of the Radiation of Heat in the Atmosphere. 69 
less so than the fig or orange tree ; possibly, having been formed 
by nature for intertropical climates, its powers of life may be- 
come fatigued and exhausted by the length of a bright English 
summer’s day in a high temperature. When we reflect on the 
constitution of the natural climate of the pine-apple, we can 
easily understand the utility of this suggestion. For whatever 
be the intensity of the scorching rays to which the plant is ex- 
posed in its native country, the long sleep through a tropical 
night is sufficient to restore its energy. On the contrary, in the 
fruiting-house, in which the heat is as great as ever experienced 
in the Brazils, it is exposed to a blaze of light during a sum- 
mer’s day of seventeen hours ; while, on the other hand, the short 
and imperfect refreshment which it can receive in a midsum- 
mer’s night, is by no means sufficient to restore its active powers. 
It is certain, that, if more pains were used to equalize in this, as 
in other respects, the situation of the plants with that of their na- 
tive soil, botanists would have it more frequently in their power 
to examine the fructification of many plants which, at present, 
shew no inclination to put forth their flowers. 
Within the Tropics, the productions of the vegetable kingdom 
are never endangered, by any interruption in the regular alter- 
nation of atmospherical variations. The undeviating regularity 
in the succession of the agents which influence organized beings, 
induces extreme sensitiveness in plants to small changes in the 
condition of the circumambient medium. Hence, when the tem- 
perature of the air declines towards evening, the irritability of 
the plants is excited by the approach of cold ; and, before the sun 
is set, flowers have closed their petals, and the delicate pinnated 
foliage has collapsed to present a further loss of heat by radia- 
tion. Even in the Temperate Zone, in those parts where con- 
tinental climates prevail, or climates distinguished by a great 
difference between the summer’s heat and the cold of winter, as 
in Russia, and in the central lands of Asia and America, plants 
are endowed with a similar constitution. Being subject during 
the winter to a degree of cold far below that at which their vital 
powers are suspended, they acquire a high organic susceptibility 
to the stimulus of light and heat, so that no sooner is the frost 
relaxed, than vegetation is renewed with a force and celerity un- 
known in this country. It is on this account that the mildness 
