomycetes 

 was probably the cause of the disease in asters. It will be seen that 

 all the later work, involving a careful comparison of the morphological 

 characters of the mycelia of the Phytophthora and the Fusarium as 

 well as experimental infections with both fungi, confirmed the correct- 

 ness of the above suspicion. 



Methods. Observations were carried out on living material from 

 diseased asters of different age, on the fungus grown in pure culture, 

 and also upon carefully fixed material. The last named was prepared 

 by fixing very small pieces of diseased tissue in Flemming's weaker 

 solution, or in chromo-acetic acid weak solution diluted with water 

 to one half strength. These pieces were embedded, cut in serial section, 

 and stained either with Flemming's triple combination or Heidenhain's 

 Iron-alum Haematoxylin followed by Orange G or Bismark Brown. 

 Delafield's Haematoxylin was also found useful for bringing out the 

 cellulose walls of the hyphae in the tissues. Sections from living material 

 were fixed in Iodine and examined in Schultze's Chlor-zinc-iodide 

 solution. 



Pure cultures were made on several different media, viz. Aster agar, 

 Beerwort agar, Quaker Oat agar, French Bean agar, Tomato agar, 

 and Salep agar. The aster agar was made by cutting up four healthy, 

 almost full-growm aster plants, and boiling for half an hour in 500 c.c. 

 water. The mixture was then filtered, 10 grams of strip agar added, 



