BORTHWICK—WARTY DISEASE OF POTATo. 117 
the winter, and experiment’ has proved that they are able to 
propagate the disease from year to year. 
The extent of damage done varies. Fig. 3 shows the diseased 
tubers in various stages of attack. On the upper offset to the 
left, the oldest or first-formed tubers have been totally destroyed. 
It is interesting to note, as may be seen in this figure and also 
in Fig. 6, that the deeper-lying tubers have escaped, which would 
suggest that the fungus confines itself to the upper layers of soil. 
Tubers which have been badly attacked rot away in the ground 
or dry and shrivel up when kept. Figs. 1 and 2 are photographs 
of specimens which had lain in a box for a few weeks. 
On the tubers themselves the first symptoms of attack appear 
at the “eyes,” where warty excrescences of various sizes may be 
seen (see Fig. 18). As regards the nature of these excrescences, 
Professor Potter says :—“ Judging from some sections in an early 
“ stage, the attack appears to commence at the ‘eyes,’ the parasite 
“easily gaining an entrance into the outer cells of the young and 
“tender structures which normally would develop into leaves. In 
“these the cells are readily stimulated to divide, and, as a result of 
“the injury caused by the parasitic invasion, irregular cell-division 
“is set up. The destruction of any one cell causes those in 
“contact with it to divide in the attempt to heal the wound; 
“when these latter cells are attacked in their turn, a further cell- 
“division is induced, and by a repetition of the process the leaf- 
“protuberances become converted into an irregular cell-mass 
“which in the initial stages may be seen as finger-like out- 
“growths. From these points the irritation spreads along the 
“cork-cambium, so that the cells over a large portion of the 
“surface of the potato gradually undergo this irregular division 
“and multiplication, which is extended also into the internal 
* tissues,” 
I entirely agree with the statements of the author just quoted, 
and in support of the view of the foliar origin of these protuber- 
ances I may point out that they are to be found in the foliage 
leaves themselves—a fact which, so far as I know, has not been 
recorded previously. In Fig. 4 one or two groups of excres- 
cences may be seen on the separate lobes which on close examina- 
tion appeared as branches from the leaf. The nature of these 
branches is shown in Fig. 5 ; and Fig. 9, which is also a photo- 
