Longevity of reference slides of pollen mounted in silicone oil

A reference collection of microscope slides of modern pollen grains was established at the University of Minnesota in 1958 and has grown to nearly 10 thousand accessions prepared in our laboratories. The pollen grains are mounted in silicone oil, and the slides have been sealed with a variety of compounds: nail polishes, adhesives, paints, and paraffins. Each has deleterious effects on the contained pollen; these effects increase with time and thus limit the usable lifetimes of the slides. One effect, pollen pox, slowly destroys the shape and structure of the pollen wall until the grain becomes a featureless sphere; it characterizes nail polish and some adhesives. Other sealants, notably paraffin, result in the growth of crystals in the silicone oil that obscure the pollen grains. Of 20 compounds tested, fewer than half made satisfactory seals, and an estimated half-life of slides sealed with most of those is 10 to 20 years. Latex paint has the best performance thus far: fewer than 5% of slides made over the past 20 years have become unusable. Although a bad slide can often be replaced by a new one made from the stored preparation, the restricted longevity of slides requires continued monitoring and imposes a steady drain on the value of a large reference collection.