18 DISEASES OF GLASSHOUSE PLANTS 
upon crop production and the ability of the plant to 
resist disease. Not only are the physical, chemical, 
and biological factors of the soil of great importance, 
but a whole series of others which are related 
to the supply of heat and light are equally important. 
Among these may be cited the aspect of the house, 
angle of the roof, size and quality of the glass, size, 
position, and number of purlins, posts, bars, etc., amount 
of air space, and system of ventilation. 
(a) Light in Relation to Plant Growth.—The importance 
of the light factor will be immediately realized when 
one considers that practically 95 per cent of the plant 
substance is produced from the carbon dioxide of the 
air by the chemical activity of light, working in con- 
junction with the chlorophyll, or green colouring matter, 
of the leaves. All parts of the spectrum rays do not 
have the same effect upon plant development, for the 
orange and red rays are especially concerned with the 
assimilatory processes, while the blue, indigo, and 
violet rays stimulate growth. 
Contrary to the general idea, plants grow. most at 
night and least in the day; but during the day they 
manufacture their food from the air under the influence 
of light. Investigations in the past have indicated that 
light has a markedly inhibiting effect upon plant growth, 
but, on the other hand, that it assists the development 
of the supporting tissues. When grown in darkness 
plants become yellowish white in colour owing to reduced 
chlorophyll production. They become etiolated and 
develop long, weak leaves and shoots. It is thus obvious 
that the light intensity and the nature of the light rays 
which penetrate the glass have an important bearing 
upon success in glasshouse culture. This being so, it 
is important to consider the many factors in glasshouse 
construction which influence the quality and quantity of 
light available to the plants the houses may contain. 
(6) Glasshouse Construction in Relation to the Light 
Factor—When compared with the modern types, the 
