490 Scientific Proceedings. Royal Dublin Society. 



In one case at least a second parasite (a species of Pythium) was found along 

 with the Phytophthora ; but although in this particular instance the former 

 may have contributed to the death of the plant, the detailed work carried 

 out by us on numerous cases of the disease under consideration shows clearly 

 that the real cause is not a Pythium but a Phytophthora. 



III.— The Causative Parasite. 



Specific proof that the fungus, to which the mycelium described above 

 belongs, is the actual cause of the disease will be adduced in a later section 

 of this communication. Before proceeding to this it will be desirable to give 

 an account of the fungus and of its development on the host plant and in 

 pure cultures. 



Scrapings made from the superficial tissues of diseased stems below ground 

 and examined with the microscope sometimes, but by no means always, show 

 the presence of a few sporangia of a phycomycetous fungus of the Nozemia or 

 the Phytophthora type. If affected plants be removed from the soil and 

 allowed to remain in a moist atmosphere for a day or two, such sporangia are 

 developed more freely. Better still, if the plants are placed with their roots 

 and the bases of their stems in water, mycelium bearing these sporangia (but 

 not in great abundance) grows out into the water from the diseased parts of 

 the plant. This mycelium sometimes bears peculiar swellings such as are 

 illustrated in fig. 9, PL XLVIL, and these may be looked upon possibly as 

 abortive attempts to produce sporangia or perhaps sexual organs. Some of 

 them at any rate are suggestive of oogonial incepts which have not met with 

 antheridia, but which, in spite of this, have made considerable growth. Similar 

 bodies are also sometimes developed in pure cultures. 



The sporangia are borne on sporangiophores which are branched in a 

 sympodial manner. They are inversely pear-shaped, but often much elongated 

 and somewhat irregular in form (fig. 2, PI. XLVIL). The apex of the ripe 

 sporangium is always blunt, but its wall in this region is always slightly 

 thicker and more hyaline than elsewhere. In this respect the sporangium 

 resembles that of Phytophthora erythroseptica, there being no apical papilla in 

 either case such as is present in the sporangia of P. infestans. (See fig. 3, 

 PI. XLVIL) 



The sporangia are rather firmly attached to the sporangiophores, and do 

 not break off easily. When forcibly detached each sporangium carries with it 

 a short length of the branch of the sporangiophore on which it was borne, 

 which forms a very short stalk. The sporangium is filled with a granular 

 mass of protoplasm. In size the sporangia vary considerably, ranging from 

 24^u to 50/a in length and 17^ to 30ju in breadth at the broadest part. On 



