ON THE DAHLIA. 
267 
become blackened ; and if the frost continues for a whole night, 
the plants never rally again for that season, though they may con¬ 
tinue to languish as unsightly objects. In consequence of this 
perfect hardiness down to a certain degree, and speedy destruction 
when that degree is arrived at, a bed of Dahlias often shows a 
very remarkable contrast on two consecutive days. During the 
first of these days they may be all in vigour of growth and fulness 
of beauty, and on the morning of the second, their leaves may be 
all black and shrivelled, and their flowers withered and drooping. 
One circumstance is worthy of notice, and that is, the greater 
injury that they suffer in low situations than in high ones, even 
although the latter are considerably the colder. We find, that 
after a severe night, the Dahlias in a low and humid valley are 
completely destroyed, while those upon the higher grounds are 
hardly touched. This shows us that it is not the mere cold, but 
the humidity of the atmosphere floating in small spiculae of ice, 
which does the mischief to these plants. It does this, even though 
the quantity is too small for forming a hoar-frost, or even a per¬ 
ceptible fog ; and the destructive effect is always increased by 
wind. Thence it is obvious, that the cold damp of the atmo¬ 
sphere is the real cause of the injury ; and therefore, if it is wished 
to have Dahlias late in bloom, and some of them bloom onward 
iill November is pretty far advanced, they should never be placed 
in situations where damp is liable to accumulate in the atmo¬ 
sphere. All plants which are natives of dry climates suffer from 
even slight frosts in moist situations. We find this exemplified 
in the potato, which is a native of the same regions as the Dahlia, 
and bears some general resemblance to it in habit. Both have 
vigorous annual stems ; and both produce a great quantity of un¬ 
derground substance during the season. Both are also instinct 
with life, and will form those underground productions at any 
node or joint of the stems, if they are properly earthed up. 
There is, however, this difference between them, that the under¬ 
ground production of the potato consists of distinct and separate 
tubers, which have no crown or general connexion from which a 
stem can be sent up in the following or any future season. The 
vitality is in the tubers, and the germs in the eyes of the tubers, 
and in these only. The potato is therefore a true annual. The 
Dahlia, on the other hand, though it multiplies greatly in the 
tuberous or fusiform part, has that part consisting of roots, con- 
