94 A PhytoplitJtora Rot of PeavR and Apple h 



smaller ones were almost spherical, but others were elongate with a 

 slight constriction near the middle. Their dimensions were in general 

 35-65/Li X 22-35 /LA, the papillae being 4-6 /x in length, but occasionally 

 sporangia 70-80 /Lt in length were observed. 



The sporangiophores were 20-40/Lt in length and emerged through 

 the epidermis in small tufts; each sporangiophore bore a single spor- 

 angium terminally. In certain coverglass cultures on films of prune agar, 

 liowever, hyphae were produced which developed sporangia sympodially 

 in the manner generally typical of the genus Phytoplithora, series of 3, 

 4 and 5 sporangia respectively being formed in this way; no localized 

 swellings such as are characteristic of the sporangiophores of P. infestans 

 were seen on these sporangiferous hyphae. 



The rich development of sporangia on pears afforded a good oppor- 

 tunity of studying their mode of germination. On agar they always 

 germinated directly by giving rise to several germ tubes. In hanging 

 drops of tap water they germinated in one of three ways, viz. 



(1) Some produced branched germ tubes often swollen at the apex, 

 with a tendency for the protoplasm to become aggregated into the 

 swollen ends of the branches. 



(2) Others gave rise to zoospores which were liberated into the 

 water. 



(3) In a few cases the protoplasm became rounded off to form 

 zoospores which however did not escape into the water but germinated, 

 by developing germ tubes, within the sporangium^ 



In distilled water the sporangia sometimes germinated by producing 

 germ tubes directly but usually zoospores were formed, and generally 

 numerous zoospores could readily be obtained by transferring sporangia 

 from an infected pear to a drop of distilled water on a shde. The number 

 of zoospores produced by a sporangium is about 30 or 40 ; four sporangia 

 of which the zoospores were counted as they emerged gave rise respec- , 

 tively to 31, 43, 31 and 38 zoospores. 



The time taken for the sporangia to become emptied was ascertained 

 in a few instances by means of a stop-watch. Two sporangia, in which 

 the number of zoospores was not determined, were emptied in 33 and 

 36 seconds respectively; in two others 31 zoospores took 31 seconds 

 while 43 required 51 seconds. In some cases the liberation of zoospores 

 was more irregular, thus from one sporangium 18 escaped in 20 seconds, 

 after which 12 others, emerging more slowly, required 70 seconds, and 

 there was still one zoospore left which remained there for some minutes. 

 From another sporangium 25 zoospores escaped in 75 seconds; 2^ minutes 



