Chapter IX 



171 



Uptake and Movement 



Variations in the Rate of Sap Flow: — Use of the thermoelectric 

 method permitted the measurement of velocity of sap flow in roots, stem 

 bases, and tips. A striking lessening of velocity was observed by Huber 

 and Schmidt (1936) in going from base to tip of trees. This is explained 

 on the relationship of number of conducting elements to number of leaves 

 drawing water through these elements. Baumgartner (1934) found that 

 the slender branches of birch exhibited the opposite state, i.e., the rate in- 

 creased rapidly toward the tip. In this case the sum of the effective cross 

 section of the branches must be less than that in the stem. Roots are 

 especially adapted to rapid flow of material in the xylem as pointed out by 

 RiEDL (1937). 



/60 



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Fig. 48. — Diurnal variation in rate of sap movement in Larix, Picea, and Fagus. 

 The vertical axis is rate in centimeters per hour. The solid line through the center 

 represents the null point readings on an excised shoot. From Huber and Schmidt 

 (1937). 



Huber and Schmidt (1937) investigated the velocity of the transpira- 

 tion stream in a number of trees and classified them into three groups : 1 ) 

 oak-type, in which the velocity diminishes toward the tip, being greatest at 

 the base of thick limbs; 2) birch-type, where the velocity increases toward 

 the tip and from stem to bough to twig; 3) the normal type, where the 

 differences in velocity between different parts of the tree were absent. This 

 last type, according to these authors, was postulated by the Leonardo- 

 Jaccard theory of constant water conductivity as being a normal situation. 

 The majority of the trees investigated fell into the oak-type. 



Ring porous trees such as oak, ash, and locust have their xylem-con- 

 ductive area confined in general to a relatively small portion of the outer 

 part of the annual rings. According to Huber and Schmidt, this is a re- 

 sult of air entering the wide vessels during the course of vegetative develop- 



