Genus Polygonum. 241 
Burck (4) showed that the floral nectaries in many instances act as 
hydathodes, causing dehiscence of the anthers even in saturated air. 
The frequent occurrence of extra-floral nectaries in tropical plants, 
where very sudden changes of humidity are often daily phenomena, cannot 
be without significance. 
There does not, therefore, seem any difficulty, so far as known facts are 
concerned, in deriving all nectaries originally from osmotic hydathodes, 
subserving a physiological function, which have in certain cases secondarily 
acquired biological importance. 
In conclusion, I should like to take this opportunity of recording 
my thanks to Professor F. W. Oliver, at whose suggestion and under whose 
direction the present work was undertaken. 
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