i54 
Dale. — A Bacterial Disease of Potato Leaves. 
PLATE XVI. 
Fig. ii. A long tube is here seen from the point of entrance in the angle between the vein and 
the lamina of a leaf to its ending in the phloem of the vascular bundle. On each side of the tube, 
along its tortuous course, are diseased cells containing Bacteria, and a few Bacteria may be seen 
lying outside the leaf round the mouth of the tube. Incidentally this section shows how the tubes, 
so to speak, avoid the stomata, one of which lies close to the outer end of the tube. 2 oc. D obj. 
Fig. 12. The section is of a considerably diseased leaf with several branching and anastomosing 
tubes passing from the exterior into a small vein. The Bacteria have only attacked the cells and 
not the vessels. Many cells of the parenchyma are filled with Bacteria. 4 oc. D obj. 
Fig. 13. This shows a few cells filled with Bacteria and containing the endings of the tubes. 
One tube is seen in transverse section in an intercellular space. 4 oc. 3 fnm. obj . 
Fig. 14. Section of potato tuber, sterilized by steaming , and infected with Bacteria. The 
Bacteria are very numerous between the cells, which, after being steamed, are only loosely attached 
to one another. There are only a few Bacteria inside the cells, whose starchy contents are apparently 
only slightly attacked. 2 oc. §- inch obj. 
Fig. 15. Section of living potato tuber infected with Bacteria. Rotting was only beginning 
and the tissues were still firm. As in the dead tuber, the Bacteria are most abundant between the 
cells, whose walls have here been separated by the organism, which has destroyed the middle 
lamella and chemically changed the cell-walls so that they stain red. The organism has formed 
spores in the intercellular spaces. The large red spheres are starch grains, which have not been 
attacked by the Bacteria. 2 oc. obj. 
Fig. 16. Formation of spores in the bacilli, a 1. Ordinary vegetative bacilli, a 2. Very long 
rod. b. Rods enlarging to form spores, c. Contents contracting from the wall. A faintly staining 
halo of zoogloea surrounds these rods. d. The contents breaking up into short lengths, e. Ripe 
spores showing deep apical staining, f. Two spores in a rod. This is very frequent, g. Rod 
with two spores, only one of which has matured, h. Portion of a zoogloea. 4 oc. 2 mm. 
Fig. 17. Section of a leaf of a seedling potato plant , artificially infected with Bacteria from a pure 
culture. Two tubes are seen entering at the lower surface and growing towards the vascular tissues. 
The plant was grown and infected in a cool greenhouse. 2 oc. D obj. 
Fig. 18. An artificially infected leaf of a mature plant from a tuber, grown and infected in the 
open. Many of the Bacteria used for infection are still clinging to the upper epidermis. A single 
tube is seen passing along a palisade cell. Bacteria are also seen in the intercellular spaces and 
inside the epidermal cells. 4 oc. 2 mm. 
Fig. 19. Represents a case of infection without definite tubes. Numerous Bacteria may be seen 
passing between the cells and also inside an epidermal cell. 4 oc. 2 mm. 
Fig. 20. Several tubes from a leaf of ‘ White City' variety of potato, artificially infected on the 
University Farm. The tubes are typical and exactly like those found in naturally infected plants. 
The swellings where the cell-walls of the host plant are pierced are very marked. 4 oc. 2 mm. 
N.B. — It should be noted that in all cases the tubes run across the veins and lamina, and never 
in any case parallel to the surface of the leaf. 
