WATER UTILIZATION RY TREES 



27 



developing leaves is determined by the wood structure of the stem 

 axis formed the preceding year, and if conditions were unfavorable 

 for its development, the decreased conduction facilities result in a 

 more xeromorphic leaf structure. 



Brenner (21) thought that the sun leaves of oak were generally 

 smaller than the shade leaves, but Burgers tein (28, p. 55) states that 

 this was not the case in many species, and that in Q. pedunculata as well 

 as in Carpinus, Fagus, Acer, etc., the sun leaves were larger than the 

 shade leaves. However, Brenner's findings that the sun leaves gen- 

 erally had thicker outer epidermis walls than the shade leaves and 

 that there were more layers of palisade cells agree with results of 

 other workers. 



Because of this thickening, it was held that sun leaves should tran- 

 spire less than shade leaves and this was the opinion of Von Hohnel, 

 who found that the mean transpiration per 100 g air-dry weight in a 

 broad-leaved tree from April 1 to October 31, 1879, was: Shade speci- 

 men, 94.35 kg; half shade, 88.78 kg; sun 62.68 kg. When conifers 

 and hardwoods were compared, he found that the hardwoods tran- 

 spired more in the shade than in the sun, while the reverse was true 

 of conifers, as shown in table 7. 



Table 7. — Relative amounts of water transpired by conifers and hardwoods in the 



sun and in the shade 



Species 



Sun 



Shade 



Species 



Sun 



Shade 



Beech 



Hornbeam 



Kg 

 76.2 

 86.3 



Kg 



107.8 

 98.9 



Fir 



Pine - 



Kg 

 13.9 

 19.1 



Kg 



4.8 

 5.0 







As has been mentioned above, however, Von Hohnel's method of 

 procedure is open to considerable objection and, if there had been an 

 abundance of water and plenty of air, doubtless the sun leaves of the 

 hardwoods would have transpired more than those in the shade, the 

 reason for the decrease being that water was the limiting factor. 



Table 8 shows the difference between larch plants in sunlight 

 and in half shade from April to October. Here also it will be noted 

 that most of the time these plants transpired more in the shade than in 

 the sun, but here, again, water was probably a limiting factor in the sun 



Table 8. — Transpiration per 100 g of air-dry weight of leaves of larch plants in 

 sunlight and in half shade, April to October 



Month 



April 

 May. 

 June. 

 July. 



Sunlight 



Half 

 shade 



Grams 



Grams 



2.82 



2.97 



7.16 



6.00 



10.82 



14.42 



16.10 



24.46 



Month 



August 



September 

 October. . . 



Sunlight 



Grams 

 21.80 

 44.66 

 21. 96 



Half 

 shade 



Grams 

 33.11 

 16.90 

 6.56 



The view that shade leaves transpired more than sun leaves was 

 held also by the other workers in this period until the careful experi- 

 ments of Geneau (68), who showed that the intensity of transpiration 

 of sun leaves was not less but even considerably greater than that of 



