22 MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



many other factors have to be taken into account that the relation 

 becomes no loDger a direct one. In Tilia europea, in which the upper 

 side contains no stomata, only two-sevenths of the water loss occurs 

 through this side of the leaves, but this is probably a higher proportion 

 than in most trees. Thus in Citrus species Bartholomew (10) found 

 85 to 95 percent of the transpiration was from the lower surface 

 where the stomata are located, while Haas and Halma (78) found that 

 two to three and one-half times as much water was lost through the 

 under surface as the upper; and according to Haberlandt (81) the 

 cuticular transpiration is only one seventy-sixth of the total in 

 Aesculus hippocastanum L., one eighty-first in Corylus avellana L., 

 and about one-eleventh in Pyrus communis. Even cutimzed epidermis 

 is not completely impermeable to water. 



Age of Leaves 



The age of leaves likewise causes a change in their transpiration 

 activity. Von Hohnel (97) cut the leaves from shoots of Tilia and 

 Ulmus, placed them in water, and followed the course of subsequent 

 transpiration by weighing. The youngest leaves transpired the most, 

 and as the leaves matured the evaporation gradually diminshed; but 

 later a second maximum was reached when the leaves were completely 

 developed. He explained this by assuming that in the youngest 

 stage only cuticular transpiration was present; and since the cuticle 

 and epidermal walls are very tender the transpiration was accordingly 

 great. As the cuticle becomes thicker, the transpiration decreases, the 

 minimum occurring when cuticular transpiration is still small and the 

 stomatal transpiration has not yet become large. 



Burgerstein (28, p. 59) confirmed these results with the leaves of 

 Aesculus hippocastanum. Three leaves of different ages were cut 

 from the same shoot, placed with the petioles in a closed beaker, and 

 weighed after 2 and 24 hours. The youngest leaf lost 35.2 percent of 

 its original weight, the middle-aged leaf 37.4 percent, and the oldest 

 leaf 45.5 percent. But when calculated on the basis of 100 cm 2 of 

 surface, the losses were 31.7, 21.5, and 34.5 g, respectively. 



Extensive researches with various woody plants were carried out 

 by Seeliger (188), who used a method similar to that described above. 

 In every case maximum transpiration was found in leaves that had not 

 reached half their final size. After reaching this maximum, the 

 transpiration decreased and reached a constant value (in Acer psuedo- 

 platanus L. and Prunus cerasus L.) or it continued to a minimum 

 which was not reached until the leaf was almost mature (Populus 

 delioides Marsh.). This was followed in completely grown leaves by 

 an increase in transpiration which continued for a long time with 

 slight fluctuations. 



Bergen (14) found, however, that branches of the evergreen-hard- 

 wood vegetation of the Mediterranean with leaves 15 to 18 months old 

 gave off more water per unit area or per unit of weight than leaves 3 

 to 6 months old. Thus, old Quercus ilex L. leaves transpired three and 

 one-half times as much as younger leaves. Von Guttenberg (77) 

 found similar results, although his ratios differed in some cases. 



In order to explain this greater transpiration in older leaves Bergen 

 covered the underside of old and young leaves with cocoa butter. 

 Since the ratio of the transpiration "without butter" to that "with 

 butter" was greater in young leaves than in old ones, he concluded 



