39 



SUMMARY. 



The experiments described in this bulletin not only have a 

 bearing on the physiology and pathology of crops, but also on 

 greenhouse construction and management. 



Light has an important influence on the configuration of plants. 

 Since most of the plant energy is derived from the air through 

 sunlight, the optim-um light conditions are important, and there is 

 a m.arked difference in plants as regards their light requirements. 



During the short winter days many greenhouse crops do not 

 obtain sufficient light, therefore any factor in greenhouse con- 

 struction which will increase the amount of light is important. 



Lack of light is responsible for m.any greenhouse diseases; on 

 the other hand, too intense light may prove harmful in some in- 

 stances. 



The old type of greenhouse was crude in construction, especially 

 as regards light. The modern tendency is to build larger houses, 

 to use stronger material casting less shade, and larger and. better 

 quality glass. 



Large houses can be constructed relatively m.ore cheaply and 

 m.anaged more easily because there is a less rapid change of atmos- 

 pheric conditions, etc. which helps to eliminate many greenhouse 

 troubles. 



Morning light is miore intense than afternoon light, our experi- 

 m.ents showing a difference of 10 percent., and ranging as high as 

 30 percent, for some months. Th^'s difference is not constant from 

 day to day, from m.onth to month or from year to 3'ear. 



The difference between morning and afternoon light has a 

 practical bearing on the location of greenhouses as regards points 

 of com.pass. 



The location of a house as regards points of the compass has a 

 bearing on the practice of syringing plants, the yield of the crop, 

 and to a certain extent on fungous infection. 



To obtain the best results in a house running east and west, 

 the house should be from 15 to 30 degrees north of east. This 

 enables the plant to take advantage ot the more intense morning 

 light and the crop can be syringed with less danger from infection. 



The difference in morning and afternoon light m,ay be shown by 

 various chemical methods, as well as by the growth of trees, and of 

 crops of cucumbers in north and south houses. Other things being 

 equal, a crop will show greater development on an east than a west 

 exposure. 



There is considerable difference in the light transmitting prop- 

 erties of glass. Second quality, double thick greenhouse glass 

 transmits 18 percent, less than No. 1, double thick, and the third 

 quality double thick transmits 33 percent, less than first quality 

 and 15 percent, less than second quality. 



