WATER UTILIZATION BY TREES 23 



that the reason for the greater transpiration in the older leaves lay in 

 their greater cuticular transpiration. 



Von Guttenberg repeated the experiment of Bergen and got contrary 

 results: The ratio of the transpiration "without butter" to that "with" 

 was greater in old leaves than in young ones. Since in July the young 

 epidermis is more permeable than the old, the reasons for the greater 

 transpiration of the old leaves must rest entirely on their stomatal 

 behavior. The reason Von Guttenberg's results differed from those 

 of Bergen probably lies in the fact that Bergen carried on his experi- 

 ments in late summer and thus with older leaves; it seems that leaves 

 which persist more than 1 year become more permeable the second 

 summer. With leaves lasting only one season, however, the results of 

 various workers agree in general that transpiration from younger 

 leaves is greater than from older ones. The cuticular transpiration 

 of a very young leaf is so large that the total transpiration at this time 

 is at a maximum. If these youngest leaves are covered with hairs or 

 are protected otherwise against transpiration, the maximum may be 

 postponed to a later period. As the cuticle thickens in the growing 

 leaf, the transpiration rapidly diminishes, but at this time the stomata 

 commence to form so that transpiration again increases. 



However this may be, most workers in this field are agreed, as 

 witness the recent results of Schmucker (186) for linden, that the 

 cuticular transpiration of freshly unfolded leaves is high. Schmucker 

 also reports that the transpiration of young conifer shoots is high in 

 comparison with that of more mature branches, when measured in 

 terms of absolute water loss, but that in terms of green weight it is 

 comparatively low. 



In this connection it may also be mentioned that the number of 

 stomata per given area is greater in young leaves than in old leaves, 

 as shown in the following tabulation by Seybold (189): 



Number of stomata per mm i on 



Ficus elastica Roxbg.: under side of leaf 



Young 260 



Old 145 



Olea europaea L.: 



Young 1,072 



Old 625 



Acacia decipiens R. Br.: 



Young 153 



Old 96 



Pinus cedrus [probably P. cembra L.]: 



Young 96 



Old 32 



Red leaves of Cornus sanguined L. containing anthocyan transpire 

 more slowly than green leaves, according to Linsbauer (137). Similar 

 results were found by Stahl (203) in yellow and red autumn leaves. 

 Since the stomata of such leaves are generally closed, this may explain 

 the decreased transpiration. Even in Salix, which ordinarily does not 

 close its stomata when wilting, the stomata were found partly closed in 

 the autumn leaves. Molisch (156) obtained the same results, but it is 

 doubtful whether this closing of the stomata can explain the decreased 

 transpiration in the red Cornus leaves. Here the explanation is rather 

 to be sought in the higher osmotic concentration of the sap of cells 

 containing anthocyan. The higher the osmotic pressure the less the 

 transpiration, other factors being equal. 



