“ Predisposition ” and “Immunity’* in Plants. 313 
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. interrupts 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. 
Bevies 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 36° 
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° C. 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— 175° 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 : 
