In conclusion it may be stated that lack of light induces the 

 formation of weak tissue by restricting its development, and 

 many plants are rendered less immune to attacks from saprophy- 

 tic and parasitic fungi by exposure to improper light conditions. 



THE RELATION OF LIGHT TO GREENHOUSE CONSTRUC- 

 TION AND MANAGEMENT. 



The most important changes in connection with greenhouse 

 construction have been based on an increased knowledge of meth- 

 ods of obtaining better light. The early houses were notable for 

 the great amount of timber used, the very small, inferior glass, 

 and other light obstructing features. As the greenhouse industry 

 increased, larger glass and lighter frames were employed ; and de- 

 velopment in this line has been so marked that the light conditions 

 in a modem house are quite unlike those common forty years ago. 

 One of the principal results of improving the light conditions 

 has been to greatly hasten maturity, to say nothing of the better 

 crops secured. Since photosynthesis or carbon assimilation is in 

 general proportionate to light intensity, any increase in the light 

 would enable plants to assimilate more food material and to devel- 

 op more rapidly. Even with an increased amount of heat it 

 would be impossible to mature crops in the old type of house in 

 the same time required in the modern house, the light being in- 

 sufficient during the dark months of winter to build up the plastic 

 substances required for the growth of the crop. Some growers 

 who own old, poorly lighted houses have attempted to substitute 

 heat for light in forcing, but diseases of all sorts usually follow such 

 a practice. 



Cucumbers, lettuce, etc. have a relatively high light require- 

 ment, and any unfavorable change in the light, intensity retards 

 their development. For instance, a few days of cloudy weather 

 in the winter months so lessens carbon assimilation that little 

 growth and often considerable financial loss results. 



Larger houses and larger glass have had a very important 

 bearing not only on crop production and the better control of dis- 

 ease, but on the economy of construction and management. 

 Briefly, the evolution in greenhouse construction has assured more 

 and better light, more air space, more even m.oisture conditions, 

 less susceptibility to disease, improved crops and greater economy 

 in construction and management, since a large house can be built 

 relatively more cheaply and managed more 'economically than a 

 small one. 



At the present time a good grade of double thick, second quality 

 glass is used in greenhouse construction, but formerly much double 

 thick, third quality was used, — an inferior glass. In the primitive 

 sashbar houses small glass of poor quality was used, and with 



