142 Dale . — A Bacterial Disease of Potato Leaves. 
The size aiid shape of colonies . Plate cultures were made in gelatine 
and agar. The colonies obtained were round or oval, and in some cases 
showed pseudopodial-like prolongations, but this may be due to the 
pressure of the cover-slip on the gelatine. In the colonies the Bacteria are 
quiescent, but when liberated they are actively motile. 
The organism grows well at 25 0 C. and also at 35 0 C. There is no 
growth between 35 0 and 45 0 C., and the thermal death point is about 50° C. 
The cultures on the various media were examined microscopically at 
different ages, fresh, stained alive with water methylene blue, and in per- 
manent preparations made on cover-slips and stained with various aniline 
dyes. The organism differed considerably according to different conditions, 
so much so that doubt was felt as to whether the cultures were the same. 
But the point was settled by cross cultures from one medium to another, 
the results being constant. 
The organism is much more motile in liquid than on solid media. It 
is rod shaped, but the rods vary greatly in size, and especially in length, 
and sometimes grow into filaments which may be as much as 5 j u long. 
The average size is 37 /x x 1-23 /x, the minimum 2-4 /x x o-8 /x, the maximum 
5 /xx 17 /x. 
In cultures on potato tuber, which are extraordinarily vigorous, rods 
of all lengths were found, some very small and scarcely motile in the 
slimy masses of zoogloea, others longer and wider in the more liquid parts 
of the culture. In this same culture, after only three days growth in an 
incubator at 25 0 C., there were quantities of ripe spores as well as spores in 
process of development. 
Formation of spores . The spores and spore-bearing individuals are so 
much larger than the average vegetative bacillus that at first sight they do 
not seem to belong to the same organism. But similar preparations have 
been made from different cultures, and in one and the same preparation all 
stages may be traced from the ordinary small vegetative bacillus to the ripe 
spore. The development is shown in Fig. 16. The ordinary bacilli are 
shown at a 1 and a long filament at a 2 . Larger and somewhat more deeply 
staining individuals are seen at b. Still larger, and in some cases longer, 
individuals are represented at c y where the contents may be seen to be 
contracting and leaving the wall. In the next stage, d y the protoplasm is 
breaking up into segments, which vary in number according to the length of 
the spore-bearing rods. At this stage or earlier the rods may break up 
into smaller pieces. The spore-bearing rods are often seen to be sur- 
rounded by a faintly staining halo ( c and d) which shades off gradually, 
separating the rods more or less widely from one another. This is no 
doubt the slimy matrix in which the rods are embedded if they are in 
a zoogloea, as is frequently the case in subaerial cultures. Inside the rods 
the spores are formed by the rounding off of the protoplasm, which becomes 
