

1906.] On the Transpiration Current in Plants. 43 



The assumption that the velocity in the terminal branches is as great or 

 greater than in the trunk is also not justified. The lower leafy branches 

 have to do work against a smaller hydrostatic head, and against a smaller 

 resistance than the outer and upper branches, and consequently the flow will 

 be faster through the lower parts than above. Furthermore, in many cases 

 the effective cross-section supplying unit transpiring area is greater above 

 than below. A good instance of this was brought under my notice by 

 Dr. J. Joly last summer, in a young specimen of Abies excelsa, which had just 

 been felled. The tree was 550 cm. high. The section at its base showed 

 17 year rings. The lateral branches were almost uniformly clothed with 

 leaves, so that the length of the branches was approximately proportional to 

 the leaf-area it supported. At 52 cm. from the apex the area of the cross- 

 section of the wood was 0*63 sq. cm. and the sum of the length of the leafy 

 branches above this was 170 cm. At 134 cm. from the apex the cross-section 

 was 4*29 sq. cm. and the sum of the lengths of the leaf-bearing branches 

 above was 1500 cm. At the higher level the cross-section of the supply 

 conduits was 0*37 sq. cm. per 100 cm. of leafy branch, at the lower 

 point the cross-section of the supply was 0*286 sq. cm. per 100 cm. of 

 branch. Therefore, in the case of this tree, if all the leaves were transpiring 

 uniformly the velocity at the lower level must be greater than above. Hence 

 in Ewart's experiments it is quite possible that the velocity below was 

 considerably in excess of that in the upper parts of his severed branch, and 

 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. 



The second method of determining the maximal 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. 



* Loc. cit., p. 56. 



