23 



clear glass and 273 for the la|)pcd glass, or a difference of 1 1 per- 

 cent. ' In the older houses, where the la])pino; is often wider and 

 considerable dust has accumulated, the percentage of loss would 

 probably be higher than in these tests. 



The lapping in a house amounts to quite an area in all. In a 

 house 30 feet wide there would be 440 inches in one section of the 

 roof 2 feet wide, provided 24 x 24 inch glass were used, and each 

 glass lapped 3-8 inches. In a house of this same type, 100 feet 

 long, the lapping would amount to 7200 square inches, or 50 square 

 feet. Lapped glass, however, is better than butted glass with the 

 present methods of construction and material. 



The loss of light from shadow casting material is quite marked, 

 and affects to a considerable extent the photosynthetic process and 

 the amount of carbohydrates formed in the leaves. Two experi- 

 ments were made in the greenhouse to determine the loss from 

 shadow casting material, and the table below gives the average 

 of both. In one experiment the exposed recording apparatus re- 

 ceived the direct light through the greenhouse roof, with no 

 shadows, and in another the apparatus was placed in the shade of a 

 2x4 wooden beam, some 8 feet distant from it, where it was in 

 the shade of the timber all the time. 



Table IX showing amount of light lost from shadows. 

 Average of two experiments. Duration of experiment, two 

 hours. 



Direct sun- 

 Rcadin5i;s. i light 



100% 



Greenhouse experiment, — no shiide, 



Greenhouse experiment, — in shadow of 2 x 4 timber. 



The difference in the two readings shows a loss of 34 percent, 

 from shadows. The loss of light caused by shadows and its effects 

 on photosynthesis can be dctemiined by various experiments. 

 For instance, two pieces of cork pinned to a leaf, one on each side, 

 cause starch to disappear from the leaf, and even the shadow cast 

 by the cork will affect m.aterially the starch fonn.ation of the part 

 of the leaf where it falls. (See fig. 3). A photographic negative 

 placed on a leaf also affects the starch formation, the leaf acting 

 as sensitized photographic paper. Under the opaque part of the 

 negative little starch will foiTn, but there will be more under the 

 light part. A leaf so exposed, when bleached and treated with 

 iodine, will bring out a reproduction of the negative in black and 

 white, and this would be a good method to demonstrate that 

 photosynthesis is in a general way proportionate to light intensity. 



