Jan. 13. 1919 Morphology of Urediniospores of P. graminis 45 



(i) Quantity of inoculum to be used; (2) optimum length of incu- 

 bation; (3) condition of urediniospores necessary to insure uniform 

 measurements; (4) number of measurements to be made of a given 

 strain; and (5) method of computation to be employed. 



In order to determine the amount of inoculum to be used, eight sets of 

 inoculations were made with very heavy, with moderate, and with 

 exceedingly light applications of inoculum. The very heavily inocu- 

 lated plants produced 132 successful infections out of 142 inoculations, 

 or 93 per cent; 130 out of 142, or 91 per cent, of the leaves inoculated 

 with a moderate amount became infected; and 104 out of 118, or 88 per 

 cent, of those which had received a small amount of inoculum became 

 infected. Whenever infection resulted, there was no perceptible differ- 

 ence in the size of either the uredinia or the urediniospores, or in the 

 virulence of attack in general. 



A liberal amount of inoculum was used whenever possible in all subse- 

 quent experiments. 



Jaczewski (5, p. 330) found that the germination of urediniospores 

 begins two or three hours after placing them in water or on the surface 

 of the plant blades, after which it progresses very rapidly, provided the 

 spores are fresh. Fromme (4, p. 513) points out that in order to obtain 

 a successful infection on plants a saturated atmosphere is necessary. 



To determine the optimum length of the incubation period, wheat 

 plants in 10 pots, each containing 10 wheat seedlings 6 days old, were 

 inoculated with an equal and liberal amount of viable urediniospore 

 material of P. graminis tritici and placed in two pans containing a 

 small amount of water and then placed under glass bell jars. Equal 

 amounts of water were put in both pans, and all other conditions were 

 kept uniform. 



At the end of 12 hours two pots were removed from under the bell jars 

 and set out on the bench; after 24 hours the second pair of pots were 

 set out; and the rest were taken out from the pans in pairs every 24 

 hours following the second pair— that is, 48, 72, and 96 hours after 

 inoculation. 



The first observation was made 80 hours after inoculation, and no 

 signs of infection could be detected on the plants incubated for 12, 24, 

 or 48 hours; but 6 plants of those which had been under for 72 hours 

 and 12 of those that were still under the bell jar showed very indistinct, 

 but apparently typical, rust flecks. These were later found not to be 

 infection flecks, but the result of supersensibility, due to the long con- 

 finement in the moist chambers. At 128 hours after inoculation clearly 

 defined rust flecks appeared on all plants which had been incubated for 

 48, 72, and 96 hours. Of the 20 plants that had been under the bell jar 

 for 24 hours, 18 plants were flecked, while only a single fleck showed on 

 one plant of those that had been under only 12 hours. 



