WATER UTILIZATION BY TREES 



29 



shaded plants was 5 to 21 times that of the plants in full shade and 2 

 to 3 times that of the plants in half shade, which transpired 3 to 8 

 times as much as those in full shade. The evaporating power of the 

 air, as measured with both black and white atmometers, showed that 

 the plants followed the black atmometers more closely than the white. 



Weaver and Thiel {230) placed in various environments potted 

 seedlings of Acer saccharinum that had been grown in the sun and 

 shade. In the sun, the shade plants lost 35 percent as much water as 

 the sun forms in a given unit of time. Similarly a cut branch from the 

 center of an elm tree lost 11 percent as much as one from the edge. 



Bartholomew (10) reported that Citrus plants subjected to total 

 darkness for 2 hours in the middle of the day lost only one-third the 

 amount transpired in the same period before or after in indirect 

 sunlight. Among the most recent results in this field are those of 

 Biswell (16) j who measured the water loss in grams per square inch 

 of leaf area, in full insolation and in half shade, of five species of trees 

 in Nebraska. The mean annual precipitation during the last 20 

 years in this section was 37 inches, which is generally well distributed 

 throughout the growing season. The wind measurements and 

 evaporation losses from atmometers were recorded. Table 9 shows 

 that in every case transpiration in half shade was less than in full 

 sunlight. 



Table 9. — Average water loss per square inch of leaf area of leaves of 5 hardwood 

 species, in full sunlight and in half shade, in 2 weekly periods 



Species 



Water loss 



July 4-11 



Sun 



Half 



shade 



July 12-19 



Sun 



Half 

 shade 



Average 



Sun 



Half 

 shade 



Boxelder 



Sycamore.- - 

 Buckeye — 



Red oak 



Hard maple 



Grams 

 6.66 

 8.04 

 7.24 

 8.38 

 11.00 



Grams 

 5.24 

 6.82 

 6.62 

 4.87 

 4.46 



Grams 

 5.03 

 7.95 

 7.25 

 6.28 

 11.08 



Grams 

 2.93 

 6.45 

 5.25 

 4.16 

 4.43 



Grams 

 5.85 

 8.00 

 7.25 

 7.33 

 11.04 



Grams 

 4^08 

 6.64 

 5.99 

 4.52 

 4.45 



While these results all show that transpiration is greater in the 

 light than in the shade, it is surprising how variable results may be. 

 Thus, Stahl (203) found that a potted Ficus plant in Europe transpired 

 102 times faster in the sun than when on the back wall of a room 

 facing the north, while Haberlandt (SO) found that a Ficus leaf in 

 Java transpired in a sunny forenoon only 12 times as much as in the 

 evening, per unit of time. It is, of course, very difficult to compare 

 results in two different localities unless one has accurate measure- 

 ments of all the other factors involved. 



Some work has been done (though not much on trees) on the effects 

 of different colored light on transpiration. These experiments agree 

 in general in showing that the rays which influence photosynthesis 

 most, i. e., the ones that are most absorbed by the leaf, increase 

 transpiration most; these are the rays in the orange-red and blue 

 regions. 



