WATER UTILIZATION BY TREES 3 



definite degree of turgor in the cell, which means that the cell vacuoles 

 must contain a minimum amount of water. 



A third function of water in the tree is that of acting as a transport- 

 ing medium. The minerals absorbed from the soil and needed by the 

 stems and leaves, as well as the elaborated, organic foods manufac- 

 tured in the aerial portions and required by the various organs and 

 tissues of the tree, are all carried in water as the solvent medium. 



Furthermore, water plays a part in practically all of the chemical 

 reactions that take place within the cell. Tree organic chemistry, 

 like most chemistry, is water chemistry, in which water either enters 

 directly into the reactions or else is required as a solvent for the re- 

 actions to take place. Thus respiration, enzymatic processes, etc., 

 all require the presence of water, which is thus seen to be of the high- 

 est importance in the life and growth of trees. The total amount of 

 water utilized by a plant consequently depends upon (1) the amount 

 absorbed, (2) the amount stored and used within the plant for the 

 various purposes mentioned above, and (3) the amount lost in various 

 ways. 



ABSORPTION OF WATER BY TREES 



The water the tree uses enters its immediate environment via the 

 air and soil in the form of rain, snow, or other forms of precipitation. 

 Before it can be of use within the tree, however, it must first be ab- 

 sorbed by the organism. This absorption process may take place, 

 therefore, either through the aerial portions (the shoots) or through 

 the subterranean portions (the root system). 



WATER ABSORBED BY SHOOTS 



Of the water absorbed, small quantities may be taken in by the 

 leaves and branches during times of rainfall or from dew. Although 

 the actual amount absorbed in this fashion by most plants is very 

 small, dew may be extremely important in dry periods because of its 

 effect upon the humidity and the consequent checking of water loss, 

 and, according to Rubner (178, p. 11 8), l in dry summers it may serve 

 as almost the sole source of moisture, especially for young plants. 



WATER ABSORBED BY ROOTS 



By far the greater proportion of the water absorbed by plants is 

 taken in through the root system, the character of which for any given 

 plant species depends upon a great many factors, including (1) the 

 kind of soil, (2) the water content and distribution in the soil, (3) the 

 amount and distribution of nutrients in the soil, (4) the aeration of 

 the soil, and (5) the temperature of the soil. 



SPECIFIC DIFFERENCES IN ROOT SYSTEMS 



While these are the chief physicochemical factors in the environ- 

 ment determining the growth and form of root systems, two other 

 important factors must be taken into account in any consideration of 

 this subject, namely, the competition of other species and the inherent, 

 hereditary characteristics of the plant, i. e., the so-called "specific 

 differences" in root systems. Some trees, such as the pine, form tap- 



i Italic numbers in parentheses refer to Literature Cited, p. 85. 



