406 FIRST FORMS OF VEGETATION. 
spreads rapidly down the stem, till in a very 
short time the whole of the plant above ground 
is destroyed. The disease still spreads its 
ravages, until ultimately it reaches ‘and penetrates 
with its spawn the tubers, the substance of which, 
when affected, speedily turns brown, emits a 
very peculiar and unpleasant odour, and soon 
decays to a fetid watery matter. The diseased 
tuber is strongly alkaline, from the presence 
of ammonia; the sound potato having an acid 
reaction. The starch of the diseased potatoes 
is often unaffected by the parasite, retaining 
its nutritive properties, and may be separated 
easily by rasping the peeled tubers upon a grater 
into a tub of cold water. 
The potato-blight belongs to a large genus of very 
destructive fungi, affecting many of our vegetables 
and fruits ; but as a species it is a comparatively 
recent introduction. Facts derived from numer- 
ous sources lead to the conclusion that it did 
not exist in this country previous to the autumn 
of 1844. All the naturalists who examined it then 
declared it to be quite new to them. It is con- 
sidered by the most eminent botanists to be of 
American origin, peculiar to the potato, and ac- 
companying it wherever it grows wild in its native 
country, as the smut accompanies the corn in this. 
From South America it was first brought to St. 
