Plant and animal taxonomists today designate a "type specimen" as THE representative of a new species that they are naming. Plant types are generally housed in important museums or herbariums where plant taxonomists have access and can compare the original type to modern specimens for validation. See detailed account in Wikipedia; http://en.wikipedia.org/wiki/Type_(biology
Type specimens are designated for Genera, species, and lesser categories and serve as the final criterion of the characteristics of that category. Linnaeus's herbarium specimen of Iris germanica serves as the type for the Genus Iris and as the type for the species.
For over a couple of hundred years taxonomists have created herbarium specimens by drying plants in a plant press and then adfixing them to standard size herbarium sheets of rag paper that holds up through time. Information is included on the sheet such as the collector's collection number which referred back to their notes made for the collection. Early information on these sheets was often sketchy. Iris missouriensis was named based upon its location in the Missouri river basin but that extended from the rocky mountains to the Missouri river and the iris only occurs in the western part of that area and in adjacent mountain ranges. Today we are more specific with GPS information providing locations within a few feet. It is also encouraged today to include a description of the environment along with photographs.
Type specimens began being designated around 1850. Specimens designated by the species author are known as holotypes and those that are designated later when no holotype had been specified or had been lost are known as lectotypes. Some type specimens such as Iris germanica go back as far as 1758. These physical records of species can divulge information that could not be noticed at the time they were preserved but now with our modern technology can be recognized.