306 



GROWTH OF PLANTS 



same amount of light. This was found to be due in part to the low CO2 

 supply as a result of low rate of air exchange in the insulated house. Three 

 sources of CO2 were used for increasing the concentration of this gas in 

 greenhouse air: lumps of solid CO2, steel cylinders of liquid CO2, and the 

 CO2 respired by fowls. When the last source was used it was found neces- 

 sary to place the fowls in an outside chamber and to filter the respired air 

 through water to remove ammonia which arose from fermentation of the 

 droppings. 



Other mechanisms were used to turn on 500-watt Mazda lamps over 

 plants in greenhouses at night. An electric clock mechanism proved 

 especially valuable for it could be adjusted to any alternate periods of illu- 

 mination and darkness desired. 



Table 31. Dry Weights of Buckwheat Plants Produced in Each Type of Greenhouse 

 during Periods Indicated. Season of 1935-1936 



Table 31 shows the increase in dry weight in buckwheat plants gro^vn in 

 the control greenhouse, in an ordinary greenhouse with lights turned on 

 intermittently during the night, and for the insulated greenhouse with the 



