WATER UTILIZATION BY TREES 



31 



constructed to favor transpiration, they can transpire at the necessary 

 rate even in the moist, calm interior of the crowns. 



Another factor which helps to explain the increased transpiration 

 in sun leaves is their increased number of veins, as shown in table 11 

 (also from the work of Schramm). This greater nervature permits 

 an added flow of water to the leaf; otherwise the increased number of 

 stomata could probably not be supplied with sufficient water. 



Table 11. — Length of veins per square millimeter of shade and sun leaves of various 



tree species 



Species 



Shade 

 leaves 



Sun 

 leaves 



Species 



Shade 

 leaves 



Sun 



leaves 





Mm 

 8.8 

 9.9 

 10.5 

 3.6 

 6.9 



Mm 

 12.2 

 14.3 

 17.2 

 8.1 

 9.8 



Acer pseudoplatanus 



Mm 

 5.6 

 9.3 

 8.4 

 3.6 

 7.9 



Mm 



7.8 



Quercus sessitiflora 



Ulm us campestris 



11.8 





12.1 



Sambucus nigra 



Cornus mas - 



6.1 





11.4 







These differences in structure probably explain the results of Huber 

 (110) j who found that, when left to dry, the shade leaves of Quercus 

 pedunculata checked transpiration more rapidly than did the sun leaves, 

 especially during the first hour; but it took the sun leaves 2 days 

 longer to become air-dry. The shade leaves, in other words, close 

 their stomata more rapidly in case of water deficit, but the greater 

 cuticular protection along with other physiological factors determines 

 the better resistance of the sun leaves to prolonged drought. Kisselew 

 (122), however, found that while the shade leaves of Syringa and Cara- 

 gana wilted more rapidly than the sun leaves, this was not the case 

 with Tilia and Acer. 



In connection with the increase of the nervature in sun leaves, it is 

 interesting to note that this increase is very marked with increasing 

 height above ground in sun plants but is very slight in the case of 

 shade individuals, with the result that translocation of nutrients as 

 well as absorption of water is impeded in shade plants. This accounts 

 for the fact (which Burgerstein regarded as paradoxical) that the thin, 

 tender leaves of shade plants transpire less than the better protected 

 leaves of sun plants. As Maximov (148, p. 353) has pointed out, it 

 is the deficient water supply rather than the intense transpiration 

 which explains the ease with which shade plants wilt; and it is this 

 insufficient water supply which Zalensky (248) has called "internal 

 physiological dryness." 



Huber (105), as stated above, has been much interested in the water 

 supply of sun and shade plants and has pointed out that for the 

 former the ability to transfer water rapidly to the evaporating cells is 

 of great importance. He thus calculated the conductivity of xylem in 

 sun and shade branches of oak, comparing the conductivity with the 

 transpiration loss, and found that a sun branch transpired 75.7 mg 

 per square decimeter per hour, while the shade branch transpired 45.9 

 mg, or only about two-thirds as much. On the other hand, the cross 

 section of the conducting tissues per square decimeter of leaf surface 

 in the shade shoot was 0.20 mm 2 and in the sun shoot 0.42 mm 2 ; that 

 is, the area of the conducting tissue in the former is only one-half 

 what it is in the latter. But since the transpiration of the shade leaf 

 is two-thirds that of the sun leaf, the rate of flow in the vessels must 



