135 
Dale . — A Bacterial Disease of Potato Leaves . 
A leaf of * plant i a * was cut to see if any change had taken place in 
the tubes in the veins. They were still quite distinct, only slightly branched, 
and they nearly all ended in cells filled with stainable granules. 
Plants with this disease almost invariably show marked symptoms 
of ‘ leaf-curl 5 and £ leaf-roll \ but { leaf-curl ’ may occur in plants which 
have not, as far as could be determined, yet been attacked by the 
organism causing the disease. A plant showing marked ‘ leaf-curl but 
no traces of browning or wilting, was cut down, and each internode and 
its adjoining leaf put into separate bottles in absolute alcohol. A micro- 
scopic examination of these leaves and stems showed no traces of tubes, 
granules, Fungi, or any other sign of disease beyond the actual ‘ curl ’. On 
the other hand, the brown veins and tubes may occur in leaves which are 
flat and remain so. 
In the spring of 1910, and again in 1911, the disease appeared sponta- 
neously in plants grown in pots in a cool greenhouse, some from the original 
‘blind’ stock and some from tubers which were apparently healthy. Attempts 
were also made to grow seedlings for infection in sterile soil, but, though the 
seedlings readily germinated, further growth would not take place in sterile 
soil, so that plants grown in ordinary soil had to be used. The tubes obtained 
were in all cases similar. As before, some of the plants, but not all, showed 
symptoms of ‘ curl \ 
a. Histological Characters of the affected Potato Plants. 
Leaves from the three plants described above were fixed in chrome- 
acetic acid. Various stains were used, but by far the best method was 
found to be double staining with diamond fuchsin and light green. The 
sections were placed for about two minutes in fuchsin, washed in water, then 
very rapidly stained with light green by pouring the stain on the slide and 
washing it off again at 07 ice with absolute alcohol, which must not be left 
on the slide more than a minute before the sections are cleared in oil of 
cloves. Hand sections, however, require a longer time in absolute alcohol 
after the light green, in order to differentiate the stains and wash out the 
excess of fuchsin. 
In the sections thus treated the tubes and Bacteria were stained bright 
red, while all the tissues of the host plant, with the exception of those which 
were lignified and of the nuclei, became green. It is therefore quite easy to 
trace the distribution of the parasite in the host plant by means of the 
coloration. 
The tubes also stained well with alkaline methylene blue. In some 
cases the sections were treated with alcoholic caustic potash, before staining, 
to swell the tubes. 
In stained microtome sections the tubes are to be found in all parts of 
the leaf, though they are largest and most numerous in the large veins. In 
