S alm on , On Specialization of Parasitism in the Erysiphaceae. 267 



Some general remarks on tlie germination of the conidia 

 and on tlie incubation period and details of infection may be 

 given here. 



As mentioned above, some of the conidia used in every inocu- 

 lation experiment were reserved and allowed to germinate floating 

 on the surface of distilled water in a watch-glass. The watch- 

 glasses wei-e covered over with a glass plate, and placed by the 

 side of the inoculated plants. The details of gerinination were found 

 to Vary only very slightly in the different Oidia on grasses used, 

 and the few differences shown were not constant, nor did they seem 

 even characteristic of any form. Althoiigh the conidium on germi- 

 nation often puts forth several germinal tubes (flgs. 8, 11, 12) it 

 was noticeable that all the specially vigorous ones were produced 

 either singly from the conidium, or more rarely had one, or two, 

 very short germinal tubes accompanying them. A marked and con- 

 stant feature of germinating conidia floating in water is the more 

 or less vertical direction taken by the long vigorous germinal hyphae, 

 which rise directly at a right angle into the air from the floating 

 conidia. In the case for example, of the conidia used in inocnla- 

 ting 1 b in Experiment 1 (Table I) no less than 30 conidia of about 

 50 sown in a watch-glass produced after 48 hours long tubes 

 (4 — 6 times the length of the conidiam) which rose vertically up- 

 wards and waved in the air. Tlie same behaviour is found in 

 germinating Vredo spores. Eipe conidia germinate almost immedia- 

 tely when floated on water. Conidia were taken from the Oidium 

 on a leaf oi Bromus interruptus, and sown at 11 o'clock in distilled 

 water, the temperature of the laboratory being 20 " C. After 3 '/^ hours 

 most of the conidia had produced germinal tubes, which reached in 

 many cases to a length equal to that of the conidium. After 

 20 hours the germination was very vigorous, with the germinal 

 tubes long and rising vertically into the air. Germinating conidia 

 sometimes produce in the place of the normal long simple or very 

 rarely branched hyphae (which usually arise singly from the conidium) 

 a number of very short usually branched ones (figs 8, 11 — 13). 

 Conidia showing this abnormal germination are often found side by 

 side with ones that behave normally, so that it seems clear that 

 the diiference is not due to different external conditions. Nor does 

 this mode of germination seem to be characteristic of any one 

 Oidium; it was noticed in the conidia of the Oidium on Bromus 

 arvensis, B. interruptus, and Triticum vidgare, and in each CHSe 

 conidia germinating normally were found in the same or other 

 sowings of the Oidia in question. It is very possible that the pro- 

 duction of these short branched germinal hyphae is due to the 

 conidium being not quite ripe. The long vigorous germinal hypha 

 is, as a rule, produced from near one of the ends of the conidium; 

 sometimes lateral tubes are produced, but these are nearly always 

 Short, although exceptions to this rule occur (tig. 13). Tf a chain 

 of spores is sown, no geri..ination usually occurs; rarely, however, 

 a Single spore of the chain will germinate freely, sending out a long 

 tube. Occasionally abnormally shaped conidia — of much greater 

 length — occur among ones of the normal shape ; these are capable 

 of germination (fig. 17). 



