Transpiration and tlie Ascent of Sap. 59 



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 act like those in the glass 

 chamber, the number of leaves on the whole tree will give the total 

 weight of water transpired and consequently the amount of water 

 which passes up the tree in a given time. Finally by measuring the 

 effective cross section of the trunk, the velocity of the transpiration 

 current may be estimated. 



The results obtained by these investigators vary between wide 

 limits. A head equal to from 1—33 times the length of the traversed 

 stem is needed to produce the maximal transpiration rate. For many 

 reasons, however, the average is to be regarded as greatly excessive.^) 



The velocities indicated by these experiments must be greater 

 than those actually obtaining in intact trees. In the experiments the 

 same lifting forces are developed in the leaves but they have to lift 

 only a x^rj short column of water and to overcome the resistance of 

 only a short piece of stem, consequently the rate must be increased. 



In Janse's first method for obtaining the amount of transpiration 

 — successive weighings of a severed branch — towards the end of 

 the experiment the true transpiration rate may be more closely 

 approximated to, but the exposure of the opened conducting tracts 

 allows air gradually to enter and clog the tracheae. Consequently 

 when the resistance comes to be ineasured later on it will be found 

 to be excessive. 



E wart's method is also exaggerative. The small desiccated 

 branch will be able to draw off water from the supplies in the stem 

 much more readily than if all the branches were transpiring at the 

 same abnormal rate. Consequently it is not justifiable to assume that 

 the rate, at which water is given off by an isolated branch under 

 conditions of exceptional desiccation and supply, is ever attained 

 b}' all the branches when all alike are exposed to conditions most 

 favourable to evaporation. 



But the gravest errors seem to have been introduced in deter- 

 mining the head required to produce a given rate of flow. This 

 is due to the fact that in the experiments high pressures were used 

 and rapid flows attained. It is evident that the rapid currents 

 produced will tend to sweep every contamination on to the cut surface 

 and so clog the tracheae. Exudations from the cut cells form an 



^) H. H. Dixon, On the Transpiration Current in Plants. Proc. Roy. Soc. 

 Lond., B, Vol. 79. 1907. 



