137 
Dale — A Bacterial Disease of Potato Leaves. 
tortuous course, and passes between two cells on the other side. Occasion- 
ally tubes branch (Fig. 9) and, more rarely, anastomose (Fig. 8). Sooner 
or later the tube enters a mesophyll cell (Figs. 8, 9, and 12), in some cases 
near the epidermis (Fig. 8), or even into an epidermal cell, but more usually 
into a cell near the vascular tissue (Figs. 8 and 9). Though frequently occur- 
ring in the vascular tissues, tubes are scarcely ever seen in the vessels, 
though often in the phloem. The cells in which the tubes end are filled 
with Bacteria which usually stain red. 
The tubes themselves are very irregular in outline and vary considerably 
in thickness. Irregular constrictions are common (PL XV, Figs. 5, 7 ; PL 
XVI, Fig. 11), and these are independent of the cell-walls of the host plant, 
as they are clearly visible in tubes crossing intercellular spaces (Fig. 7). The 
walls often present a corrugated appearance (Figs. 5 and it). On the other 
hand, some tubes may be very straight and cylindrical (Figs. 6, 10, and 20). 
There is usually a lumen which is relatively narrow, though it varies in 
diameter (Figs. 6 and 7). The two sides of a tube are in many cases by 
no means parallel (Figs. 7, 11, 13). 
The growing end of a tube is generally swollen and pointed (Fig. 5). 
Such swollen ends may be seen in intercellular spaces (Fig. 7) and also in 
the cells containing Bacteria. In some cases the tubes appear to pass right 
through the Bacteria-containing cells, or several tubes may end in one cell 
(Figs. 10, 12). 
In the later stages of the disease the tissues of the host become 
extremely disorganized, especially in the neighbourhood of the veins. In 
these stages the affected tissues of the host plant, as well as the parasite, 
take up the fuchsin, so that the tubes cannot be so clearly followed. 
It is noteworthy that sections of stems show no tubes or Bacteria, or at 
most very slight traces. The disease is confined, as far as could be deter- 
mined, to the leaves. The explanation of this fact seems to be that the 
cuticle of the stem is too thick for the tubes to penetrate. In this connexion 
it may also be noted that in pure cultures of the organism grown on sterilized 
potato there was little or no growth on the c skin 5 of the tuber, whereas the 
cut surface was rapidly covered with a thick mass of Bacteria. 
3. The Parasitic Organism and its Culture. 
The appearance of the tubes and of the granule-containing cells 
strikingly recalls, in many respects, those occurring in the nodules of legu- 
minous plants, and shown by Miss Dawson 1 to contain, and to be formed by, 
organisms resembling Bacteria. Sections of nodules of Pisum sativum , 
fixed and prepared in the same manner as those of the potato leaves, 
showed similar reactions towards dyes. Morphologically there is also 
1 Dawson : ‘ Nitragin ’ and the Nodules of Leguminous Plants. Phil. Trans., Series B., 
vol. cxcii, 1899, p. 1. 
