vi TENSION REOUIRED TO RAISE THE SAP 121 



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 maximum transpiration. 



It will be of interest to quote one 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 weighings gave the 

 amount of water transpired. At the same time, a branch 

 still attached to the tree was introduced into a hermeti- 

 cally 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 correc- 

 tion 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 



