VENTILATING FORCING-HOUSES. 79 



be secured by means of sash at the peak alone, thus obvi- 

 ating the danger of currents of cold air which arises when 

 there are ventilating sashes in both the sides and top. In 

 very large houses, particularly in those of the shed roof 

 pattern, it may be necessary to place ventilating openings 

 in the walls, more especially on the back or high side of the 

 house. Ventilating openings should be removed as far as 

 possible from the plants in order to reduce the danger of 

 cold draughts to the utmost. 



Particular care should be taken with the ventilating dur- 

 ing dull, cold weather, when the plants become soft and are 

 very quickly injured by draughts. It is not necessary to 

 ventilate primarily for the purpose of securing fresh air, but 

 to regulate the temperature of the house. When the house 

 becomes over wet and close, it is often necessary to ventilate 

 for the purpose of drying it out. The larger the house, the 

 less, as a rule, is the necessity of ventilating. 



Houses are shaded to prevent the sun from scorching the 

 plants. The shading is supplied by coating the glass with 

 some white covering, like whitewash. The necessity for 

 shading may be largely obviated by not allowing the plants 

 to become over-vigorous, sappy, and soft. The greatest 

 danger from sun-scald occurs after a spell of dark and wet 

 weather. It is then essential to keep the house rather cool 

 when the weather brightens, and it may be necessary to 

 shade it. Plants which are suffering from root-galls or other 

 disease of the roots, or those which are growing in very 

 leachy soils or on very shallow benches, may have to be 

 shaded in order to check the evaporation from their tops 

 and thereby prevent them from wilting. Many plants thrive 

 best under shaded roofs, but amongst the forced vegetables 

 there is only the English or frame cucumber which appears 

 to thrive best under a tempered light. This plant was 

 developed in the humid and soft climate of England, and it 

 seeems to be impatient of our violent suns ; yet it may be 

 made to withstand the sun if grown rather slowly. 



For plants which require permanent shading, a paint 



