14 



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



Schroder also studied the water content of leaves during wilting 

 and dying, as well as the changes in the leaf structure at this critical 

 period. The first portions of the plant to suffer are generally the 

 margins or tips of the leaves and then the parenchyma regions be- 

 tween the main veins, in other words, the regions most remote from 

 the water supply. Death occurred when the water supply was re- 

 duced to the percentages shown in table 3. 



Table 3. — Percents of water content of leaves at time of death 



Genus 



Percent of 

 normal 

 water 

 content 



Percent of 



fresh 



weight 



Genus 



Percent of 

 normal 

 water 

 content 



Percent of 

 fresh 

 weight 



Betula __. l 75 



45 

 46 

 35 



Plaianus _ 



48 

 46 

 82 



30 





74 

 70 





30 



Fagus 



Tilia 



50 











The water content of both leaves and twigs of 11 evergreen and 

 11 deciduous species including Picea excelsa Link., Carpinus betulus 

 L., Pseudotsuga taxijolia (Lamb.) Britt., Pinus strobus, Fagus sylvatica 

 L., Betula verrucosa Ehrh., Quercus robur L., Acer platanoides L., and 

 Ulmus campestris from the botanical garden at Sophia, were studied 

 by Stefanoff (207). The "water capacity" was measured by the 

 difference between the weight after standing in water under glass 

 jars 48 to 72 hours and the weight after drying for 7 hours at 105° C. 

 The "water content" was the difference between the fresh and dry 

 weights, and the "water deficit" the difference between the water 

 capacity and the content. Measurements were made in the morning 

 and in the afternoon from March to September in the conifers, and 

 from April to September in the deciduous species. It was found that 

 the water capacity, which varied with the age and structure of the 

 leaves and twigs, was highest in the deciduous species in May and 

 in evergreens in June. The maximum capacity was about the same 

 for all species (70 to 80 percent' of the dry weight). The deciduous 

 species decreased more rapidly with age than the evergreens. Actual 

 water content, which was always less than the capacity and varied 

 not only with the species (especially with the internal conductive 

 tissues) but also somewhat with the soil moisture, temperature, 

 humidity, etc., was usually greater in the morning than in the after- 

 noon, and was less in June and July. The deficit exceeded 10 percent 

 in only a few species at any time, the maximum being about 16 

 percent with Thuja orientalis L. 



WATER CONTENT RELATED TO SITE 



Since the density of the cell sap (that is, osmotic pressure) may bo 

 a measure of a plant's ability to absorb water from the tissues below 

 it and ultimately from the soil itself, Korstian (129) tried to correlate 

 the osmotic pressure in leaves with site conditions in the Wasatch 

 Mountains of Utah, where he studied the freezing-point depression 

 (A) of the cell sap of leaves of a number of tree species. The osmotic 

 pressure of the leaves increased in winter, and the more drought- 

 resistant species generally had higher sap concentrations. Conse- 

 quently, the more favorable the site, especially from the point of 



