20 DISEASES OF GLASSHOUSE PLANTS 
the yield and health of the crop, and suggests that houses 
built so that the crop receives the maximum morning 
light will give the best results. Another point of im- 
portance, in view of the uniform construction of glass- 
houses in this country, is the effects produced by roofs 
of different angles. Past work has shown that the more 
nearly the roof is placed at right angles to the sun’s 
rays the more light is transmitted. Records made by 
Stone in February showed that a house with a roof 
angle of 46° gave 18 per cent more light than one with a 
32° roof angle. 
Considerable differences exist in the light-transmitting 
properties of various types of glass, as was evident to 
purchasers of this material shortly after the war years ; 
and it is common knowledge that irregular surfaces, 
flaws, bubbles, etc., in the glass are detrimental. 
Experienced growers are extremely careful in selecting 
glass for their houses, and panes with flaws are invariably 
rejected. Bubbles and irregularities act as lenses and 
may cause “ scorching ”’ in leaves near to the glass, and 
the former frequently are painted over by growers of 
such delicate plants as lilies. Stone has shown that the 
amount of light excluded by different samples of glass 
may vary from 13 per cent to 36 per cent of the total 
light. 
Researches of some interest to practical glasshouse 
growers were performed by, Flammarion (21), who 
compared the light and heat values of white glass with 
those of red, green, and blue glass. He showed that the 
temperature within the house varied with the colour 
of the glass. Blue, in virtue of its great absorbing power, 
gave the lowest temperature, while green, red, and white 
gave increasingly higher temperatures. The height of 
sensitive plants grown in a red house was fifteen times 
as great as that ina blue house. Plants in a red house 
came into bloom first, and their foliage was lighter in 
colour than those grown in a white house, while the 
foliage under blue glass was still darker incolour. After 
