A herbarium (pl. herbaria; lat. herba=herb, plant) is a collection of pressed and dried plants that are mounted on paper sheets. In addition, fruits, seeds, and wood samples are stored separately in special collections. Besides flowering plants, ferns, mosses, algae and fungi are preserved and stored in herbaria. These plant specimens are arranged based on taxonomic criteria (families, genera, species) and filed either alphabetically or based on a certain classification system. Morphological and anatomical features can easily be investigated and observed on specimens after centuries.
Herbarium specimens should present an image of the conserved plant as complete and detail-rich as possible. Therefore, specimens are supplemented with additional information about the geographical locality and the life conditions, as well as about growth form, color and smell. Basically, any features that cannot be retrieved later on from the preserved individual should be noted. For many plant groups, it is helpful to dissect parts of the flowers before drying and pressing.
Originally, the term "herbarium" was the name given to a book about herbs. The first herbal book, which was published by Otto Brunfels in several volumes starting from 1530, had the title "herbarium vivae eicones" (= "images of living plants"). The first herbarium collections were compiled during the first half of the 16th century along with the foundation of botanical gardens in central Italy. At that time, these collections were called "hortus siccus" (= "dry garden") or "hortus hiemalis" (= "winter garden"). This innovation allowed one to preserve plants for scientific investigations and the assessment of plant diversity, while providing material for future comparisons. Different sources indicate that Luca Ghini (1490-1556), founder of the botanical gardens in Pisa and Padua and professor of Botany in Bologna, was the first who dried and pressed plants and compiled a large collection of herbarium specimens. The herbarium of Gherardo Cibo, which was initiated in 1532, is still stored in Rome. The oldest herbarium specimens in our herbaria were collected by Johannes Scheuchzer (1684-1738). His extensive grass collections were used by himself for the publication of "Agrostographia" (= first scientific publication on grasses) in 1719.
What is a herbarium meant for?
The opportunity of preserving plants for future investigations constituted the prerequisite for the comprehensive research on plant diversity on our planet. Since the end of the Middle Ages, explorers have gathered collections of plant specimens from increasingly remote areas of the world and brought them back to Europe. For example, Carl von Linné, the founder of scientific nomenclature, has sent a large number of his pupils to all around the world, to enlarge his plant collections.
Each dried and labelled plant constitute the evidence that a species has occurred at a given time in a given place. A herbarium therefore constitutes a database of plant diversity in space and time. It is estimated that more than 300 million preserved plant specimens are stored worldwide in botanical institutes and museums; about a hundredth, namely about 3.8 million specimens, belong to the United Herbaria of the University and ETH Zurich. Every year, several thousand new specimens are added. A large part of these specimens result from donations and exchange programs with other herbaria, but field investigations in connection with ongoing research projects at our institutes also provide a large number of new specimens every year.
This large database of herbarium specimens is of great scientific value for various research questions related to plant diversity:
- Identification: An extensive collection of herbarium specimens is helpful for the identification of 'difficult' species. Many species complexes from the tropics and subtropics are still taxonomically insufficiently treated. In such cases, comparison with herbarium sheets may help identifying the genus or even the species of the unknown plant material, even if no recent taxonomic treatments with identification keys are at hand. The United Herbaria of the University and ETH Zurich encompass herbarium specimens of about a third of the 280'000 species of flowering plants described so far.
- Naming: If a Latin name is suggested for a newly discovered plant species, a selected, preserved individual must be deposited in a public herbarium according to the rules of botanical naming (nomenclature). The name of the corresponding species is then always associated with this one individual. The use of such "calibration points" for species names contributes to the stability of scientific names. The United Herbaria of the University and ETH Zurich have more than 10'000 such types.
- Morphological variation: Herbarium specimens of a given species document the range of morphological variation encountered in natura. Deciphering the reasons underlying biological diversity constitute a fundamental question in biology.
- Distribution in space and time: Data on sampling sites constitute inform about the geographical distribution of a given species. Distributional shifts over time can be revealed when data from various collections are compared. Accordingly, the spread or decline of a particular species can be easily assessed. Such findings are of great importance in the fields of conservation biology.
- Biodiversity: The identification of all species within a particular area provide information on the diversity of organisms ("biodiversity") of this region.
- Voucher repository: Depositing the specimens investigated during scientific investigations of biological compounds (phytochemistry), cellular or chromosomal features (cytology, karyology), or genes (molecular biology) is fundamental, because the vouchers allow a subsequent verification of the plant material used in the study. Any doubts about species identification can only be clarified, if the examined individuals have been preserved and deposited.
- Stock material: Dried plants can serve as a source of material for a variety of scientific investigations, e.g. for micro-morphological and anatomical investigations. It is even possible to isolate DNA from herbarium specimens, in order to sequence certain genes, even in species that are now considered extinct. Yet, such destructive investigations are only possible to a limited extent, because herbarium specimens are irreplaceable and therefore cannot be completely used up.
In the age of molecular biology, herbaria still constitute an indispensable source of information for the study of plant diversity. The "biodiversity crisis" (= rapid decline in biodiversity, often equated with mass extinction) has strengthened the awareness that the compilation of biodiversity is far from being completed. The data stored in herbaria are indispensable for the study of plant diversity worldwide.