When I took botany and taxonomy of vascular plants in college, we spent many an hour poring over specimens under dissecting microscopes pulled with tweezers from smelly jars of preserving liquid. I was always a bit dubious about this prospect, although the result generally justified the effort. We also had some fresh specimens or live plants to examine, and on one occasion we even had frozen magnolia flowers to dissect. But the backbone of our laboratory learning program -- since there was no way the greenhouses could contain good specimens of all of the dozens of plant families we were learning -- were 11 x 17" sheets of acid-free paper on which plants pressed flat had been taped, glued, and labeled. They were called herbarium sheets.
Often the plants were decades old. One day I looked down at a sheet I was examining and noticed the plant had been collected in 1902. That was the year my grandfather was born, and I gazed in wonder at a plant that had been alive and growing in that year. And yet, like all the plants in the herbarium, it looked like it could have been pressed just a few decades ago at most. Such is the magic of the herbarium sheet -- manmade plant fossils that preserve color, detail, seeds, spores, and DNA in a way art cannot.
Herbarium sheets may also preserve the "type" specimen for a given species. This is usually the first specimen collected and named by a scientist. There's often a piece of red tape or another indicator on the page to mark it as type. They are very valuable, since they are the specimen against which all future specimens -- and the dividing lines between other species -- will be judged.
Some herbarium sheets contain little treasure chests too -- folded paper packets containing seeds or other structures that don't press well or fall off too easily. I once even saw an herbarium sheet for a fir tree with a fir cone wired into its own snug little cage on the sheet (fir cones fall apart at maturity, which is why you never see one lying on the ground off the tree).
Duke University has created a charming little slideshow, whence the images on thsi page come, highlighting some of the 800,000+ sheets in their herbarium, or library of preserved botanical specimens, which they claim makes it the second largest in the nation. Put the slide show in full-screen mode to fully appreciate the details.
With a focus on passion flowers (arguably (when unpressed) the coolest flowers in the world and the source of my favorite tropical fruit -- passion fruit) and a few neglected plants like sedges (at the top of this page -- monocot flowering plants related to grasses; the brown clusters are either the flowers or the developing fruits) and ferns (in the slide show, note the sporangia (spore houses) at the edges of the leaflets of the maidenhair ferns, and in clusters called sori on the edges of the smaller leaflets of the Christmas fern), it's a fun way to spend a few moments appreciating plants. And really, I think most people -- perhaps even you -- don't do that enough. : )