The lodgepole pine dwarf mistletoe, Arceuthobium americanum Nutt. ex Engelm., is a parasitic angiosperm thatinfects conifers in western Canadian forests. A striking feature of the Arceuthobium life cycle is explosive seed dispersal,triggered by hydrostatic build-up within the fruit, localized in a tissue called ‘‘viscin.’’ Viscin is composed of two cell types: ‘‘viscin cells’’ that accrue pressure in their hydrated mucilage, and ‘‘vesicular cells’’ with a previously undetermined function. The objective of this work was to investigate the development and composition of vesicular cells using microscopy, cytochemistry, and gas chromatography – mass spectroscopy. We found that vesicular cells are initiated from mesocarp cells lining the viscin cells. As development of the seed progresses, vesicular cells form a single layer, enlarge, and become isodiametric. Later, the cells make up all of the remaining 3–4 outer mesocarp layers. Concomitant with enlargement, intracellular triacylglycerides (TAGs) largely composed of 16-carbon and 18-carbon fatty acids, accumulate. Immediately prior to discharge, vesicular cell boundaries become indistinct, and the cell contents become confluent, creating a lipid mass between the viscin cells and the exocarp. Vesicular cells and their resulting lipid mass likely function in dis-charge by acting as a hydrophobic barrier to water loss and (or) as a repellant, resistant layer.
