The proportions of partial (incompletely acylated) glycerides present in oils from ripening seeds at different oil development stages were determined by the gravimetric method for 24 plant species belonging to 16 families. 133 samples were examined and 17 of these represented oil development stages below 10 %. No detectable amounts of partial giycerides were present in any case; this suggests the operation of the ‘quantum’ mechanism of fat synthesis in all the species studied. Copyright © 1969 John Wiley & Sons, Ltd