164 VEGETABLE FORCING 
properly ventilated soon become tender and spindling, 
and they are then especially subject to the attack of 
fungous diseases. Abundant ventilation is necessary in 
order to grow strong, vigorous plants. The direct effects 
of ventilation are to reduce the humidity, to lower the 
temperature and to increase transpiration of water from 
the leaves and evaporation from the soil. Good judgment 
must be exercised in ventilating. Too much ventilation 
under certain conditions may be just as harmful as too 
little. Admitting fresh air increases the circulation of 
air in the houses, and this may be of special value in re- 
ducing humidity and in preventing or checking fungous 
diseases. "Too much ventilation is impossible during the 
summer months. In the winter time care should be taken 
to prevent cold drafts from coming into direct contact 
with the plants. 
The usual custom is to open the ridge ventilators as 
much as may be possible in the forenoon, when the tem- 
perature in the houses is rising, and to close them some- 
time in the afternoon. If the weather is fairly mild, they 
should be opened early in the morning and closed late 
in the afternoon. In the summer they are left open day 
and night, except on the approach of severe storms, when 
they should be closed to protect both crops and houses 
from possible damage. 
Shading the houses is sometimes an advantage or even 
a necessity. Whitewash made from air-slaked lime is 
generally used. Cucumbers seem to be most benefited by 
shading. 
