Palynologists count on reference collections. These are hundreds of slides from modern samples. Flowers were collected and pollen mounted on slide, for long-term storage. In case of any doubt when identifying a pollen grain, these reference slides can help. Of course, it works when you already have an idea about this pollen grain, but you're just not quite sure. Such collections are organised in families.
The many slide plates, organised by families, containing the many slides of the reference collection of the Institute of Botany of the University of Innsbruck.
Sometimes, I don't remember to which family belongs the species I have in mind, so I check it online. Unfortunately, nomenclature often changes, and it causes me regularly not to find the excepted reference slide in the newly attributed family. So, should we update the sorting of our reference collections? If yes, how often? Again, official classification is not really stable...
Then, at least, I came to an easy fix to make the process easier: I just add a blank slide in the reference to point to the family where the species was sorted at the time of the old-fashion classification!
The slide plate of Adoxaceae, where I expected to find slides of genus Sambucus, and a link to the plate of Caprifoliaceae, where its species were originally sorted.
You can think about it like a material equivalent to hypertext links :D