139 
Dale. — A Bacterial Disease of Potato Leaves. 
tuber, at a temperature of 25 0 C. Under such conditions the Bacteria form 
a dirty white film which quickly grows in thickness, but which is almost 
exclusively confined to the cut surface. The cultures are very slimy and 
pull out into strings. As they grow, they give off a quantity of gas which 
raises bubbles on the surface of the cultures. The cultures, when old and 
becoming dry, sometimes turn a purplish or reddish-brown colour, recalling 
that of the veins of potato leaves in which this organism is parasitic. 
The organism is very dependent on an abundant supply of water, and 
growth soon ceases, even on potato tuber, when the substratum becomes in 
the least dry, even though the food supply is by no means exhausted. 
Pieces of sterile (boiled) potato tuber, with cultures of the organism, were 
fixed in chrom-acetic acid, cut with a microtome, and stained with 
diamond fuchsin and light green (Fig. 14). Sections showed that the 
Bacteria were present in large numbers between the cells, filling up the 
intercellular spaces, whereas only a few, even in a culture a month old, were 
seen inside the cells and attached to their contents. The starch was 
apparently not attacked at all, as the contents of the cells gave a dark 
purple colour with iodine. These contents were chiefly the starch grains, 
boiled to a paste in the process of sterilization. The distribution of the 
Bacteria in the tuber was very clearly shown, as they took the usual bright 
red stain, while the walls and contents of the cell became a uniform green, 
with the exception of the starch grains and nuclei. It is noteworthy that 
the abundant proteid crystals resisted all the processes of sterilization, 
fixing, washing, and staining. They still retained their crystalline form, 
and stained red with fuchsin. 
The organism was also grown on living potato tuber, which was 
sterilized as far as possible by cutting pieces from the centre of a sound 
tuber with a sterilized knife. The pieces of tuber were put in a sterile Petri 
dish, and sterile distilled water containing the organism was poured over 
them. A control section was placed in a dish containing sterile distilled 
water only. In about a week the control piece of tuber was unchanged, 
and the infected slice was brown and rotten and smelt strongly ammoniacal. 
Sections of the living material showed Bacteria between the cells, and few or 
no Bacteria inside the cells, which contained unchanged starch grains, 
stained red with fuchsin. One of the pieces of rotting tuber was fixed in 
chrom-acetic acid, cut with a microtome, and then stained with fuchsin and 
light green. In the parts where rotting was only beginning and the tissues 
were still firm (Fig. 15), the Bacteria were seen to be abundant between the 
cells, which were widely separated so that they easily fell apart. Apparently 
the middle lamella is first attacked and dissolved, and then the cellulose 
walls, which stain red as in the leaves. In some sections spores were 
abundant, as well as ordinary Bacteria, in the intercellular spaces. Some of 
the spores are shown in Fig. 15, in the angles between the cells. The 
