24. Diseases of the Potatoe. 
Mr Jobn Shirreff takes a general and philosophical view 
of the cause of disease in the potatoe crop, and though, no 
doubt, his observations are particularly applicable to the curl, 
still they will apply equally well to the tavnt ; for the con- 
nection between the two diseases is so intimate, that you have 
seen Mr. Dickson’s observation is, that some sets “did not 
vegetate at all,” that is, failed, “or produced curled plants.” 
Mr. Shirreff adopts the general doctrine broached by Mr. 
Knight. “The maximum of the duration of the life of any 
individual vegetable or animal,” he says, “is predetermined 
by nature, under whatever circumstances the individual may 
be placed; the minimum, on the other hand, is determined 
by these very circumstances. Admitting, then, that a pota- 
toe might reproduce itself from tubers for a great number of 
years in the shady woods of Peru, it seems destined to be- 
come abortive in the cultivated champaign of Britain, inso- 
much that not a single healthy plant of any sort of potatoe 
that yields berries, and which was in culture twenty years 
ago, can now be produced.” Mr. Shirreff concludes, there- 
fore, that the potatoe is to be considered a short-lived plant, 
and that though its health and vigor may be prolonged by 
rearing it in elevated or in shady situations, or by cropping 
the flowers, and thus preventing the plants from exhausting 
themselves, the only sure way to obtain vigorous plants, and 
to ensure produetive crops, is to have frequent recourse to 
new varieties from seed. The same view had occurred to Dr. 
Hunter, who, in his Georgical essays, has lintted the duration 
of a variety in a state of perfection to fourteen years. 
The fact ascertained by Mr. Knight deserves to be noticed. 
That by planting late in the season, an exhausted good 
variety, may, in a great measure be restored; that is, the 
tuber resulting from the late planting, when again planted at 
the ordinary season, produces the kind in its pristine vigor 
and of its former size. IJtis obvious that all these opinions 
