1 36 Dale . — A Bacterial Disease of Potato Leaves. 
sections of fresh leaves the tubes are easily overlooked in the mesophyll on 
account of the presence of chlorophyll. They are also inconspicuous in 
unstained hardened material because of their small size compared with 
those in the veins. 
It was at first uncertain whether the tubes were formed inside the leaf 
and grew outwards, or whether they originated at the periphery. Various 
considerations led to the conclusion that the latter is the case. 
In early stages of the disease, when there are few or no cells containing 
Bacteria, the tubes are seen to occur at the periphery and to extend inwards 
for varying distances. They were never observed in the deeper tissues only. 
In many cases the tubes are narrow at the surface of the leaf and widen 
irregularly towards the centre, but not infrequently they show a large swell- 
ing at the surface. In the case of branching tubes the branching usually 
occurs in the neighbourhood of the vascular tissues and not near the surface. 
It is much more probable that the tubes branch than that they anastomose, 
though this does seem to occur occasionally. 
It may therefore be concluded that the tubes begin at the surface and 
pass inwards. How is the entrance effected ? In no single case , amongst 
the hundreds of sections examined, was a tube seen to enter the leaf through 
a stoma. This point has been studied carefully both in surface view and in 
transverse sections. In every case the tube pierces the epidermis , almost 
always where two adjacent cells are in contact. This may be seen by 
looking at any of the diagrams representing tubes at the surface, either in 
surface view or in section. 
A horizontal section of a vein is shown in PI. XV, Fig. 1, where the 
epidermal cells are seen in surface view. Two stomata are represented sur- 
rounded by epidermal cells, on the walls of which, in almost every case 
where two adjacent cells are in contact, are numerous tubes in early stages 
of formation. The unaltered tissues of the host are stained green, the parts 
attacked by the disease take up the red colour. 
Figs. 2 and 3 show two later stages. In surface views there is usually 
a deeply staining, approximately circular mass, often with an irregular 
lumen in the centre. Comparison with a section (Fig. 4) shows that this 
deep red area is a more or less hemispherical mass at the mouth of the 
tube (cf. Figs. 2 and 3 with Fig. 4). 
In other cases the tube is extremely narrow at the surface (Figs. 5 and 
8, &c.). Whether narrow or wide, it frequently shows one or more points 
projecting above the surface (Figs. 4, 5, 10, &c.). 
After piercing the epidermis the tube grows at right angles to the 
surface along the middle lamella of two adjacent cells. Rarely it seems to 
cross the centre of a cell. At the point where the tube pierces a wall there 
is often a swelling (PL XV, Fig. 6, and PL XVI, Fig. 20). On arriving at 
an intercellular space the tube crosses the space (Fig. 7), often with a very 
