Nature and Cause of the Potato Disease. 
311 
Plate 1. 
DisejaeJ Cell as observeJ in stem, at a joint, in this season's plant. 
Pow er of object-glass used, •ISO diameters. 
afterwards exposing it to a damp, dark, and moderately warm 
atmosphere. A temperature of from 40° to 60° will do very w ell. 
The third kind, which I have called radiated, from its form 
and general appearance, is found only internally among the 
vessels of the tuber and plant. I have never observed it until 
disease has made considerable progress, when it frequently ap- 
pears as an elongation of the vessels and fibres. It is difficult to 
determine whether many of the masses that resemble fungi be so 
or not ; for the brown colouring matter which always accom- 
panies the decomposition of the vessels, not only renders an ex- 
amination of them burdensome, but presents such a diversity of 
form as renders it vexatiously troublesome to distinguish the 
ultimate fibre from the fungus itself; but although in some cases 
this difficulty occurs, yet in others, as a compensation, we have 
none whatever, for we perceive the vessels and the fungi growing 
from them, as represented in plate 7, figs. 2 and 4. Fig. 2 is the 
form in which it is found in the stem in the first stages of the 
disease, and fig. 4 in that of the tuber. 
Putrescence produces this kind as well as the boleti ; and 
wherever the boletus appears, the radiated fungus always accom- 
panies it, the radiated being internal, and the boleti external. 
The radiated form, fig. 4, plate 7, grows from, and is generally 
