WATER UTILIZATION BY TREES 21 



internal factors 



Number and Location of Stomata 



Among these internal factors, to be mentioned first of all are the 

 number and location of the stomata. Since the stomata are openings 

 in the epidermis through which the moisture from the inner cells of 

 the leaf can escape, it is to be expected that, other things being 

 equal, leaves or regions with more stomata will show more transpira- 

 tion. Not all of the water, however, evaporates through the stomata; 

 it has been estimated by Haberlandt (81) that about 10 percent of 

 the total water lost from leaves is through the cuticle. These figures 

 differ from species to species, since the percentage of the area taken 

 up by the stomata also differs. In the horsechestnut it is estimated 

 that the proportion of water transpired by the cuticle to the total 

 evaporation of the leaf is as 1 is to 7.6, and in the birch as 1 is to 10.6. 



The stomata are located for the most part on the under side of the 

 leaves, although leaves of some trees contain stomata on both sides. 

 Some maple species have stomata only on the under side but a few 

 have stomata also on the upper side, according to Warsow (225). In 

 Tilia grandifolia Ehrh., Fraxinus excelsior, and Acer pseudoplatanus L. 

 they are most plentiful in the center of the lower side of the leaf, 

 diminishing towards the margin. In these species stomata are com- 

 pletely absent from the upper side, as is also the case in T. europea L. 

 and T. silvestris Desf. In conifers the stomata appear as white 

 points in longitudinal rows on the needles. In the silver fir they 

 occur only on the under side; but in Pinus strobus and in certain 

 species of Thuja and Juniperus, they are found only on the upper 

 side. Out of 1,359 species of broad-leaved trees studied by Koehne 

 (126), stomata were found on the upper side of the leaves in only 

 222 species. The following average numbers of stomata per square 

 millimeter of leaf surface were found by Espe (57) on the under side 

 ^ leaves: Number 



Fraxinus 494 



Aesculus 318 



Tilia grandifolia Ehrh 222 



Prunus avium L 203 



Juglans regia L 195 



Acer pseudoplatanus L 112 . 



Malus malus Britt 106 



Prunus laurocerasus L 76 



Oaks have an unusually large number of stomata, varying in dif- 

 ferent species, according to Yocum (246), from about 500 to 1,200 

 per square millimeter. This seems somewhat surprising in view of 

 the comparative drought resistance of oaks, and indicates that number 

 of stomata alone is not a determining factor in transpiration. This 

 is supported further by his findings that on a sprout which grew 44 

 inches during one season the number of stomata increased from 410 

 per square millimeter at 8 inches above the ground to 533 at 40 inches. 

 All were mature leaves, indicating that it is possible, under certain 

 conditions, for the number of stomata to vary directly with the 

 drought resistance rather than inversely, as is usually the case in 

 trees. This question, however, like many others, needs much further 

 study. 



Also most workers are agreed that, in general, leaves transpire 

 more from the side possessing the greater number of stomata, but so 



