80 J. H. Priestley and Dorothy Armstead 
its passage through the xylem. This equilibrium concentration would 
vary in the same plant at different seasons. 
In the tree stems investigated the evidence obtained agreed with 
the conclusion of Dixon and Joly that the highest value for the 
equilibrium concentration would be found in the xylem at the time 
that the leaf buds were bursting. 
Botanical Department, 
The University, Leeds. 
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