AGRICULTURAL METEOROLOGY. 181 
cised upon it. This author cites, for example, the lilac, because its 
elasticity is very remarkable. 
When plants die only after having endured the action of cold during 
several days, their destruction is not owing to congelation, but more to 
the arrest of nutrition and transpiration. Finally, Hoffmann remarks 
that in the observations on the degree of cold to whieh plants may be 
submitted, it is necessary to beware of concluding from the temperature 
indicated by the thermometer that which the plants can support, 
because their exposition and their radiation may considerably modify 
their thermal condition. After his observations on the barley, the flax, 
and the water-cress, it resulted that the same plants may be affected in 
different manners, relative to the frosts, at different moments of their 
existence. He specifies, as a fact worthy of remark, that constantly 
all or nearly all the stalks, proceeding from one seed-plot, are affeeted 
in the same manner by the frosts, while they support its influence in 
different ways when they proceed from different seed-plots of the same 
species. Sometimes the young stalks are most sensitive to the action 
of the cold; sometimes, on the contrary, the oldest suffer most; and 
then, again, the tallest are affected most readily.* 
DIFFERENT METHODS FOR PROTECTING PLANTS FROM FROST, 
For protecting the plants of warm countries from frost, Thouin has 
employed several methods with success. The first consisted of placing 
these plants in inclesures and shelters from strong light, the day 
of the frost, and some moments before the appearance of the sun. 
Plants which had passed a very cold night in the open air, and whose 
leaves, covered with white frost, had become stiff and brittle, thawed 
very slowly, and did not experience the accidents which occurred to the 
same species of plants not placed within these shelters. 
The second method consisted of showering the frozen plants, using a 
watering-pot for the small ones, a skimmer-syringe for shrubs, and a 
hand-pump for large trees in boxes, the elevation exceeding fourteen 
feet. We sprinkled only at the instant when the first rays of the sun, 
which were very brilliant the first two days of the frost, began to fall 
upon them. The water, less cold than the air, in melting the frost 
which covered the leaves, prevented the injurious effects of the rays 
of the sun; but if the plants were sprinkled before the rising of the 
sun, the water congealed on them and increased the intensity of the 
cold. The leaves of some trees with large tops, which were in the shade 
at the instant of the sprinkling of the water, were covered with a slight 
layer of ice, which thawed only after some hours. All these leaves and 
branches grew yellow or blackish at the end of some days, and after- 
ward fell off entirely. 
The third method consisted of interposing between the plants touched 
by the frost and the rays of the sun a thick cloud of smoke. At various 
distances in the vicinity of the plants, and above the reach of the wind, 
were established piles of half-dry grass, damp leaves, or partly ground 
manure, which were fired as the sun was about to appear, and sustained 
until the frost melted and the water which was produced fell at 
the foot of the trees. The effect of this mode of thawing will be more 
prompt and sure if the smoke is carried by the wind directly uvon the 
trees, which result is desirable, but not indispensable; for it suffices if 
the rays of the sun are broken, or obscured, so that they do not act upon 
ns und Wachsthum, oder Grundziige der Pflanzenklimatologie, Leipzig, 
, 8vo. 
