vi TENSION REQUIRED TO RAISE THE SAP 1 19 



The subsequent wilting of the leaves of cut branches 

 shows that clogging afterwards greatly reduces the flow ; 

 but, of course, in the experiment quoted it is only the 

 initial stage which is recorded. 



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 conse- 

 quently 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, 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 approxi- 

 mately proportional to the leaf-area they supported. At 

 52 cm. from the apex the area of the cross-section of the 

 wood was 63 sq. cm., and the sum of the lengths of the 

 leafy branches above this was 170 cm. At 134 cm. from 

 the apex the cross-section was T29 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 



this error, but probably gives too small an amount after transpiration has 

 proceeded for some time. Strasburger considers that this is corrected more 

 or less accurately by his over-estimate of the resistance. 



