146 Dale. — A Bacterial Disease of Potato Leaves. 
withering of the leaves often begins at the apex or round the edges, and 
that the veins are the parts chiefly affected, seems to be explained by 
the fact (already pointed out, p. 144) that the water excreted at night round 
the edges of the leaves at the ends of the veins serves as a medium for the 
Bacteria, which thus obtain the large amount of moisture necessary for 
their growth. On the lower surface of the leaves the angle between the 
projecting vein and the lamina is also a common point of infection, doubtless 
because it affords shelter for the parasite (cf. Fig. 11). 
The Seedling Plants. Only one case of really clear and unmistakable 
tubes was found in cutting sections of seedling potato leaves. This was in 
a hand section of a comparatively large vein (Fig. 17). But other sections 
showed Bacteria in the cells of the affected parts of the leaves. 
The Mature Potato Plants. These gave better results, though the 
experiments had to be stopped sooner than was desirable on account 
of the attack of Pliytophthora. But definite tubes were obtained, and also 
early stages of infections were observed with less well defined tubes. In all 
cases, as in the spontaneously infected plants, the entrance of the Bacteria 
was through the epidermis and never by means of the stomata. Fig. 18 is 
an example of a tube which shows clearly the contained Bacteria, whereas 
in the naturally infected tubes distinct Bacteria were rarely if ever seen, 
perhaps because the tubes were older and emptied of their contents. This 
section also shows many Bacteria between the cells and outside the tube 
from which they may have escaped. It will be noticed that the tube 
is lying on the surface of the wall, on which are also Bacteria. Inter- 
cellular spaces are also to be seen with bacterial contents. Apparently the 
tubes are in an early stage of development, and the Bacteria have not 
yet penetrated into the interior of the cells, nor far into the tissues. 
Fig. 19 represents a case of infection without definite tubes, in which the 
bacteria make their way between the epidermal cells and the palisade cells 
and cross the intercellular spaces. As in the last figure the epidermal cells 
are injured, apparently by the entry of the Bacteria. In both cases Bacteria 
occur outside the section on the surface of the leaf. The Bacteria, both in 
spontaneously and in artificially infected leaves, cling so tenaciously to the 
outer surface of the leaf that they are not washed off by the various processes 
involved in making permanent microtome sections. It is therefore often 
possible to determine the exact spot where the infection was made. 
In the case of the tomato seedlings no trace of tubes was found, but 
Bacteria were abundant inside the leaves, especially in the intercellular 
spaces and in the thin-walled cells of the larger veins. In both potato and 
tomato plants the epidermal cells were attacked very seriously. This is 
not usually the case in naturally infected plants. Perhaps the contact of 
the larger mass of material, often including some of the potato tuber under- 
lying the Bacteria, may account for the greater injury. 
