Collecting biological material requires care and attention to detail on locality, environment and general facies of the organism, all registered in the field-book, nowadays often directly in the computer.
The techniques used to prepare material for herbarium are time-consuming but quite simple and extremely effective. The specimens can be used for decades, even centuries, in different fields of research.
The characters used to distinguish between species, genera, etc, and to establish classifications are, then, available for study for a long time. For these purposes the amount of plant material needed is very small.
After careful re-hydration, all organs can be dissected. Anatomical sections of even old hard seeds can be made.
Pollen grains can be collected from anthers and its outer ornamentation studied under a scanning electron microscope. The importance of pollen morphology in taxonomy has been enhanced more recently as it has been shown that, in many groups of plants, it points to the same phylogenetic direction as molecular data.
DNA itself can also be extracted from herbarium specimens. Nowadays, it is possible to extract DNA from quite old specimens.
Modern electronic microscopes can operate at low vacuum. We recently investigated the ultrastructure of structures dependent of water, such as mucilage. The nutlet mucilage in the Lamiaceae is of taxonomic importance at species level.
We also investigate syndromes of pollination using exclusively herbarium specimens! This requires careful handling of material to guarantee as little as possible destructive sampling of data. This has been done on African specimens of difficult access in the wild.