82 THE PRINCIPLES OF FLORICULTURE 



are sometimes used in place of pipe columns (Fig. 9, 

 center). 



As these columns cast considerable shade, which is 

 objectionable, they have been eliminated in many houses 

 and span-trusses used to support the roofs. When 

 these iron trusses are attached to iron, the results are 

 highly satisfactory, but when attached to wood, as in 

 the early forms of truss-work, decay of the wood quickly 

 weakened the supporting strength of the truss. Where 

 many trusses are used, however, and these are of small 

 dimensions, as they usually must be in order not to detract 

 from the light of the house, much time must be spent in 

 keeping them adjusted. The cost of painting is also con- 

 siderable, and these items should be taken into considera- 

 tion in estimating the value of a house. 



65. Ventilators. Ventilators are usually placed on 

 each side of the ridge and frequently on each side of the 

 house below the eaves. Sometimes the side ventilators 

 are below the glazing-sill, in which case panel ventilators 

 are often used. They are advantageous when benches are 

 placed against the sides of the houses, for cold air does 

 not blow directly on the plants. If the fresh air passes 

 through the heating pipes which are along the sides of the 

 houses and below the benches, the atmosphere is modified 

 before it comes in contact with the plants. Orchid houses, 

 which require an abundance of moderately heated, fresh 

 air, are frequently ventilated in this way. 



When violets and sweet peas are grown, there should be 

 wide ventilators below the eaves, and it is generally con- 

 sidered that such should be in carnation houses, but in rose 

 houses such ventilators are never used. They are usually 

 wide enough so that one and one-half panes of glass, six- 

 teen by twenty-four inches, are used. 



