Laffektv and Pethybridge — On a Phyiophthora Parasitic on Apples. 33 



are removed to u:edia, or even to sterilised moist tilter paper in Petri dishes, sexi^al 

 organs are produced within the tissues in much greater abundance. Possibly this 

 may be the result of an increased supply of oxygen; but the matter was not 

 further investigated. 



The fungus is also pathogenic to pears. When inoculated into unripe fruits a 

 firm brown rot quickly develops. On keeping them in a moist atmosphere tufts 

 of mycelium, sometimes bearing sexual organs, but not, so far as was observed, 

 sporangia, arise from some of the lenticels. Sexual organs are also developed 

 within the rotting tissue. 



As regards potato tubers, repeated inoculations invariably gave negative results ; 

 hence the fungus is not pathogenic to the potato. 



III. — Cultured Characteristics and Morphologij of the Fungus. 



The fungus has been cultivated on a number of media, parallel cultures having 

 been made with stocks raised by the various methods already described. On a 

 given medium, as was to be expected, the behaviour of the fungus has always been 

 the same, regardless of the particular method by which it was isolated. 



On Quaker Oat agar the growth was submerged, creeping, or very slightly 

 raised above the surface of the medium. Sporangia were few, and occurred in 

 small localized tufts on long and rather ill-defined sporaugiophores. Oospore 

 production was very scanty, or even entirely suppressed. The oogonial wall was 

 yellow where embedded in the substratum, but hyaline when formed on or above 

 its surface. Yellow-walled oogonia generally imparted a faint tinge of yellow to 

 the walls of the oospores within them. 



On oat-extract agar the growth was sparse, creeping, or submerged. Sporangia 

 were few, and sexual organs absent. On this medium the mycelium, when 

 examined microscopically, had a characteristic appearance. Towards the centre of 

 the growth the hyphae were distinctly tuberculate, with numerous short lateral 

 branches ; while at the margins each radiating hypha branched copiously, pro- 

 ducing a dense fan-like growth. 



Cooked prune-extract and cooked apple-extract agars proved to be unfavourable 

 media for the fungus. The limited growth on each was submerged or raised 

 slightly above the surface. Sporangia were absent, and sexual organs almost 

 completely so. 



On cooked potato stalk and cooked potato phigs the fungus was completely sterile 

 and confined to the tissues, while on cooked potato agar a very sparse and sterile 

 growth was produced, which resembled that on oat-extract agar. 



Cooked or raw appile plugs and cooked carrot plugs were the media on which the 

 best production of oospores was obtained ; but on none of these media were 

 sporangia observed. 



The sporangia, which, as has already been stated, are produced only sparingly 

 by this fungus, are borne on long hyphae which branch in a sympodial manner, 

 typical of the genus Phytophthora. Each sporangium is spherical when young, 

 but becomes inversely pyriform as it matures. The wall is thin, except at the 

 narrower free end, where it is thickened, but distinctly hyaline. The free end is 

 blunt, and not papillate ; whilst at the basal end of the detached sporangium there 

 remains a small portion of the parent hypha on which it was borne. In size the 

 sporangia varied greatly, but were found to average 40ju long by 27 fx broad. A. 

 single normal sporangium is illustrated on Plate I, fig. 15, and Plate II, fig. 16. 



When mature sporangia were placed in tap-water, a certain number of them 



