5G MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



twigs from the 12-m level. In one series the hourly loss of water 

 varied at different heights as follows: 



Loss per gram of 



fresh weight in 



Height (meters): milligrams 



2 29 



4 . 27 



6 15 



8_____ 15 



10 10 



12 7 



14 14 



The cause of this lies not in differences in structure but in the greater 

 difficulty of supplying water to the upper part of the tree owing to the 

 resistance offered to the transpiration stream by the water-conducting 

 tissues. The transpiration of the twig immediately after it is cut from 

 the tree is generally greater than it is a few hours after cutting. The 

 transpiration rate falls off rapidly at first and then slowly approaches 

 a constant value, which apparently corresponds with the purely 

 physical cuticular evaporation as opposed to the physiologically 

 regulated stomatal transpiration at the start. 



Later, Huber (108) studied the fall in transpiration with increasing 

 height in many other trees, including Fagus, Tilia, Picea, Acer, 

 Quercus, Fraxinus, Larix, and Pinus, all of which showed a decrease in 

 transpiration rate with height above ground. Beech leaves, for 

 example, taken from the tree at 20 m from the ground, transpired less 

 than half as fast as leaves from a height of 10 m. In a still later paper 

 (110) Huber showed that the transpiration of mesophytic, tolerant 

 trees decreased with height more than that of intolerant ones. Thus, 

 there was a greater decrease in Sequoia, Tilia, Fagus, and Picea, than 

 in Acer, Quercus, and Fraxinus; and the decrease in Larix and Pinus 

 was still less. Blaydes (17) also found with Fagus and Carya that the 

 transpiration rate from leaves 4 feet above the ground was much 

 greater than that from leaves 20 to 30 feet above the ground. 



TRANSPIRATION AS A FUNCTION 



Xo discussion of transpiration could be considered complete without 

 reference to the question of the value of, or reason for, transpiration. 

 The early students of plant physiology considered that transpiration, 

 like photosynthesis and respiration, was a function, that is, it served 

 some useful purpose in the life of the plant. In recent years, however, 

 there has arisen a decided reaction against this point of view, and there 

 are many workers who consider that transpiration is only a necessary 

 evil. The leaf is constructed to manufacture food. Moist cell sur- 

 faces which permit the exchange of gases necessarily permit water 

 vapor to escape — an inevitable result of the leaf structure and not in 

 any sense purposeful. 



Those who hold that transpiration fulfills a useful function believe 

 that the transpiration stream aids in the distribution of minerals 

 throughout the plant. Water absorbed by the roots carries with it 

 the essential minerals, which flow along with the transpiration stream 

 to the different organs of the plant; therefore, the more transpiration, 

 the more water is carried from the roots to the leaves and the greater 

 the amount and the speed with which the minerals are distributed. 



