44 Prof. H. H. Dixon. [Nov. 7, 



The objections to this method are obvious. 



1. When the branch is first introduced into the desiccated 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 rate as those on the single desiccated 

 branch, whereas the supply, which under the conditions of the experiment 

 is available for the very actively transpiring branch, would be largely 

 encroached upon if all the branches were under equally favourable conditions 

 for transpiration. In fact, the single branch in the desiccated chamber and 

 still attached to the rest of the tree, which is under normal conditions of 

 moisture, is under conditions of supply approximating to those of a cut 

 branch set in water and, for the same reasons, cannot be assumed to give a 

 correct estimate of the velocity of the transpiration current throughout the 

 whole tree. 



The validity of this objection may be demonstrated experimentally by 

 weighing the amount of water given off by a given number of leaves in 

 a desiccated chamber, and comparing this amount with the quantity of water 

 transpired by the same number of leaves on the same tree exposed to normal 

 conditions of maximal transpiration. 



I have made several of these experiments : A small yew-tree was removed 

 from the flower-pot in which it had been grown, and the roots, and their 

 surrounding soil, enclosed in a rubber bag ; to prevent loss of water, except from 

 the leaves, the opening of the bag was tied tightly round the stem. Periodic 

 weighing's gave the amount of water transpired. At the same time a branch 

 still attached to the tree was introduced into a hermetically closed flask 

 containing calcium chloride. The flask could be removed and weighed 

 periodically. A rubber bag, similar to that enclosing the roots, filled with 

 moist earth and closed, was exposed to the same conditions and weighed 

 simultaneously, thus giving a small correction for loss through the bag. In 

 one of these experiments the unenclosed branches supported approximately 

 9500 leaves, the enclosed branch 520, i.e., the proportion of leaves on the 

 single branch to those on the whole tree was 1 : 18. When the tree was 

 exposed in a hot sun and brisk breeze, air temperature varying between 31° 

 and 24° C. in the month of July, the amount transpired was 5*171 grammes 



