"Predisposition" and "Immunity" in Plants. 31 3 



It should be noted that in this series, the seedlings of No. 741 

 — the first named of each pair — were twenty days old, and, after 

 infection, were more exposed to the air and sun. Moreover, they 

 were infected two days earlier, with spores gathered the same 

 morning : whereas No. 744 were infected with the same crop of 

 spores, kept in a moist box for two days. 



Here, again, it seems impossible to explain the results as due 

 to anything but differences of susceptibility on the part of the 

 host. For even if we lay stress on the irregularities of infection 

 of B. mollis, B. interruptus and B. racemosus, we are still in face 

 of the fact that in these two series, differing slightly in treatment, 

 the spores growing on B. secalinus pick out that species first and 

 especially, and also its near ally B. arvensis, and that they do not 

 successfully infect any of the allies of B. sterilis. We cannot put 

 this down to any lack of germinating power on the part of the 

 spores : they were well tested and proved to be of excellent 

 germinal capacity in every case. 



Series Numbered 774 and 775. (Table II.) 



In the following series I only used few plants as before so as to 

 allow plenty of room in each pot, and infected them on the face of 

 the leaf below the tip, in water only, and at a moderate tempera- 

 ture carefully watched. The infecting material (uredospores) was 

 also carefully selected from rich green leaves early in the morning 

 of a, hot day, and allowed to develope further in glass dishes on 

 damp filter paper kept near the optimum. 



Moreover, the germinating capacity of every infection ivas 

 separately tested, at the same temperature as that prevailing 

 during infection, thus disposing of the criticism that failure to 

 infect might be due to a failure of the spores to germinate. 



During the first 24 hours the inoculated plants were kept outside 

 at near 20° C, and the bell-jars lifted after another 24 hours, the 

 plants thenceforth being on a table freely exposed to the mid-day 

 sun. The weather was close and dull on the 21st, with hot sunny 

 intervals : the shade temperature reached 26° C, and in the sun 30 J 

 to 39° were registered, but the infected seedlings were kept in the 

 shaded area at 20 — 22° C. 



On the 22nd a sharp fall of temperature occurred, and 20 — 

 21° G. in the shade was the highest recorded. The weather was 

 dull and a cool wind prevailed. 



On the 23rd it was still colder, and the shade temperature out- 

 side fell to 15-5— 17-5° C. 



On the 24th it rained heavily, and the shade temperature was 

 17 — 18° C, but a little sun appeared after 5 p.m. 



The results are summarised in the following table : 



