H. WORMALD 91 



experienced in isolating it, for by transferring, on the point of a sterile 

 needle, a few of the highly refractive granules from a diseased pear to 

 agar, mycelium grew out and developed numerous oospores, no other 

 organisms appearing. The fungus grew readily on such culture media 

 as prune agar, celery agar, maize meal agar and French bean agar, and 

 numerous oospores developed, but no sporangia were observed on these 

 media; on nutrient starch jelly there was feeble growth and no repro- 

 ductive bodies were found. 



Inoculation Exjjeriment 2. The Ph>/(ophthora was cultivated in the 

 laboratory during the winter, and in the following summer two pears 

 were inoculated with mycelium from a plate culture. Within seven days 

 both pears were discoloured over an area extending 3-5 cm. from the 

 pomt of inoculation; one showed many sporangia and sexual organs 

 also, the other bore sexual organs only. 



In order to prove conclusively the cojinection between the sporangia 

 and the oospores the following cultural experiment was carried out. 

 From one of the pears sporangia were taken and diffused in a little sterile 

 distilled water and drops transferred with a platinum wire loop to the 

 surface of prune agar in a Petri dish. Three of these drops, examined 

 under a low power of the microscope, were found to contain but one 

 sporangium each; these three were marked on the bottom of the dish, 

 with a ring of ink round each, and kept under observation. Within 24 

 hours two of them had germinated and produced a number of germ 

 tubes; in one case the germ tubes formed a diverging tuft at the non- 

 papillate end of the sporangium, while in the other they were scattered 

 and not localized to any particular part of the surface. The longest 

 germ tube was 0-9 mm. in length. The two sporelings were transferred 

 to other plates and within five days from the isolation of the sporangia, 

 which under these conditions germinated as conidia, both had developed 

 sexual organs similar to those observed on the pears, thus showing that 

 the two types of reproductive bodies belong to the same fungus. 



Inoculotion Experiment 3. Two pears were inoculated by making a 

 cut through the skin and placing in the wound mycelium from one of 

 the sporelings. Within 10 days both pears had become infected and the 

 fungus had developed the glistening particles characteristic of the dis- 

 ease on pears. The particles were most numerous towards the edge of 

 the affected area, and again each was found to be a cluster of sporangia; 

 no superficial sterile mycelium was present at such places. On the other 

 hand, in the immediate neighbourhood of the point of inoculation 

 myceliun) was seen on the surface as a white felt; here the sporangia 



7—2 



