842 BIOLOGICAL EFFECTS OF RADIATION 



The average reflection for the visible region was approximately 25 per 

 cent at X6000 A for a number of leaves measured. A tulip-tree leaf had a 

 reflection of 38 per cent at X9500 A and 5.6 per cent at 44,000 A. Reflec- 

 tion was determined with the leaf at an angle of 45 deg. For other data 

 on the absorption, transmission, and reflection of leaves in the visible 

 region the reader is referred to the section in this work by Spoehr and 

 Smith (Paper XXII) and that by Popp and Brown (Paper XXXI). 



In order to have some specific data on the relative transmission of 

 leaves in the infra-red and visible region the following tests were made. 

 A hole 1 in. in diameter was made in a piece of cardboard. This was 

 placed over a Coblentz vacuum thermopile with the hole above the 

 sensitive elements. The leaf was placed over the hole in the cardboard. 

 A sheet of metal was placed over this and supported at a distance of 

 about 1 in. above the cardboard. The sheet of metal was fitted with a 

 hole in the center 5 in. in diameter over which glass filters were placed. 

 The infra-red only filter was a piece of Coming's heat-transmitting glass 

 4 mm. in thickness. The visible-region filter, used for absorbing infra- 

 red, consisted of a circular glass dish containing 1 cm. of water and a 

 piece of Coming's heat-absorbing glass approximately 3 mm. in thickness. 

 The transmission of the two filters has been given in a previous publica- 

 tion (1). A 1500-watt lamp was used as a light source, suspended 

 directly above the thermopile. Allowing for a 20 per cent reflection in 

 both the infra-red only and visible region, the percentage transmission of 

 various leaves was as shown in Table 1. 



Table 1. — I.vfra-red and Visible Transmission of Leaves in Per Cent after 

 Allowing 20 Per Cent for Reflection 



Plant Infra-red Visible 



Sunflower 30 23 . 3 



Tomato (Magnus) 22 19.0 



Tobacco (Turkish) 30 22.0 



The figures indicate that the transmission as determined in this way is 

 slightly less in the visible region than in the infra-red. In addition to the 

 errors arising from the selective reflecting power in the two regions 

 already discussed, these figures when applied to sunlight are open to 

 another error in that the incandescent lamp has considerably more 

 infra-red and less visible energy than sunlight. Much more work needs 

 to be done on the effects of photosynthetic materials, water, and mineral 

 salts present on the infra-red absorption of various leaves. It is evident, 

 however, that a high percentage of infra-red is absorbed by the leaf. 

 This being the case, interest is directed toward a further study of what 

 becomes of the energy absorbed. 



