no TRANSPIRATION AND ASCENT OF SAP ch. 



certainly without special measurements it is not legitimate 

 to assume that the velocity throughout the length of the 

 branch was uniform. This is equally true with regard to 

 intact large trees. 



With regard to the resistance, it will be seen later on that 

 Ewart's results are considerably too high. 



Second method. The second method of deter- 

 mining the maximum velocity of the transpiration 

 current employed by Ewart is also open to objection. 

 It is described by him as follows : A small branch 

 bearing a small number of leaves, while still attached 

 to the tree, is led through a split rubber cork 

 into an air-tight glass chamber containing a weighed 

 quantity of calcium chloride. The gain in weight of the 

 latter gives the amount of water transpired by the leaves 

 in a given time. Assuming, then, that all the leaves of 

 the tree may act like those in the closed glass chamber, 

 the number of leaves on the whole tree will give the weight 

 of water transpired by the tree, and consequently the 

 amount of water which passes up the tree in a given time. 

 Then, by measuring the effective cross-section of the 

 trunk, the velocity of the transpiration current may be 

 estimated. 



Criticism. The objections to this method are obvious. 



1. When the branch is first introduced into the desic- 

 cated chamber it will lose water more rapidly than when 

 it was transpiring into the less dry external air. It will 

 continue this abnormally rapid rate of transpiration until 

 the concentration of the vacuoles of the evaporating cells 

 reaches a steady state, depending on the freedom of 

 supply from the water conduits and the vapour pressure 

 in the chamber, but, until this steady state is attained, 

 the amount of water entering the calcium chloride may 

 be largely in excess of that passing up the stem. 



2. But a more serious source of error is the assumption 

 that all the leaves of the tree can transpire at the same 



