6 ORCHIDS. 
require no shading, the exceptions being principally a few 
Dendrobiums from tropical Australia. The effect of the 
sun’s rays passing through glass is to reduce the amount 
of light, and to create an increase in the temperature. 
The result is that those Orchids which, in a natural state, 
are exposed to the full blaze of a tropical sun, are unable 
to withstand the scorching heat of an unshaded glass-house 
in bright sunshine. Attempts have been made to grow 
Cattleyas without shading, but this can only be done in 
comparatively lofty houses, where the plants stand several 
feet from the glass, and a free circulation of air passes 
over them. Even then, during hot summers, the practice 
has been found disadvantageous. 
For amateur cultivation, and for mixed collections, every 
house must be supplied with shading. The best material 
for the purpose is a thin, white canvas or netting. This 
should be nailed or otherwise fastened along the top of 
the house, and attached at the bottom to rollers, which 
can be let down and drawn up at will. It is a great advan- 
tage to have strips of wood or iron rods strong enough to 
support roller and blinds running from the top of the roof 
to the bottom, at a distance of about 6in. above the glass, 
thereby allowing a free circulation of air underneath. 
When the canvas lies flat on the glass it prevents the 
outward passage of the heated air at the top of the house. 
For the intermediate and tropical houses fixed shading 
is not advisable, except at the ends and other places not 
covered by the blinds. For the cool Odontoglossum and 
Masdevallia house it may be used. In some establishments, 
indeed, it is the practice to thickly coat the glass with 
one of the various mixtures prepared for permanent shad- 
ing, and thus the use of roller blinds is dispensed with. 
But where the saving of time is not so much an object, 
it is better to cover the glass with a thin film, and to use 
