312 
Nature and Cause of the Potato Disease. 
found attached to, minute vessels in the tuber ; sometimes it is 
seen vegetating from the channel at the end of a broken vessel, 
and at others from the side ; and it may be seen in various other 
forms also. Its chief characteristics are its adherence to a vessel, 
its fibrous form, its appearance with putrescence, and its secret 
habits, growing only internally. 
The fourth is the botritis or grape-like fungus. It grows on a 
hollow flat oval stem externally, but on a cylindrical tubular one 
internally. Externally, the stem at its apex is loaded with seed, 
arranged sometimes in the form of grapes. The seed is white, 
and so is the stem generally, but at times it is found dark- 
coloured. The form of the seed is oval, and they are so minute 
that I found them to measure from -000001 to '000005 of an 
inch in length only. The structure of this fungus is simple : its 
peduncle is a hollow membranous stem, to which the seed, 
formed of the same substance, is attached. The seed has a re- 
fractive power somewhat superior to water, and analogous to that 
of potato-starch, with which it agrees in density, form, and 
colour ; and so well does it resemble potato-starch, that if mixed 
with it in the tuber it could only be distinguished by the hilum, 
which granules of potato-starch do not possess. 
There is no difficulty, however, in detecting this fungus in the 
stem, leaves, and hairs of a plant, as the seed and stem are gene- 
rally found united to each other, in which state they cannot be 
mistaken. 1 have discovered it in all parts of the present year's 
plants, but not in the tubers. I have traced it from the stem, as 
it issues from the set, through its entire length to the leaves, in 
the leaves themselves, and even in the hairs also ; but by the 
most minute examination I have failed to discover even a trace of 
it in the young tubers, or the sets from which the plant grew. 
Plate 8 represents a cell infested with it. This cell was found 
in a cutting taken from the base of a stem immediately above the 
old set. The procreative power of this fungus is so great that it 
exceeds calculation. Myriads of seeds are contained in a single 
cell of the potato plant ; and they are so minute that a pea of one 
line in diameter would, according to my measurement and figures, 
contain the astonishing number of 800 billions of them. I have 
discovered it in this season's plants, both externally and internally; 
and I have seen its small fibrous tubes adhering to the hairs of a 
plant until the apex of the hair has become so loaded with its 
seeds as to be no unapt representation of a knotted club. This 
fungus seems to precede all the other kinds we have noticed, 
and to disappear as they become sensible. I'hus we fmd tlie 
Ijotritis in the apparently sound ]iarts of the leaves and stem 
accompanied by no change of colour ; then we find the radiate*! 
in the stem and tuber, with change of colour; and, lastly, the 
