Smithsonian Report, 1919.—Coville. PLATE 17. 
BLUEBERRY LEAF EXUDING SUGAR FROM GLANDS INTERPRETED AS OSMOTIC-PRESSURE 
: SAFETY VALVES. 
This is a leaf of the high-bush blueberry, Vacciniwm corymbosum. The photograph was taken May 19, 
1916, and isenlarged four diameters. The sugar-secreting glands, sometimescalled extrafloralnectaries, 
are situated in this plant on the back of the midrib and along the margins of theleaf, toward its base. 
The drops of sugar solution have been wiped away from the glands on theleft-hand margin and from 
two glands on the midrib at the base ofthe second and fourth lateral veins above thesugar drop shown 
nearthe middle ofthe picture. Thisexudation ofsugarisinterpreted as a means ofrelieffrom excessive 
internal pressure that might burst the cells ofthe plant or derangeits physiological activities. 
