56 Diseases of Greenhouse Crops 



Construction and Management of Hothouses 

 AS Affected by Light Conditions 



From the previous discussion, it is now evident 

 that to improve light conditions indoors will tend to 

 produce normal growth and to hasten maturity. 

 The greater the photosynthesis, the more rapid the 

 assimilation of plant food, hence the quicker the 

 growth. An increased amount of heat can never 

 replace the normal effect of an increase of light for 

 those crops which most require it. Many growers,- 

 especially those who possess poorly constructed 

 houses, often attempt to substitute heat for light 

 in forcing. The result is generally a failure, because 

 diseases of all sorts find the tender weak plants an 

 easy prey to their attacks. The modem greenhouse 

 man is partly solving the light problem by con- 

 structing larger houses and using larger glass. As 

 a result of this, more air space and more uniform 

 moisture distribution is assured. The double-thick, 

 third quality glass, used in previous years, is now 

 being replaced by a good grade of double-thick, sec- 

 ond quality glass. Improvements are also being 

 introduced in the roof angles, for these, too, mean 

 added and better light. The more closely the angle 

 of the roof coincides with the right angle cast by 

 the sun's rays, the greater the amount of light that 

 may reach the indoor plants. In the old form of 

 houses, many of which are still in existence, the 

 glass used was from two to seven inches long and 

 two to five inches wide, and was often lapped more 



