WATER UTILIZATION BY TREES 13 



dropped, the osmotic pressure very suddenly decreased from 26.53 

 atmospheres to 8.43 in Larix occidentalis Nutt. and from 19.5 to 5.4 

 atmospheres in Fraxinus pennsylvanica lanceolata Sarg. 



So many workers have shown the relationship between osmotic 

 pressure and hardiness in herbs, as well as in shrubs and trees, that 

 the connection between these two phenomena can hardly be questioned. 

 Meyer (161), however, believes that other factors besides osmotic 

 pressure must be taken into account. He finds the water content of 

 the needles of Pinus rigida highest in summer and lowest in late 

 spring and admits that the maximum osmotic pressure (25.73 atmos- 

 pheres) occurs in January, but he thinks the differences in osmotic 

 pressure are not sufficient to explain the hardiness of this species, 

 which he attributes to the presence of pentosans and water-holding 

 colloids. In a later paper (152) he records that the total amount of 

 water in needles of this species was about 5 percent less in winter 

 than in summer, with a corresponding difference in osmotic pressure. 



The research in this field indicates, in general, that the total water 

 content of leaves is higher in summer than in winter, when there is 

 not only a lower absolute water content but also a lower relative 

 water content, inasmuch as the water at this time contains a greater 

 percentage of soluble materials. 



OTHER FACTORS INFLUENCING WATER CONTENT OF LEAVES 



Furthermore, the water content of leaves varies not only with the 

 season but also with the local climatic factors, age of leaf, etc. Hardtl 

 (84) showed that the water content in the blade and petiole varies 

 with the developmental stage of the leaf, and as the cell walls increase 

 in thickness this change in water content is associated with the torsion 

 stresses which help to maintain the leaf in its proper position. 



Other workers have emphasized the water content of leaves in 

 relation to wilting. Schroder (182), for example, who studied the 

 water loss of detached leaves over a period of time in order to observe 

 the maximum amount of water which the leaves could lose and still 

 recover when placed in water, found the relative water content of 

 leaves to be as follows, in percent: Fagus 50.5, Prunus 56, Pinus 56, 

 Salix 58, Quercus 59, Acer 63, Platanus 63. Alnus 63, Aesculus 66, 

 Betula 59, Tilia 61. 



Since these figures are obtained from drying in the air and hence 

 do not include the chemically and hygroscopically bound water, they 

 are somewhat smaller (but not much) than those reported by Eber- 

 mayer (56), who gives the following average water content of fresh 

 leaves: 



Water content {percent) 



Lombardy poplar 70. 49 



Ash, horse chestnut 65. 90 



Black alder, black locust 64. 10 



Oak, elm, white alder, linden 63. 32 



Birch, mountain ash, sycamore 61. 66 



Willow, Norway maple, aspen 69. 48 



Beech, hornbeam, field maple, white poplar 57. 02 



These figures also agree essentially with those of Clark (36), who 

 found in Populus leaves a water content of about 60 percent. This 

 student also noted only very slight changes in the water content of 

 leaves during the course of the day, but did find that the content 

 varied somewhat with the season. 



