inch glass, and in some of the modern houses 20 by 30 inch glass 

 is used. The use of this larger glass results in giving a house 

 considerably more light, thus increasing the photosynthetic* activi- 

 ties of the plant or capacity for carbon assimilation and growth. A 

 house glazed with 20 by 30 inch glass requires only five sash bars to 

 cover a space demanding six sash bars where 16 by 24 or 16 by 28 

 inch glass is used, and the inferior light conditions due to more fre- 

 quent lapping are avoided. 



A modern even span lettuce house constructed with 20 by 30 inch 

 glass is indicated in the following table as number 13. According to 

 our estimate a house of this size, constructed with sash bars 2 inches 

 wide, would by the use of 16 inch width glass furnish about 745 

 square feet more opaque surface, due to the use of light obstructing 

 material, than the same house constructed out of 20 by 30 inch 

 glass ; or in other words there would be about 19 per cent less sash 

 bar surface to obstruct light in this house if glazed with 20 by 30 inch 

 glass than if glazed with x6 by^2 4 inch glass. There would also be an 

 improvement in the light conditions owing to the less frequent lap- 

 ping reqijired by the larger glass. Assuming the glass to lap one- 

 half inch, there would be about 129 square feet more lapped glass with 

 the 16 by 24 inch size than with the 20 by 30 inch, or again of about 

 22 percent, which, together with the smaller number of sash bars 

 used, would amount to an appreciable gain in the light conditions for 

 the whole house. That this increase in the amount of light is of 

 great advantage in the growing of such crops as cucumbers and let- 

 tuce under glass in winter there is no doubt. It has long been 

 known to vegetable physiologists that, where the light conditions 

 fall below the normal requirements of the plant, increasing the 

 amount or intensity of the light by artificial means, such as the use 

 of electricity, etc., considerably accelerates the process of assimila- 

 tion. It is indeed not difficult to observe practical and conclusive 

 demonstrations of the positive beneficial effects which superior light 

 conditions have upon such crops as lettuce and cucumbers in green 

 houses. Many experiments have been made by the writer which 

 show this. cf. p. 20. 



*Photosynthesis is now universally used in all elementary text-books of botany to desig- 

 nate the processes bv which the chlorophyll grains (green granules of the leaves), aided by 

 liglit, manufacture carbohydrates from carbon dioxide and water, or in other words, to 

 designate carbon assimilation. 



