WINTER MANAGEMENT OE PLANTS IN FRAMES. 229 
flower earlier than they would flower naturally, they must have 
free exposure to light as well as fresh air at such times as may 
be necessary ; the degree of which depends greatly on the habit 
of the plant, and that again upon the atmosphere which is natural 
to it; which in general may be judged of from the physical cha¬ 
racter of its native region. Commonly speaking, the more tro¬ 
pical the plant is, and the more dry the atmosphere when the 
plant comes into flower, the more light is necessary in forcing 
it, but if the plant flowers in a clouded atmosphere or under the 
shade of thick trees, much less light is requisite. Thus in the 
forcing of plants from different physical localities, even though 
they have the same length of day and the same average tempe¬ 
rature, there is some little difficulty in doing justice to both; it 
is by experience alone the cultivator is able to determine the 
exact amount required by each. 
Frames, especially if there are several of them, possess ad¬ 
vantages over a greenhouse for the winter management of half- 
hardy plants, because all degrees of heat may be given them 
from the mere protection from frost to a much higher degree than 
that of the warmest climate in its hottest season. Simply placing 
a frame over the plants and raising it at the corners with brick¬ 
bats is a sufficient protection for many plants, especially for 
such as require much,air during the winter; a common frame, 
if close at the top, has a temperature considerably higher than 
the atmosphere without, even though it is elevated for the ad¬ 
mission of air as described. 
The reason is this, the earth when not exposed to the imme¬ 
diate action of the sun is continually giving out heat by radia¬ 
tion ; this heat naturally warms the air nearest the surface, and 
if a substance of far less substantial texture than the glass of 
any frame is so placed as to prevent the heat from ascending 
intQ the wide expanse, the region below the intercepting surface 
is always higher in temperature than it otherwise would be; 
clear nights are always colder than cloudy ones, although the 
general temperature of the twenty-four hours should be exactly 
the same. It is from this reason that though the air is other¬ 
wise well disposed for its formation, dew is not produced in 
cloudy nights so readily or so copiously as on nights which are 
clear. This is a wise provision of Nature, as it protects plants 
