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. 



