Nature's Pharmacy: Ancient Knowledge, Modern Medicine

Herbarium Specimens

Herbarium specimens are prepared by pressing and drying plants, and subsequently employing various techniques to attach them to sheets of stiff paper. The most common method is to use an adhesive similar to white glue and, if necessary, heavier portions of the plant are supported additionally by linen thread or narrow strips of gum-backed linen tape. If there are loose seeds or fruits, these are placed in a small fragment packet, which also is glued to the sheet. A label, which generally includes the Latin name of the plant, the locality where it was collected, information on habitat, name of the collector and date of collection, is attached in the lower right corner of the sheet. If someone subsequently examines the specimen, they may add an annotation, a small strip of paper on which they indicate agreement with the original identification or suggest an alternative. Usually, the sheet is stamped or embossed with the name of the herbarium (including the official herbarium acronym - IA, for the University of Iowa Herbarium), and a unique number, which reflects total collections at the time each specimen is added.

Potentially, herbarium specimens will last in perpetuity. As noted above, the earliest specimens in the University of Iowa Herbarium are over 100 years old, and older herbaria have some that are 300 years old. At present, to ensure longevity, every effort is made to utilize archival materials; for example, pH-neutral paper and glue. However, the deleterious effects of acidic materials were not recognized until relatively recently, so specimens from the first half of the 20th century usually are mounted on acidic paper with various acidic glues. A perpetual threat to the maintenance of herbarium specimens is insects, a number of which find dried plants palatable. Historically, various methods have been used to kill insects, which come in on the plants, before a specimen is inserted in the collection. For example, in the 1950s and early 1960s, University of Iowa Herbarium specimens were painted with an alcoholic solution of mercuric chloride, and these are stamped "poisoned." In addition, moth balls were placed in the storage cabinets to discourage insect activity; however, both substances are no longer used because they pose a health hazard. Presently, new specimens are frozen for one to two weeks prior to insertion and an air handling system maintains the temperature of the storage room at 60° F year-round. Over the last 10 years, these two methods have been extremely effective, virtually eliminating insect predation.