INFLUENCE OF CLIMATE ON PLANTS. 
109 
abruptness and violence, very injurious to the members of the vegetable world. The 
abstractions of winter leave them in a completely inversed condition in the spring ; 
and their average temperature is decidedly below that of the atmosphere in the 
day, so that they are the less able safely to part with any caloric at night. 
We conclude, therefore, that shelter is equally essential to tender plants at both 
these epochs ; but that a much thinner covering is required in the autumn than 
in the spring, because vegetation is thoroughly furnished with internal heat in the 
former, and retains a small amount only in the latter season. 
Inferior only in degree to the radiation from plants fully exposed to the atmo- 
sphere, is that of the more delicate kinds which are confined in houses. Glass has 
been before declared to be a most liberal radiator of heat ; and hence, the greater 
the amount of glazed surface a house presents, the more speedy and perfect will be 
its radiation. The temperature of a plant -house being diminished, that of the 
plants will rapidly be reduced likewise ; and as vegetation is always more or less 
susceptible according to the circumstances to which it has been habituated, those 
plants which are kept in an artificial condition will suffer considerably from the 
most trifling degree of radiation to which they may be subjected. 
To retard radiation in hot-houses, and prevent it from entailing any injurious 
consequences on the plants which they protect, recourse is generally had to the 
introduction of artificial heat. In this respect, cultivators err most egregiously. 
Radiation is effected from the external surface of the house, and the means professedly 
employed to counteract it are usually arranged near or beneath the lower surface. 
Notwithstanding the lightness and ascension of heated air, the porosity of the glass 
combined with the numerous fissures which occur at the junction of the panes, 
invariably maintains the superior stratum of atmosphere at a low temperature. The 
upper, and most tender portions of plants, thus come in contact with the coldest 
air ; and these, being rendered more susceptive by the excitation of heat from 
below, are kept in a perpetual state of conflicting exertion and endurance. 
Did plants require to be supplied with a uniformly high temperature throughout 
the winter season, the practice here denounced would be in some measure defensible. 
But this is not the case. "We hesitate not to affirm, that the total exclusion of frost 
is all that is desirable with even tropical species. How much more easily, safely, 
and effectually, then, could this be ensured, by an exterior covering to the roof! 
This appears to us the only rational mode of procedure, seeing that it is through 
the roof alone that frost can be admitted, or, more strictly speaking, heat dissi- 
pated. And though we grant the utility and propriety of applying fire heat 
in extreme cases, it should never be regarded otherwise than as an inevitable 
auxiliary. 
If it be urged as an objection to the above method, that such a measure would 
exclude light as well as frost, the torpidity of the plants under the circumstances 
recommended will render the continued action of light unnecessary. Complete 
dormancy, and partial darkness, are by no means incompatible ; nor is the latter, 
when coincident with the former, at all detrimental. When maturation is duly 
