28 MtBG. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



shade leaves. Using six different methods, he studied very carefully 

 the relative transpiration of sun and shade leaves of the same species 

 placed in the same environment. The method which he considered 

 the best was that of weighing rooted plants grown in watertight 

 vessels from which soil evaporation was prevented. The results of 

 all six methods, however, were consistent and in every case the leaves 

 developed in the sun showed a higher transpiration intensity than 

 leaves developed in the shade, whether in light or in darkness. Sun 

 and shade leaves of the oak transpired, during 1 hour, 170 and 110 

 mg per square decimeter; beech 240 and 110; and yew, 110 and 40, 

 respectively. At the time, these results were viewed with a great deal 

 of skepticism, since it was thought that plants in the sun should have 

 methods of checking transpiration. 



Geneau's results have been confirmed by many workers, including 

 Bergen (13), who used evergreen broad-leaved species. Twigs were 

 cut, placed with the cut end in water, and weighed at regular intervals. 

 When the sun leaves were in the sun and the shade leaves in the shade, 

 the former transpired about three to six times as much as the shade 

 leaves: but when both were in the sun or both in the shade, the sun 

 leaves transpired only one to two times as much as the shade leaves 

 per unit of surface. These leaves, on the whole, evaporated only 

 about 25 percent less than Ulmus campestris, a distinctly thin-leaved 

 species. 



Huber (109) reported that sun leaves of oak always transpire as 

 much as shade leaves and generally more. Since their area is smaller 

 they really transpire more per square centimeter. Kamp (119) found 

 that the loss of water from sun leaves of various species was much 

 more than that from shade leaves of the same species, and that the 

 dorsal surface of the sun leaves lost much more than that of the shade 

 leaves. Hesselman (95) also has shown that in sun leaves of beech 

 with many palisade cells there was more water transpired in propor- 

 tion to the leaf surface than from shade leaves of the same locality. 



Hanson (83), working with Ulmus, Acer, and Fraxinus in Nebraska, 

 concluded that five-sixths of the transpiration from the tree was from 

 the peripheral branches and only one-sixth from equal areas on shaded 

 branches. The evaporation at the south edge of the crowns was 

 one-half to two and one-third times that within the crown. The 

 leaves from the south edge also show a much more xerophytic structure 

 (thicker, more palisade tissue, etc.) than those inside the crown. In 

 Fraxinus pennsylcanica Marsh, the leaves at the south margin lost 

 three to six times as much water per unit area as those within the 

 crown, while in Ulmus americana the marginal leaves transpired 12 

 times as much per unit area as the interior leaves. 



Cribbs (40) in his fairly comprehensive studies on transpiration 

 in Tilia found a distinct correlation between transpiration and light 

 intensity when other factors were about equal. In ecological studies, 

 however, it is impossible to isolate individual, experimental factors; 

 in such studies one is forced, by the nature of things, to study the 

 effect of the site as a unit. 



Some experimental work in this general direction was carried on 

 by Burns and Hooper (29, 30), who planted seedlings of Pinus strobus 

 in the open in three different beds, one of which was left exposed to the 

 sun while the other two were placed in half shade and full shade, 

 respectively. Transpiration (determined by weighing) of the un- 



